Classifications

• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10D—INORGANIC ELECTRIC SEMICONDUCTOR DEVICES
  • H10D30/00—Field-effect transistors [FET]
  • H10D30/60—Insulated-gate field-effect transistors [IGFET]
  • H10D30/67—Thin-film transistors [TFT]
  • H10D30/674—Thin-film transistors [TFT] characterised by the active materials
  • H10D30/6755—Oxide semiconductors, e.g. zinc oxide, copper aluminium oxide or cadmium stannate
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03K—PULSE TECHNIQUE
  • H03K17/00—Electronic switching or gating, i.e. not by contact-making and –breaking
  • H03K17/08—Modifications for protecting switching circuit against overcurrent or overvoltage
  • H03K17/082—Modifications for protecting switching circuit against overcurrent or overvoltage by feedback from the output to the control circuit
  • H03K17/0822—Modifications for protecting switching circuit against overcurrent or overvoltage by feedback from the output to the control circuit in field-effect transistor switches
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10D—INORGANIC ELECTRIC SEMICONDUCTOR DEVICES
  • H10D30/00—Field-effect transistors [FET]
  • H10D30/60—Insulated-gate field-effect transistors [IGFET]
  • H10D30/67—Thin-film transistors [TFT]
  • H10D30/6729—Thin-film transistors [TFT] characterised by the electrodes
  • H10D30/673—Thin-film transistors [TFT] characterised by the electrodes characterised by the shapes, relative sizes or dispositions of the gate electrodes
  • H10D30/6733—Multi-gate TFTs
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10D—INORGANIC ELECTRIC SEMICONDUCTOR DEVICES
  • H10D62/00—Semiconductor bodies, or regions thereof, of devices having potential barriers
  • H10D62/10—Shapes, relative sizes or dispositions of the regions of the semiconductor bodies; Shapes of the semiconductor bodies
  • H10D62/117—Shapes of semiconductor bodies
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10D—INORGANIC ELECTRIC SEMICONDUCTOR DEVICES
  • H10D62/00—Semiconductor bodies, or regions thereof, of devices having potential barriers
  • H10D62/10—Shapes, relative sizes or dispositions of the regions of the semiconductor bodies; Shapes of the semiconductor bodies
  • H10D62/17—Semiconductor regions connected to electrodes not carrying current to be rectified, amplified or switched, e.g. channel regions
  • H10D62/213—Channel regions of field-effect devices
  • H10D62/221—Channel regions of field-effect devices of FETs
  • H10D62/235—Channel regions of field-effect devices of FETs of IGFETs
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10D—INORGANIC ELECTRIC SEMICONDUCTOR DEVICES
  • H10D62/00—Semiconductor bodies, or regions thereof, of devices having potential barriers
  • H10D62/40—Crystalline structures
  • H10D62/405—Orientations of crystalline planes
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10D—INORGANIC ELECTRIC SEMICONDUCTOR DEVICES
  • H10D62/00—Semiconductor bodies, or regions thereof, of devices having potential barriers
  • H10D62/80—Semiconductor bodies, or regions thereof, of devices having potential barriers characterised by the materials
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10D—INORGANIC ELECTRIC SEMICONDUCTOR DEVICES
  • H10D64/00—Electrodes of devices having potential barriers
  • H10D64/60—Electrodes characterised by their materials
  • H10D64/62—Electrodes ohmically coupled to a semiconductor
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10D—INORGANIC ELECTRIC SEMICONDUCTOR DEVICES
  • H10D86/00—Integrated devices formed in or on insulating or conducting substrates, e.g. formed in silicon-on-insulator [SOI] substrates or on stainless steel or glass substrates
  • H10D86/40—Integrated devices formed in or on insulating or conducting substrates, e.g. formed in silicon-on-insulator [SOI] substrates or on stainless steel or glass substrates characterised by multiple TFTs
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10D—INORGANIC ELECTRIC SEMICONDUCTOR DEVICES
  • H10D86/00—Integrated devices formed in or on insulating or conducting substrates, e.g. formed in silicon-on-insulator [SOI] substrates or on stainless steel or glass substrates
  • H10D86/40—Integrated devices formed in or on insulating or conducting substrates, e.g. formed in silicon-on-insulator [SOI] substrates or on stainless steel or glass substrates characterised by multiple TFTs
  • H10D86/421—Integrated devices formed in or on insulating or conducting substrates, e.g. formed in silicon-on-insulator [SOI] substrates or on stainless steel or glass substrates characterised by multiple TFTs having a particular composition, shape or crystalline structure of the active layer
  • H10D86/423—Integrated devices formed in or on insulating or conducting substrates, e.g. formed in silicon-on-insulator [SOI] substrates or on stainless steel or glass substrates characterised by multiple TFTs having a particular composition, shape or crystalline structure of the active layer comprising semiconductor materials not belonging to the Group IV, e.g. InGaZnO
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10D—INORGANIC ELECTRIC SEMICONDUCTOR DEVICES
  • H10D86/00—Integrated devices formed in or on insulating or conducting substrates, e.g. formed in silicon-on-insulator [SOI] substrates or on stainless steel or glass substrates
  • H10D86/40—Integrated devices formed in or on insulating or conducting substrates, e.g. formed in silicon-on-insulator [SOI] substrates or on stainless steel or glass substrates characterised by multiple TFTs
  • H10D86/481—Integrated devices formed in or on insulating or conducting substrates, e.g. formed in silicon-on-insulator [SOI] substrates or on stainless steel or glass substrates characterised by multiple TFTs integrated with passive devices, e.g. auxiliary capacitors
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10D—INORGANIC ELECTRIC SEMICONDUCTOR DEVICES
  • H10D86/00—Integrated devices formed in or on insulating or conducting substrates, e.g. formed in silicon-on-insulator [SOI] substrates or on stainless steel or glass substrates
  • H10D86/40—Integrated devices formed in or on insulating or conducting substrates, e.g. formed in silicon-on-insulator [SOI] substrates or on stainless steel or glass substrates characterised by multiple TFTs
  • H10D86/60—Integrated devices formed in or on insulating or conducting substrates, e.g. formed in silicon-on-insulator [SOI] substrates or on stainless steel or glass substrates characterised by multiple TFTs wherein the TFTs are in active matrices

Definitions

• the present invention relates to a semiconductor device and a driving method thereof. Further, the present invention relates to an electronic device provided therewith.
• semiconductor device means all devices that can operate by utilizing semiconductor characteristics.
• a power device a display device including the power device, an integrated circuit including the power device, and the like are included in the category of the semiconductor device.
• a power device manufactured with the use of a silicon material As a semiconductor device used for a power device, a power device manufactured with the use of a silicon material is widely prevalent.
• the power device including silicon has a narrow band gap; therefore, the operation range is limited at high temperature.
• a power device including SiC or GaN, which has a wide band gap has been developed (see Patent Document 1, for example).
• Patent Document 1 Japanese Published Patent Application No. 2009-010142
• a heterojunction field-effect transistor As an example of a power device including GaN, a heterojunction field-effect transistor (HFET) can be given.
• HFET a heterojunction field-effect transistor
• an A1N layer, a GaN layer, and an AlGaN layer, which are buffer layers, are stacked over a SiC substrate, and a source electrode, a gate electrode, and a drain electrode are formed over the AlGaN layer.
• a high concentration two dimensional electron gas layer is formed at the interface between the GaN layer and the AlGaN layer.
• the two dimensional electron gas layer serves as a channel in the HFET and the HFET is in a normally-on state in which current flows even when voltage is not applied to a gate, causing a problem in that a circuit configuration of a driver circuit or a protective circuit is complicated.
• a normally-off power device When an electron concentration is simply reduced in order to obtain a normally-off power device, resistance of an element is increased. Therefore, it is extremely difficult to achieve both a normally-off state of a power device and a low resistance of the power device at the same time. Further, an attempt has been made to realize a normally-off device with an innovative structure; however, a device structure becomes complicated and manufacturing cost is increased, which is a problem.
• One embodiment of the present invention is a semiconductor device including a power element which is in an on state when voltage is not applied to a gate, a switching field-effect transistor for applying first voltage to the gate of the power element, and a switching field-effect transistor for applying voltage lower than the first voltage to the gate of the power element.
• the switching field-effect transistors have small off-state current.
• each of the switching field-effect transistors is a semiconductor device in which a channel region is formed using an i-type or substantially i-type oxide semiconductor layer. With the switching field-effect transistors, the power element is turned on or off by application of a high potential or a low potential to the gate of the power element.
• One embodiment of the present invention is a semiconductor device including a power MOSFET in which a first gate and a second gate are included and a channel region is formed using an n-type oxide semiconductor layer, a switching field-effect transistor for applying positive voltage to the first gate and the second gate of the power MOSFET, and a switching field-effect transistor for applying negative voltage to the first gate and the second gate of the power MOSFET.
• a node of the first gate and the second gate of the power MOSFET is connected to the switching field-effect transistors, and a channel region of each of the switching field-effect transistors is formed using an i-type or substantially i-type oxide semiconductor layer.
• One embodiment of the present invention is a semiconductor device including a first field-effect transistor connected to a high voltage generation source, a second field-effect transistor connected to the first field-effect transistor, a third field-effect transistor connected to the second field-effect transistor and a low voltage generation source, a capacitor connected to the second field-effect transistor and the third field-effect transistor, and a power MOSFET connected to the first field-effect transistor and the second field-effect transistor.
• the power MOSFET includes a first gate, a second gate, a first insulating layer in contact with the first gate, a second insulating layer in contact with the second gate, an oxide semiconductor layer formed between the first insulating layer and the second insulating layer, and a first terminal and a second terminal which are in contact with the oxide semiconductor layer and serve as a source region and a drain region.
• a node of the first gate and the second gate is connected to the first field-effect transistor and the second field-effect transistor.
• a channel formation region of each of the first to third field-effect transistors is formed using an i-type oxide semiconductor layer.
• the oxide semiconductor layer of the power MOSFET is n-type.
• MOSFET is higher than or equal to 1 x 10 cm and lower than or equal to 1 x 10 " cm—3 , preferably higher than or equal to 1 x 1017 cm — 3 and lower than or equal to 1 x
• the carrier concentration of the oxide semiconductor layer in each of the switching field-effect transistors and each of the first to third field-effect transistors is lower than 5 x 10 14 /cm 3 .
• the first gate or the second gate of the power MOSFET overlaps with one of the first terminal and the second terminal but does not necessarily overlap with the other of the first terminal and the second terminal.
• a power device which can realize an off state and a semiconductor device including the power device can be provided without increase in power consumption.
• FIG 1 is an equivalent circuit diagram illustrating a semiconductor device of one embodiment of the present invention
• FIGS. 2A to 2C are each an equivalent circuit diagram illustrating a semiconductor device of one embodiment of the present invention.
• FIGS. 3A to 3C are each an equivalent circuit diagram illustrating a semiconductor device of one embodiment of the present invention.
• FIGS. 4A and 4B are each an equivalent circuit diagram illustrating a semiconductor device of one embodiment of the present invention.
• FIGS. 5A and 5B are a cross-sectional view and a top view illustrating a semiconductor device of one embodiment of the present invention.
• FIGS. 6A and 6B are each a cross-sectional view illustrating a semiconductor device of one embodiment of the present invention.
• FIGS. 7A to 7D are cross-sectional views illustrating manufacturing steps of a semiconductor device of one embodiment of the present invention.
• FIG 8 is a cross-sectional view illustrating a semiconductor device of one embodiment of the present invention.
• FIGS. 9A and 9B are a cross-sectional view and a top view illustrating a semiconductor device of one embodiment of the present invention.
• FIG 10 is a cross-sectional view illustrating a semiconductor device of one embodiment of the present invention.
• FIGS. llA to 11D are cross-sectional views illustrating manufacturing steps of a semiconductor device of one embodiment of the present invention.
• FIG 12 is an equivalent circuit diagram illustrating a semiconductor device of one embodiment of the present invention.
• FIG 13 is an equivalent circuit diagram illustrating a semiconductor device of one embodiment of the present invention.
• FIG. 14 is an equivalent circuit diagram illustrating a semiconductor device of one embodiment of the present invention.
• FIG 15 is an equivalent circuit diagram illustrating a semiconductor device of one embodiment of the present invention.
• FIG 16 is an equivalent circuit diagram illustrating a semiconductor device of one embodiment of the present invention.
• FIG 17 is an equivalent circuit diagram illustrating a semiconductor device of one embodiment of the present invention.
• FIGS. 18Ato 18C illustrate electronic devices.
• voltage refers to a potential difference between a given potential and a reference potential (e.g., a ground potential) in many cases. Accordingly, voltage, potential, and a potential difference can be referred to as potential, voltage, and a voltage difference, respectively.
• each of A and B corresponds to an object (e.g., a device, an element, a circuit, a wiring, an electrode, a terminal, a conductive layer, or a layer).
• a connection relation other than that shown in drawings and texts is also included without limitation to a predetermined connection relation, for example, the connection relation shown in the drawings and the texts.
• a semiconductor device illustrated in FIG. 1 includes a power element 110 and a control circuit 100.
• the control circuit 100 includes a field-effect transistor 102 (also referred to as a first transistor), a field-effect transistor 103 (also referred to as a second transistor), a field-effect transistor 104 (also referred to as a third transistor), a capacitor 105, an overvoltage detection circuit 106, a refresh control circuit 107, a high voltage generation source 108, and a low voltage generation source 109.
• the control circuit 100 switches between high voltage generated by the high voltage generation source 108 and low voltage generated by the low voltage generation source 109, which is voltage applied to the power element 110. Moreover, the control circuit 100 controls the amount of current flowing through the power element 110 when overvoltage is applied between an input terminal IN and an output terminal OUT.
• a gate is connected to the overvoltage detection circuit 106, a first terminal is connected to the high voltage generation source 108, and a second terminal is connected to the power element 110.
• the field-effect transistor 102 controls application of a high potential to the power element 110 connected to the second terminal.
• a gate is connected to the overvoltage detection circuit 106, a first terminal is connected to the capacitor 105 and a second terminal of the field-effect transistor 104, and a second terminal is connected to the power element 110.
• the field-effect transistor 103 controls application of a low potential stored in the capacitor 105 from the low voltage generation source 109, to the power element 110 connected to the second terminal.
• off-state current in this specification refers to current flowing between a source and a drain, that is, between a first terminal and a second terminal of a field-effect transistor when the field-effect transistor is not conducting.
• a gate is connected to the refresh control circuit 107, a first terminal is connected to the low voltage generation source 109, and the second terminal is connected to the capacitor 105 and the first terminal of the field-effect transistor 103.
• the field-effect transistor 104 controls charging of the capacitor 105 connected to the second terminal with a low potential.
• a channel region of each of the field-effect transistors 102 to 104 is formed using an i-type or substantially i-type oxide semiconductor layer.
• the carrier density of the i-type or substantially i-type oxide semiconductor layer is lower than 5 x 10 14
• the carrier density can be obtained by the Hall effect measurement. Lower carrier density can be obtained with the use of measurement results of capacitance -voltage (CV) measurement.
• the hydrogen concentration of the oxide semiconductor layer can be measured by secondary ion mass spectrometry (SIMS).
• the field-effect transistor 102 including the i-type or substantially i-type oxide semiconductor in a channel region can have an off-state current of 1 x 10 ⁇ 16 ⁇ / ⁇ or smaller, and further can have an off-state current of 1 x 10 ⁇ 19 ⁇ / ⁇ or smaller.
• An i-type or substantially i-type oxide semiconductor has a wide band gap and requires a large amount of thermal energy for excitation of electrons; therefore, direct recombination and indirect recombination are less likely to occur.
• holes which are minority carriers are substantially zero; accordingly, direct recombination and indirect recombination are less likely to occur and the amount of current is as small as possible.
• a circuit in a state where the field-effect transistor is in a non-conducting (also referred to as OFF) state, a circuit can be designed with the oxide semiconductor layer that can be considered as an insulator.
• the field-effect transistor when the field-effect transistor is in a conducting state, the current supply capability of the i-type or substantially i-type oxide semiconductor layer is expected to be higher than the current supply capability of a semiconductor layer formed of amorphous silicon.
• the field-effect transistors 102 to 104 are enhancement type transistors and normally-off transistors with extremely small leakage current in an off state, and thus have excellent switching characteristics.
• the capacitor 105 is an element for holding a low potential to be applied to the power element 110 when the field-effect transistor 104 is intermittently conducting (also referred to as ON).
• the capacitor 105 may have a structure in which an insulating layer is sandwiched between conductors.
• the overvoltage detection circuit 106 is a circuit for controlling conduction and non-conduction of the field-effect transistor 102 and the field-effect transistor 103 in accordance with voltage between the input terminal IN and the output terminal OUT. Specifically, when overvoltage is applied between the input terminal IN and the output terminal OUT, the field-effect transistor 102 is made to be conducting, the field-effect transistor 103 is made to be non-conducting, and application of high voltage from the high voltage generation source 108 to the power element 110 is controlled.
• the field-effect transistor 102 When overvoltage is not applied between the input terminal IN and the output terminal OUT, the field-effect transistor 102 is made to be non-conducting, the field-effect transistor 103 is made to be conducting, and application of a low potential stored in the capacitor 105 from the low voltage generation source 109, to the power element 110 is controlled.
• the refresh control circuit 107 is a circuit for controlling conduction and non-conduction of the field-effect transistor 104 in order to control charging of the capacitor 105 with a low potential from the low voltage generation source 109. Specifically, the refresh control circuit 107 is a circuit for intermittently making the field-effect transistor 104 conducting in order to charge the capacitor 105 with a low potential before releasing a low potential to the power element 110, which is held because of charging of the capacitor 105 from the low voltage generation source 109.
• a power element which is turned on without application of voltage to its gate is used as the power element 110.
• a bipolar transistor, a field-effect transistor (FET), a gate turnoff thyristor, an insulated gate bipolar transistor (IGBT), or the like, which includes Si, SiC, GaN, or an oxide semiconductor can be used as appropriate.
• a power metal oxide semiconductor FET (power MOSFET), an HFET, a junction field-effect transistor (JFET), or the like can be used as appropriate.
• An equivalent circuit including a power element 121 having three terminals is illustrated in FIG 2 A. A gate of the power element 121 is connected to the field-effect transistor 102 and the field-effect transistor 103.
• one of a source terminal and a drain terminal of the power element 121 is referred to as a first terminal and the other of the source terminal and the drain terminal of the power element 121 is referred to as a second terminal.
• the first terminal is connected to the input terminal IN and the second terminal is connected to the output terminal OUT.
• the power MOSFET 101 includes four terminals, which are typically a first gate terminal (also referred to as a first gate), a second gate terminal (also referred to as a second gate), a drain terminal (also referred to as a drain), and a source terminal (also referred to as a source).
• the first gate and the second gate are provided above and below a channel region, and a signal for controlling switching of the power MOSFET 101 is supplied to the first gate and the second gate.
• FIG 2C is a circuit symbol of the power MOSFET 101 in which a first gate 201 and a second gate 206 are provided above and below a channel region.
• the power MOSFET 101 includes the first gate 201, the second gate 206, a first terminal 204A, and a second terminal 204B.
• a signal (a signal G in FIG 2C) output from the high voltage generation source 108 or the low voltage generation source 109 is input to the first gate 201 and the second gate 206.
• the signal output from the high voltage generation source 108 or the low voltage generation source 109 switching of conduction and non-conduction between the first terminal 204A and the second terminal 204B of the power MOSFET 101 is controlled.
• the channel region of the power MOSFET 101 may be formed using an n-type oxide semiconductor layer.
• the n-type oxide semiconductor layer has a carrier density
• the power MOSFET 101 includes an n-type oxide semiconductor layer in the channel region, on-resistance can be reduced and a large amount of current can flow as compared to a power MOSFET including an i-type oxide semiconductor layer in a channel region.
• the power MOSFET including an n-type oxide semiconductor layer in a channel region is a depletion type transistor, the transistor is a normally-on transistor in which current flows even in a state where voltage is not applied to its gate.
• the power MOSFET described in this embodiment includes the second gate 206 in addition to the first gate 201. When negative voltage is applied to the first gate 201 and the second gate 206, the power MOSFET can be turned off.
• the power MOSFET in which on-resistance is low and a large amount of current can flow can be turned off.
• the power MOSFET when positive voltage is applied to the first gate 201 and the second gate 206, the power MOSFET can be turned on.
• the threshold voltage can be made further negative by making the channel region thicker and on current can be increased as compared to a power MOSFET having a single gate.
• FIGS. 3A to 3C and FIGS. 4A and 4B dotted arrows are shown to facilitate understanding of flow of signals in accordance with conduction and non-conduction of the power MOSFET 101 and the field-effect transistors.
• the channel region of the power MOSFET 101 included in a semiconductor device is formed using an n-type oxide semiconductor layer, and the power MOSFET 101 is made to be conducting by the high potential from the high voltage generation source 108 and is made to be non-conducting by the low potential from the low voltage generation source 109.
• the power MOSFET 101 in the equivalent circuits illustrated in FIGS. 3A to 3C and FIGS. 4A and 4B may be replaced with the power element 121.
• the field-effect transistor 102 is made to be conducting by control of the overvoltage detection circuit 106, the field-effect transistor 103 is made to be non-conducting by control of the overvoltage detection circuit 106, and the field-effect transistor 104 is made to be non-conducting by control of the refresh control circuit 107.
• the field-effect transistor 102 is made to be conducting, so that a high potential is applied to the first gate and the second gate of the power MOSFET 101 from the high voltage generation source 108 and thus the power MOSFET 101 is conducting.
• the field-effect transistor 102 is made to be non-conducting by control of the overvoltage detection circuit 106
• the field-effect transistor 103 is made to be conducting by control of the overvoltage detection circuit 106
• the field-effect transistor 104 is made to be non-conducting by control of the refresh control circuit 107.
• the field-effect transistor 103 is conducting, so that a low potential which is stored in the capacitor 105 from the low voltage generation source 109 is applied to the first gate and the second gate of the power MOSFET 101 and thus the power MOSFET 101 is non-conducting.
• the field-effect transistor 102 is made to be non-conducting by control of the overvoltage detection circuit 106, the field-effect transistor 103 is made to be conducting by control of the overvoltage detection circuit 106, and the field-effect transistor 104 is made to be conducting by control of the refresh control circuit 107.
• a low potential is stored in the capacitor 105 from the low voltage generation source 109.
• charge of the capacitor 105 with a low potential is performed at regular intervals controlled by the refresh control circuit 107.
• the semiconductor device keeps the state illustrated in FIG 3B while a low potential making the power MOSFET 101 non-conducting, with which the capacitor 105 is charged by the low voltage generation source 109, is held in the capacitor 105.
• the semiconductor device is made to be in a state illustrated in FIG 3C in an intermittent manner; thus, the field-effect transistor 104 is made to be conducting and a low potential is stored in the capacitor 105.
• the operation illustrated in FIG. 3C may be performed once a minute, with enough time taken for charging.
• FIG 4A a node connected to the first gate and the second gate of the power MOSFET 101, the field-effect transistor 103, and one of the terminals of the capacitor 105, which are in a state illustrated in FIG. 3B, are illustrated with solid lines and other connections are illustrated by dotted lines.
• each of the field-effect transistor 102 and the field-effect transistor 104 becomes non-conducting, whereby the node connected to the first gate and the second gate of the power MOSFET 101 is electrically in a floating state.
• each of the field-effect transistor 102 and the field-effect transistor 104 includes an i-type or substantially i-type oxide semiconductor layer in the channel region, so that off-state current is extremely small. Therefore, the node connected to the first gate and the second gate of the power MOSFET 101 can hold a low potential stored in the capacitor 105 from the low voltage generation source 109 for a long time. A low potential may be applied to the capacitor not constantly but intermittently. Further, when a low potential is applied to the first gate and the second gate of the power MOSFET 101, the power MOSFET 101 is in an off state. Therefore, the semiconductor device of this embodiment can realize an off state of the power MOSFET 101 without increase in power consumption.
• a capacitor 401 may be additionally provided to the node connected to the first gate and the second gate as illustrated in FIG 4B. Note that in the semiconductor devices illustrated in FIGS. 2A and 2B, the capacitor 401 may be provided to the gate of the power MOSFET 101 or the gate of the power element 121.
• Embodiment 1 and a manufacturing method thereof will be described with reference to FIGS. 5A and 5B, FIGS. 6A and 6B, and FIGS. 7Ato 7D.
• FIG 5A illustrates one embodiment of a cross-sectional structure of the power MOSFET 101 described in Embodiment 1 and FIG 5B illustrates a top view of the power MOSFET 101.
• a cross-sectional view taken along line A-B in FIG. 5B corresponds to FIG 5A.
• the first gate 201 formed of a conductive layer is provided over a substrate 200, a gate insulating layer 202 is provided over the first gate 201, an n-type oxide semiconductor layer 203 is provided over the gate insulating layer 202, the first terminal 204A and the second terminal 204B formed of a conductive layer are provided so as to cover part of the oxide semiconductor layer 203, an insulating layer 205 is provided so as to cover the oxide semiconductor layer 203, the first terminal 204A, and the second terminal 204B, and the second gate 206 formed of a conductive layer is provided over the insulating layer 205 so as to overlap with part of the first terminal 204A and part of the second terminal 204B.
• the substrate 200 have at least heat resistance high enough to withstand heat treatment performed later.
• a glass substrate having a strain point of higher than or equal to 730 °C is preferably used.
• a glass material such as aluminosilicate glass, aluminoborosilicate glass, or barium borosilicate glass is used, for example.
• a glass substrate containing BaO and B 2 0 3 so that the amount of BaO is larger than that of B 2 0 3 is preferably used.
• a substrate formed of an insulator such as a ceramic substrate, a quartz substrate, or a sapphire substrate can be used.
• crystallized glass or the like may be used.
• a substrate obtained by forming an insulating layer over a surface of a semiconductor substrate such as a silicon wafer or a surface of a conductive substrate formed of a metal material can be used.
• the power MOSFET 101 having high heat resistance can be manufactured.
• the insulating layer having high thermal conductivity include an aluminum nitride layer, an aluminum nitride oxide layer, a silicon nitride layer, and the like.
• the first gate 201 is formed using a metal element selected from aluminum, chromium, copper, tantalum, titanium, molybdenum, and tungsten; an alloy containing any of these metal elements as a component; an alloy containing these metal elements in combination; or the like. Further, one or more metal elements selected from manganese, magnesium, zirconium, and beryllium may be used. In addition, the first gate 201 may have a single-layer structure or a stacked structure having two or more layers.
• a single-layer structure of an aluminum layer containing silicon, a two-layer structure in which a titanium layer is stacked over an aluminum layer, a two-layer structure in which a titanium layer is stacked over a titanium nitride layer, a two-layer structure in which a tungsten layer is stacked over a titanium nitride layer, a two-layer structure in which a tungsten layer is stacked over a tantalum nitride layer, a three-layer structure in which a titanium layer, an aluminum layer, and a titanium layer are stacked in this order, and the like can be given.
• a layer, an alloy layer, or a nitride layer, which contains aluminum and one or more elements selected from titanium, tantalum, tungsten, molybdenum, chromium, neodymium, and scandium, thereof may be used.
• the first gate 201 can be formed using a light-transmitting conductive material such as indium tin oxide, indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium zinc oxide, or indium tin oxide to which silicon oxide is added. It is also possible to have a stacked-layer structure formed using the above light-transmitting conductive material and the above metal element.
• a light-transmitting conductive material such as indium tin oxide, indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium zinc oxide, or indium tin oxide to which silicon oxide is added. It is also possible to have a stacked-layer structure formed using the above light-transmitting conductive material and the above metal element.
• the gate insulating layer 202 can be formed in a single layer or a stacked layer using a silicon oxide layer, a silicon nitride layer, a silicon oxynitride layer, a silicon nitride oxide layer, or an aluminum oxide layer.
• a portion of the gate insulating layer 202 which is in contact with the oxide semiconductor layer 203 preferably contains oxygen, and in particular, the portion of the gate insulating layer 202 is preferably formed using a silicon oxide layer.
• oxygen can be supplied to the oxide semiconductor layer 203 and favorable characteristics can be obtained.
• the gate insulating layer 202 is formed using a high-A: material such as hafnium silicate (HfSiO ⁇ ), hafnium silicate to which nitrogen is added (HfSi ⁇ O y N z ), hafnium aluminate to which nitrogen is added (HfAljO y N z ), hafnium oxide, or yttrium oxide, so that gate leakage current can be reduced.
• a stacked structure can be used in which a high-A: material and one or more of a silicon oxide layer, a silicon nitride layer, a silicon oxynitride layer, a silicon nitride oxide layer, and an aluminum oxide layer are stacked.
• the thickness of the gate insulating layer 202 can be greater than or equal to 100 nm and less than or equal to 300 nm.
• a four-component metal oxide such as an In-Sn-Ga-Zn-O-based metal oxide, a three-component metal oxide such as an In-Ga-Zn-O-based metal oxide, an In-Sn-Zn-O-based metal oxide, an In-Al-Zn-O-based metal oxide, a Sn-Ga-Zn-O-based metal oxide, an Al-Ga-Zn-O-based metal oxide, or a Sn-Al-Zn-O-based metal oxide, a two-component metal oxide such as an In-Zn-O-based metal oxide, a Sn-Zn-O-based metal oxide, an Al-Zn-O-based metal oxide, a Zn-Mg-O-based metal oxide, a Sn-Mg-O-based metal oxide, or an In-Mg-O-based metal oxide can be used.
• an n-component metal oxide includes n kinds of metal oxides
• M represents one or a plurality of elements selected from Group 13, such as gallium (Ga), aluminum (Al), or boron (B).
• Ga gallium
• Al aluminum
• B boron
• the contents of In and Zn are not zero.
• the above expression may represent an In-Ga-Zn-O-based metal oxide, an In-Zn-O-based metal oxide semiconductor, and the like.
• the energy gap of the metal oxide forming the n-type oxide semiconductor layer 203 be 2 eV or more, preferably 2.5 eV or more, more preferably 3 eV or more.
• an oxide semiconductor having an amorphous structure, a microcrystalline structure, a polycrystalline structure, or a single crystal structure can be used as appropriate. Further, an oxide semiconductor having a crystal in which the c-axis is approximately parallel to a direction perpendicular to a surface can be used.
• the n-type oxide semiconductor layer 203 has a carrier density of higher than or equal to 1 x 10 16 cm -3 and lower than or equal to 1 x 10 20 cm -3 , preferably higher than or equal to 1 x 10 17 cm "J and lower than or equal to 1 x 10 20 cm -3 . Since hydrogen and oxygen deficiency serve as donors in an oxide semiconductor, it is preferable that the hydrogen concentration be higher than or equal to 1 x 10 16 cm -3 and lower than or equal to 1 x 10 " cm .
• the thickness of the n-type oxide semiconductor layer 203 is set so that a depletion layer spreads in a channel region and the power MOSFET 101 is turned off when negative voltage is applied to the first gate and the second gate.
• the carrier density is higher than or equal to 1 x 10 cm and lower than or equal to 1 x 10 " cm
• the dielectric constant is 15
• the band gap is 3.15
• a gate is provided on one surface side of the oxide semiconductor layer
• the maximum width of the depletion layer is greater than or equal to 7 nm and less than or equal to 677 nm.
• the thickness of the n-type oxide semiconductor layer 203 can be set to greater than or equal to 14 nm and less than or equal to 1354 nm. Further, in the case where the carrier density is higher than or equal to 1 x 10 17 cm—3 and lower than or equal to 1 x 10 ⁇ " 0 cm — 3 , the maximum width of the depletion layer is greater than or equal to 7 nm and less than or equal to 218 nm. In this case, the thickness of the n-type oxide semiconductor layer 203 can be set to greater than or equal to 14 nm and less than or equal to 436 nm.
• the first terminal 204A and the second terminal 204B are formed using a metal element selected from aluminum, chromium, copper, tantalum, titanium, molybdenum, and tungsten; an alloy containing any of these metal elements as a component; an alloy containing these metal elements in combination; or the like. Further, one or more metal elements selected from manganese, magnesium, zirconium, and beryllium may be used. In addition, the first terminal 204A and the second terminal 204B may have a single-layer structure or a stacked structure having two or more layers.
• a single-layer structure of an aluminum layer containing silicon, a two-layer structure in which a titanium layer is stacked over an aluminum layer, a two-layer structure in which a titanium layer is stacked over a titanium nitride layer, a two-layer structure in which a tungsten layer is stacked over a titanium nitride layer, a two-layer structure in which a tungsten layer is stacked over a tantalum nitride layer, a three-layer structure in which a titanium layer, an aluminum layer, and a titanium layer are stacked in this order, and the like can be given.
• a layer, an alloy layer, or a nitride layer, which contains aluminum and one or more elements selected from titanium, tantalum, tungsten, molybdenum, chromium, neodymium, and scandium may be used.
• the first terminal 204A and the second terminal 204B can be formed using a light-transmitting conductive material such as indium tin oxide, indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium zinc oxide, or indium tin oxide to which silicon oxide is added. It is also possible to have a stacked-layer structure formed using the above light-transmitting conductive material and the above metal element.
• a light-transmitting conductive material such as indium tin oxide, indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium zinc oxide, or indium tin oxide to which silicon oxide is added. It is also possible to have a stacked-layer structure formed using the above light-transmitting conductive material and the above metal element.
• the insulating layer 205 can be formed using the material of the gate insulating layer 202 as appropriate.
• the second gate 206 can be formed using the material of the first gate 201 as appropriate.
• the power MOSFET 101 illustrated in FIGS. 5A and 5B includes an n-type oxide semiconductor layer in the channel region, on-resistance can be reduced and a large amount of current can flow.
• the power MOSFET including an n-type oxide semiconductor layer in a channel region is a depletion type transistor, the transistor is a normally-on transistor in which current flows even in a state where voltage is not applied to its gate.
• the power MOSFET described in this embodiment includes the second gate 206 in addition to the first gate 201. When negative voltage is applied to the first gate 201 and the second gate 206, the power MOSFET can be turned off. Therefore, the power MOSFET in which on-resistance is low and a large amount of current can flow can be turned off.
• the power MOSFET 101 when positive voltage is applied to the first gate 201 and the second gate 206, the power MOSFET can be turned on. Further, since the power MOSFET 101 includes the first gate 201 and the second gate 206, the channel region can be made thicker and on current can be increased as compared to a power MOSFET having a single gate.
• the power MOSFETs 101 are connected in parallel, so that the channel width W can be made wide. Accordingly, a power device through which a large amount of current can flow can be manufactured.
• FIGS. 6A and 6B cross-sectional structures of a power MOSFET, which are different from the structure in FIGS. 5A and 5B, are illustrated in FIGS. 6A and 6B.
• a second gate 206A overlaps with one of the first terminal 204A and the second terminal 204B and does not overlap with the other.
• the first gate 201 formed of a conductive layer is provided over the substrate 200
• the gate insulating layer 202 is provided over the first gate 201
• the n-type oxide semiconductor layer 203 is provided over the gate insulating layer 202
• the first terminal 204A and the second terminal 204B formed of a conductive layer are provided so as to cover part of the oxide semiconductor layer 203
• the insulating layer 205 is provided so as to cover the oxide semiconductor layer 203, the first terminal 204A, and the second terminal 204B.
• the second gate 206A formed of a conductive layer is provided over the insulating layer 205 so as to overlap with one of the first terminal 204A and the second terminal 204B and so as not to overlap with the other. That is, a region 208 is provided so that the oxide semiconductor layer 203 is overlapped with none of the second gate 206A, the first terminal 204A, and the second terminal 204B in the region 208.
• the second gate 206A can be formed using the material and the method similar to those of the second gate 206 illustrated in FIGS. 5A and 5B.
• a first gate 201A formed of a conductive layer is provided over the substrate 200, the gate insulating layer 202 is provided over the first gate 201A, the n-type oxide semiconductor layer 203 is provided over the gate insulating layer 202, the first terminal 204A and the second terminal 204B formed of a conductive layer are provided so as to cover part of the oxide semiconductor layer 203, and the insulating layer 205 is provided so as to cover the oxide semiconductor layer 203, the first terminal 204A, and the second terminal 204B.
• the second gate 206A formed of a conductive layer is provided over the insulating layer 205 so as to overlap with one of the first terminal 204A and the second terminal 204B and so as not to overlap with the other. That is, an offset region 209 is provided so that the oxide semiconductor layer 203 is overlapped with none of the first gate 201A, the second gate 206A, the first terminal 204A, and the second terminal 204B in the offset region 209.
• the first gate 201 A can be formed using the material and the method similar to those of the first gate 201 illustrated in FIGS. 5A and 5B.
• the power MOSFET 101A and the power MOSFET 101B illustrated in FIGS. 6A and 6B each includes the n-type oxide semiconductor layer 203 in the channel region, on-resistance can be reduced and a large amount of current can flow.
• the power MOSFET including an n-type oxide semiconductor layer in a channel region is a depletion transistor, the transistor is a normally-on transistor in which current flows even in a state where voltage is not applied to its gate.
• the power MOSFET described in this embodiment includes the second gate 206A in addition to the first gate 201 or the first gate 201A. When negative voltage is applied to either the first gate 201 or the first gate 201A and the second gate 206A, the power MOSFET can be turned off.
• the power MOSFET in which on-resistance is low and a large amount of current can flow can be turned off.
• the power MOSFET when positive voltage is applied to either the first gate 201 or the first gate 201A and the second gate 206A, the power MOSFET can be turned on.
• the power MOSFET since the power MOSFET includes either the first gate 201 or the first gate 201A and the second gate 206A, the channel region can be made thicker and a large amount of current can flow as compared to a power MOSFET having a single gate.
• the n-type oxide semiconductor has the offset region 209 which is not covered with the first gate 201A, the second gate 206A, the first terminal 204A, and the second terminal 204B, so that the drain breakdown voltage can be increased as compared to the power MOSFET 101 illustrated in FIG. 5A, and high voltage can be applied to the first terminal 204A or the second terminal 204B.
• FIGS. 5A and 5B a method for manufacturing the power MOSFET 101 illustrated in FIGS. 5A and 5B will be described with reference to FIGS. 7A to 7D.
• the first gate 201 is formed over the substrate 200.
• the gate insulating layer 202 is formed over the first gate 201.
• the first gate 201 When the first gate 201 is formed by a printing method, an inkjet method, or the like, the number of steps can be reduced.
• the first gate 201 can be formed in such a manner that a conductive layer is formed by a sputtering method, a CVD method, an evaporation method, or the like, and then the conductive layer is etched with the use of a resist formed in a photolithography step as a mask.
• an end portion of the first gate 201 be tapered because coverage with an insulating layer, a semiconductor layer, and a conductive layer which are to be formed later can be improved.
• an insulating layer having high thermal conductivity is preferably formed between the substrate 200 and the first gate 201 by a sputtering method, a CVD method, a coating method, a printing method, or the like.
• the gate insulating layer 202 can be formed by a sputtering method, a CVD method, a printing method, a coating method, or the like.
• the dense and high-quality gate insulating layer 202 having high withstand voltage can be formed by high-density plasma CVD using a microwave (e.g., a frequency of 2.45 GHz), for example.
• a microwave e.g., a frequency of 2.45 GHz
• the gate insulating layer 202 formed by the high-density plasma CVD can have a uniform thickness, the gate insulating layer 202 has excellent step coverage. Further, the thickness of the gate insulating layer 202 formed by the high-density plasma CVD can be controlled precisely.
• the n-type oxide semiconductor layer 203 is formed over the gate insulating layer 202.
• the n-type oxide semiconductor layer 203 is formed by a printing method, an inkjet method, or the like, the number of steps can be reduced.
• the n-type oxide semiconductor layer 203 having an island shape can be formed in such a manner that an n-type oxide semiconductor layer is formed over the gate insulating layer 202 by a sputtering method, a CVD method, a coating method, a pulsed laser deposition method, or the like, and then the oxide semiconductor layer is etched with the use of a resist formed in a photolithography step as a mask.
• the carrier density of an oxide semiconductor layer depends on a deposition condition such as the hydrogen concentration and the oxygen concentration of a source gas and a target, a material for deposition, or composition of the material.
• a deposition condition such as the hydrogen concentration and the oxygen concentration of a source gas and a target, a material for deposition, or composition of the material.
• the oxide semiconductor layer 203 after formation of the oxide semiconductor layer 203, heat treatment may be performed so that the oxide semiconductor layer has a microcrystalline structure, a polycrystalline structure, or a single crystal structure. Further, an oxide semiconductor having a crystal structure with a crystal in which the c-axis is approximately parallel to a direction perpendicular to a surface can be used. [0084]
• the first terminal 204A and the second terminal 204B serving as a source electrode and a drain electrode are formed.
• the first terminal 204A and the second terminal 204B are formed by a printing method, an inkjet method, or the like, the number of steps can be reduced.
• the first terminal 204A and the second terminal 204B can be formed in such a manner that a conductive layer is formed over the gate insulating layer 202 and the oxide semiconductor layer 203 by a sputtering method, a CVD method, an evaporation method, or the like, and then the conductive layer is etched with the use of a resist formed in a photolithography method as a mask.
• the insulating layer 205 is formed over the gate insulating layer 202, the oxide semiconductor layer 203, the first terminal 204A, and the second terminal 204B.
• the insulating layer 205 can be formed in a manner similar to that of the gate insulating layer 202.
• the second gate 206 is formed over the insulating layer 205.
• the second gate 206 can be formed in a manner similar to that of the first gate.
• the depletion type power MOSFET 101 including an n-type oxide semiconductor layer in the channel region can be manufactured. Note that in the above manufacturing steps, a layout of the second gate is changed, so that the power MOSFET 101A illustrated in FIG 6A or the power MOSFET 101B illustrated in
• FIG 6B can be manufactured.
• Power MOSFETs illustrated in FIG 8 and FIGS. 9A and 9B are different from the power MOSFETs illustrated in FIGS. 6A and 6B in that a gate is not provided between the substrate 200 and an oxide semiconductor layer 213. [0089]
• the n-type oxide semiconductor layer 213 is provided over the substrate 200, the first terminal 204A and the second terminal 204B formed of a conductive layer is provided so as to cover part of the oxide semiconductor layer 213, a gate insulating layer 212 is provided so as to cover the oxide semiconductor layer 213, the first terminal 204A, and the second terminal 204B, and a gate 211 formed of a conductive layer is provided over the gate insulating layer 212 so as to overlap with part of one of the first terminal 204A and the second terminal 204B. That is, a region 208 is provided so that the oxide semiconductor layer 213 is overlapped with none of the gate 211, the first terminal 204A, and the second terminal 204B in the region 208.
• the power MOSFET 111A having high heat resistance can be manufactured.
• the first terminal 204A and the second terminal 204B may be provided between the substrate 200 and the oxide semiconductor layer 213.
• the region 208 may not be provided and the gate 211 may be provided so as to overlap with part of the first terminal 204A and part of the second terminal 204B as in FIG 5A.
• the first terminal 204A formed of a conductive layer is provided over the substrate 200, the n-type oxide semiconductor layer 213 is provided so as to cover the first terminal 204A formed of the conductive layer, the second terminal 204B formed of a conductive layer is provided so as to cover part of the oxide semiconductor layer 213, the gate insulating layer 212 is provided so as to cover the oxide semiconductor layer 213 and the second terminal 204B, and the gate 211 formed of a conductive layer, a wiring 214 connected to the first terminal 204A, and a wiring 215 connected to the second terminal 204B are provided over the gate insulating layer 212.
• FIG. 9B is a top view of the power MOSFET 11 IB illustrated in FIG 9A.
• a cross-sectional view taken along line A-B in FIG. 9B corresponds to FIG. 9A.
• the gate 211 is provided in the periphery of the second terminal 204B and the wiring 215 connected to the second terminal 204B. Further, the first terminal 204 A and the wiring 214 connected to the first terminal 204 A are provided in the periphery of the gate 211.
• the gate 211 is provided over a region including a region which overlaps with neither the first terminal 204A nor the second terminal 204B. Further, part (an end portion) of the gate 211 may overlap with one or both of the first terminal 204A and the second terminal 204B.
• the power MOSFET 111B having high heat resistance can be manufactured.
• the oxide semiconductor layer 213 illustrated in FIG. 8 and FIGS. 9A and 9B can be formed using a material similar to the oxide semiconductor layer 203 described in Embodiment 2. Note that in the power MOSFET 111A illustrated in FIG 8 and the power MOSFET 111B illustrated in FIGS. 9A and 9B, the gate 211 is formed on only one surface side of the oxide semiconductor layer 213. Therefore, the thickness of the oxide semiconductor layer 213 is set so that a depletion layer spreads in the channel region and the power MOSFET 111B can be turned off when negative voltage is applied to the gate 211.
• the maximum width of the depletion layer is greater than or equal to 7 nm and less than or equal to 677 nm in the case where the carrier density is higher than or equal to 1 x 10 cm and lower than or equal to 1 x 10 20 cm -3 . Therefore, the thickness of the n-type oxide semiconductor layer 213 can be set to greater than or equal to 7 nm and less than or equal to 677 nm.
• the maximum width of the depletion layer is greater than or equal to 7 nm and less than or equal to 218 nm in the case where the carrier density is higher than or equal to 1 x 10 cm " and lower than or equal to 1 x 10 " cm .
• the thickness of the n-type oxide semiconductor layer 213 can be set to greater than or equal to 7 nm and less than or equal to 218 nm.
• each of the power MOSFETs illustrated in FIG 8 and FIGS. 9A and 9B includes an n-type oxide semiconductor layer in the channel region, on-resistance can be reduced and a large amount of current can flow.
• the power MOSFET including an n-type oxide semiconductor layer in a channel region is a depletion type transistor, the transistor is a normally-on transistor in which current flows even in a state where voltage is not applied to its gate.
• the power MOSFETs described in this embodiment can be turned off when negative voltage is applied to the gate 211 and can be turned on when positive voltage is applied to the gate 211. Therefore, the power MOSFET in which on-resistance is low and a large amount of current can flow can be turned off.
• Embodiment 1 a method for manufacturing the field-effect transistors 102 to 104 described in Embodiment 1 will be described with reference to FIG. 10 and FIGS. 11A to 11D. Since all the field-effect transistors 102 to 104 can have the same structure, here, a description is given on the field-effect transistor 102 as an example.
• a gate 251 formed of a conductive layer is provided over a substrate 250, a gate insulating layer 252 is provided over the gate 251, an i-type or substantially i-type oxide semiconductor layer 253 is provided over the gate insulating layer 252, a first terminal 254A and a second terminal 254B formed of a conductive layer are provided so as to cover part of the oxide semiconductor layer 253, and an insulating layer 255 is provided so as to cover the oxide semiconductor layer 253, the first terminal 254A, and the second terminal 254B.
• the substrate 250 the substrate 200 described in Embodiment 2 can be used as appropriate.
• the gate 251 can be formed using any of the materials for the first gate 201 described in Embodiment 2 as appropriate.
• the gate insulating layer 252 can be formed using any of the materials for the gate insulating layer 202 described in Embodiment 2 as appropriate.
• the thickness of the gate insulating layer 252 can be greater than or equal to 50 nm and less than or equal to 500 nm. When the thickness of the gate insulating layer 252 is large, gate leakage current can be reduced.
• the oxide semiconductor layer 253 can be formed using the metal oxide described as the material for the oxide semiconductor layer 203 in Embodiment 2. Further, an oxide semiconductor having an amorphous structure, a polycrystalline structure, or a single crystal structure can be employed as appropriate. Furthermore, an oxide semiconductor having a crystal structure with a crystal in which the c-axis is approximately parallel to a direction perpendicular to a surface can be used. Note that since the oxide semiconductor layer 253 is i-type or substantially i-type, the carrier density is lower than 5 x 10 14 /cm J , preferably lower than 1 x 10 12 /cm 3 , more preferably lower than or equal to 1 11
• x 10 /cm 3 it is preferably that hydrogen and oxygen deficiency serving as a donor be little, and the hydrogen concentration be lower than or equal to 1 x 10 16 /cm 3 .
• the field-effect transistor 102 which includes an i-type or substantially i-type oxide semiconductor layer in a channel region, which is purified by drastically removing hydrogen and in which oxygen deficiency is reduced to satisfy the stoichiometric composition ratio, whereby the off-state current can be smaller than or equal to 1 x 10 " A.
• a circuit can be designed with the oxide semiconductor layer that can be regarded as an insulator when a field-effect transistor is in a non-conducting state.
• the field-effect transistor 102 is an enhancement type transistor, which is normally off with an extremely small leakage current in an off state; thus, the field-effect transistor 102 has excellent switching characteristics.
• the first terminal 254A and the second terminal 254B can be formed using the material for the first terminal 204A and the second terminal 204B in Embodiment 2 as appropriate.
• the insulating layer 255 is preferably formed using an oxide insulating layer.
• the oxide insulating layer As a typical example of the oxide insulating layer, a silicon oxide layer, a silicon oxynitride layer, or an aluminum oxide layer can be given. Note that the insulating layer 205 may have a stacked structure of an oxide insulating layer and a nitride insulating layer. As a typical example of the nitride insulating layer, a silicon nitride layer, a silicon nitride oxide layer, or an aluminum nitride layer can be given. In the insulating layer 255, a region in contact with the oxide semiconductor layer 253 is formed using an oxide insulating layer, whereby oxygen deficiency of the oxide semiconductor layer can be reduced and the stoichiometric composition ratio can be satisfied.
• a structure of the field-effect transistor 102 can employ various modes without being limited to a specific structure. For example, a multi-gate structure with two or more of gates can be employed. Further, a structure where gate electrodes are provided above and below a channel region can be used. Note that when gate electrodes are provided above and below a channel region, it is possible to employ a structure where two field-effect transistors are connected in parallel.
• the gate 251 is formed over the substrate 250.
• the gate insulating layer 252 is formed over the gate 251.
• the gate 251 can be formed by the method for manufacturing the first gate 201 described in Embodiment 2 as appropriate.
• the gate insulating layer 252 can be formed by the method for manufacturing the gate insulating layer 202 described in Embodiment 2 as appropriate. Since an i-type or substantially i-type oxide semiconductor layer is highly sensitive to an interface state and interface charge, the gate insulating layer 252 is formed by high-density plasma CVD with the use of microwaves, so that the interface state density can be reduced and favorable interface characteristics can be obtained.
• the substrate 200 is heated when the gate insulating layer 252 is formed, whereby hydrogen, water, a hydroxyl group, hydride, or the like contained in the gate insulating layer 252 can be reduced.
• the gate insulating layer 252 is preferably formed while hydrogen, water, a hydroxyl group, hydride, or the like remaining in a treatment chamber is removed in order that hydrogen, water, a hydroxyl group, hydride, or the like contained in the gate insulating layer 252 is reduced.
• An entrapment vacuum pump is preferably used for removing hydrogen, water, a hydroxyl group, hydride, or the like remaining in the treatment chamber.
• a cryopump, an ion pump, or a titanium sublimation pump can be given.
• a turbo pump provided with a cold trap can be used for an exhaustion unit.
• the gate insulating layer 252 is higher than or equal to 6N (99.9999 %), preferably higher than or equal to 7N (99.99999 %) (that is, the impurity concentration is lower than or equal to 1 ppm, preferably lower than or equal to 0.1 ppm), hydrogen, water, a hydroxyl group, hydride, or the like contained in the gate insulating layer 252 can be reduced.
• an oxide semiconductor layer 253 A is formed over the gate insulating layer 202.
• the oxide semiconductor layer 253A can be formed by a printing method, an inkjet method, or the like.
• the island-shaped oxide semiconductor layer 253A can be formed in such a manner that an oxide semiconductor layer is formed over the gate insulating layer 252 by a sputtering method, a CVD method, a coating method, a pulsed laser deposition method, or the like and the oxide semiconductor layer is etched with the use of a resist formed in a photolithography step as a mask.
• the carrier density of an oxide semiconductor layer depends on a deposition condition such as the hydrogen concentration and the oxygen concentration of a source gas and a target, a material for deposition, composition of the material, or a condition of heat treatment.
• a deposition condition such as the hydrogen concentration and the oxygen concentration of a source gas and a target, a material for deposition, composition of the material, or a condition of heat treatment.
• the oxide semiconductor layer becomes i-type or substantially i-type.
• the oxide semiconductor layer 253A may be either i-type or n-type.
• the substrate is heated in the case where the oxide semiconductor layer is formed by a sputtering method, whereby an impurity such as hydrogen, water, a hydroxyl group, or hydride contained in the oxide semiconductor layer can be reduced. Further, crystal growth can be promoted in first heat treatment.
• the relative density of the metal oxide in the metal oxide target is higher than or equal to 80 %, preferably higher than or equal to 95 %, further preferably higher than or equal to 99.9 %, whereby the impurity concentration in the oxide semiconductor layer can be reduced; thus, a transistor having excellent electrical characteristics and high reliability can be obtained.
• hydrogen, water, a hydroxyl group, hydride, or the like remaining on an inner wall of the sputtering apparatus, on a surface of the target, or in a target material can be removed, so that an impurity such as hydrogen, water, a hydroxyl group, or hydride contained in the oxide semiconductor layer can be reduced.
• an entrapment vacuum pump for removing hydrogen, water, a hydroxyl group, hydride, or the like remaining in the sputtering apparatus.
• hydrogen, water, a hydroxyl group, hydride, or the like is evacuated, and the concentration of hydrogen, water, a hydroxyl group, hydride, or the like contained in the oxide semiconductor layer can be reduced.
• first heat treatment is performed, so that an impurity such as hydrogen, water, a hydroxyl group, or hydride contained in the oxide semiconductor layer 253A is removed. That is, at least dehydration or dehydrogenation can be performed. Note that oxygen deficiency in the oxide semiconductor layer 253A is also formed in the first heat treatment.
• the oxide semiconductor layer from which an impurity such as hydrogen, water, a hydroxyl group, or hydride is removed by the first heat treatment is referred to as an oxide semiconductor layer 253B.
• the temperature of the first heat treatment is higher than or equal to 400 °C and lower than or equal to 750 °C, preferably higher than or equal to 400 °C and lower than the strain point of the substrate.
• a heat treatment apparatus used for the first heat treatment is not limited to a particular apparatus, and the apparatus may be provided with a device for heating an object to be processed by heat radiation or heat conduction from a heating element such as a resistance heating element.
• a heating element such as a resistance heating element.
• an electric furnace, or a rapid thermal annealing (RTA) apparatus such as a gas rapid thermal annealing (GRTA) apparatus or a lamp rapid thermal annealing (LRTA) apparatus can be used.
• RTA rapid thermal annealing
• GRTA gas rapid thermal annealing
• LRTA lamp rapid thermal annealing
• An LRTA apparatus is an apparatus for heating an object to be processed by radiation of light (an electromagnetic wave) emitted from a lamp such as a halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressure sodium lamp, or a high pressure mercury lamp.
• a GRTA apparatus is an apparatus for heat treatment using a high-temperature gas.
• hydrogen, water, a hydroxyl group, hydride, or the like be not contained in nitrogen or a rare gas such as helium, neon, or argon.
• the purity of nitrogen or a rare gas such as helium, neon, or argon introduced into a heat treatment apparatus is preferably higher than or equal to 6N (99.9999 %), more preferably higher than or equal to 7N (99.99999 %) (that is, the impurity concentration is lower than or equal to 1 ppm, preferably lower than or equal to 0.1 ppm).
• an atmosphere inside the furnace may be a nitrogen atmosphere at the time of increasing the temperature and the atmosphere may be switched to an oxygen atmosphere at the time of performing cooling.
• oxygen can be supplied into the oxide semiconductor layer, the hydrogen concentration can be reduced, and oxygen can be supplied to oxygen deficiency in the oxide semiconductor layer, whereby an i-type or substantially i-type oxide semiconductor layer can be formed.
• the oxide semiconductor layer 253A might be crystallized to be an oxide semiconductor layer including crystals.
• the oxide semiconductor layer has a crystal structure with a crystal in which the c-axis is approximately parallel to a direction perpendicular to a surface is formed over a surface portion of an amorphous oxide semiconductor layer in some cases.
• first heat treatment may be performed after the first terminal and the second terminal are formed over the oxide semiconductor layer.
• the substrate is introduced into an electric furnace and the heat treatment is performed in an inert gas atmosphere such as nitrogen or a rare gas at 450 °C for one hour.
• an inert gas atmosphere such as nitrogen or a rare gas at 450 °C for one hour.
• the first terminal 254A and the second terminal 254B can be formed in a manner similar to that of the first terminal 204A and the second terminal 204B described in Embodiment 2.
• the insulating layer 255 is formed over the gate insulating layer 252, the oxide semiconductor layer 253B, the first terminal 254A, and the second terminal 254B.
• the insulating layer 255 can be formed by a sputtering method, a CVD method, a printing method, a coating method, or the like. Note that when a silicon oxide layer is formed by a sputtering method as the insulating layer 255, oxygen can be supplied from the silicon oxide layer to the oxygen deficiency which is generated in the first heat treatment and included in the oxide semiconductor layer 253A; thus, oxygen deficiency serving as a donor can be reduced and a structure satisfying the stoichiometric composition ratio can be obtained. As a result, the i-type or substantially i-type oxide semiconductor layer 253 can be formed.
• second heat treatment (preferably, at higher than or equal to 200 °C and lower than or equal to 400 °C, for example, at higher than or equal to 250 °C and lower than or equal to 350 °C) is performed in an inert gas atmosphere or in an oxygen gas atmosphere.
• the second heat treatment may be performed after a protective insulating layer or a planarization insulating layer is formed over the insulating layer 255.
• oxygen can be supplied from the insulating layer 255 formed using an oxide insulating layer to the oxygen deficiency which is generated in the first heat treatment and included in the oxide semiconductor layer; thus, oxygen deficiency serving as a donor can be reduced and a structure satisfying the stoichiometric composition ratio can be obtained.
• the more i-type or substantially i-type oxide semiconductor layer 253 can be formed.
• the second heat treatment is performed in a nitrogen atmosphere at 250 °C for one hour.
• heat treatment may be performed in air at higher than or equal to 100 °C and lower than or equal to 200 °C for longer than or equal to 1 hour and shorter than or equal to 30 hours. Reliability of a field-effect transistor can be increased by the heat treatment.
• the enhancement type field-effect transistor 102 can be manufactured which includes an i-type or substantially i-type oxide semiconductor layer in a channel region and has extremely small off-state current.
• FIG. 12 is one embodiment of a semiconductor device in which the power element described in any of Embodiments 1 to 3 is used for a protective element.
• the protective element functions so that current flows through the power element which is a protective element and overcurrent does not flow through a circuit to be protected when overvoltage is input to a power source terminal.
• the circuit to be protected includes any circuit with low withstand voltage which is broken by application of overvoltage.
• description will be given with the use of the power MOSFET having four terminals described in Embodiments 1 and 2.
• FIG 12 is a semiconductor device including a power MOSFET 501, a control circuit 502, a circuit 503 to be protected, an input terminal 504, and an output terminal 505.
• the control circuit 502 controls operation of the power MOSFET 501 serving as a protective element by detecting overvoltage applied to the input terminal 504 or the output terminal 505.
• FIG 13 is a detailed diagram of the control circuit 502.
• the control circuit 502 includes an overvoltage detection circuit 511, an inverter 512, a positive power source 513, switch transistors 514, 515, and 516, a capacitor 517, a negative voltage generation circuit 518, an oscillator circuit 519, a divider circuit 520, a delay circuit 521, and an AND circuit 522.
• the positive power source 513 corresponds to the high voltage generation source 108 in Embodiment 1.
• the switch transistors 514, 515, and 516 correspond to the field-effect transistors 102, 103, and 104 in Embodiment 1, respectively.
• the capacitor 517 corresponds to the capacitor 105 in Embodiment 1.
• the negative voltage generation circuit 518 corresponds to the low voltage generation source 109 in Embodiment 1.
• the oscillator circuit 519, the divider circuit 520, the delay circuit 521, and the AND circuit 522 correspond to the refresh control circuit 107 in Embodiment 1. Note that the structure of the control circuit 502 is not limited to this structure.
• the overvoltage detection circuit 511 is a circuit which operates in the case where overvoltage which exceeds normal power supply voltage is input to the input terminal 504.
• the overvoltage detection circuit 511 has a function of outputting a pulse having a high potential in the case where overvoltage is input.
• An output terminal of the overvoltage detection circuit 511 is connected to a gate terminal of the switch transistor 514 and an input terminal of the inverter 512.
• An output terminal of the inverter 512 is connected to a gate terminal of the switch transistor 515.
• the negative voltage generation circuit 518 includes a charge pump circuit in FIG 17 and the like to generate negative voltage.
• the protective circuit Since the protective circuit does not frequently operate, it is not appropriate to constantly feed a large amount of current in terms of power consumption. Accordingly, it is effective in terms of reduction in power consumption that the capacitor 517 is charged with the use of a small electric capacity. Therefore, power consumption can be reduced in such a manner that the capacitor 517 is intermittently charged by the negative voltage generation circuit 518 through the switch transistor 516.
• a signal in which an oscillation signal generated in the oscillator circuit 519 is divided by the divider circuit 520, and the divided signal is supplied to a gate terminal of the switch transistor 516. That is, one of output terminals of the divider circuit 520 is connected to a first input terminal of the AND circuit 522. The other of the output terminals of the divider circuit 520 is connected to a second input terminal of the AND circuit 522 through the delay circuit 521.
• a pulse which has a pulse width equivalent to delayed time of the delay circuit 521 and a cycle similar to output of the divider circuit 520 can be obtained. With the use of the pulse, the gate terminal of the switch transistor 516 can be controlled.
• the oscillator circuit 519 a general oscillator circuit such as a ring oscillator can be used, but it is not limited thereto. Further, a flip-flop can be used for the divider circuit 520. As the delay circuit 521, a circuit using an inverter, a circuit using a CR delay circuit, or the like can be used, but it is not particularly limited thereto. Furthermore, a pulse can be generated by other methods.
• FIG. 14 is a semiconductor device in which a power MOSFET and a circuit to be protected are connected in series. Unlike FIG. 12, a power MOSFET 601 is turned off and application of overvoltage to a circuit 603 to be protected is prevented when overvoltage is applied to an input terminal.
• the semiconductor device shown in FIG 14 includes a power MOSFET 601, a control circuit 602, a circuit 603 to be protected, an input terminal 604, and an output terminal 605.
• the control circuit 602 controls the power MOSFET 601 serving as a protective element by detecting overvoltage applied to the input terminal 604 or the output terminal 605.
• FIG 15 is a detailed diagram of the control circuit 602.
• the control circuit 602 includes an overvoltage detection circuit 611, an inverter 612, a positive power source 613, switch transistors 614, 615, and 616, a capacitor 617, a negative voltage generation circuit 618, an oscillator circuit 619, a divider circuit 620, a delay circuit 621, and an AND circuit 622.
• the positive power source 613 corresponds to the high voltage generation source 108 in Embodiment 1.
• the switch transistors 614, 615, and 616 correspond to the field-effect transistors 103, 102, and 104 in Embodiment 1, respectively.
• the capacitor 617 corresponds to the capacitor 105 in Embodiment 1.
• the negative voltage generation circuit 618 corresponds to the low voltage generation source 109 in Embodiment 1.
• the oscillator circuit 619, the divider circuit 620, the delay circuit 621, and the AND circuit 622 correspond to the refresh control circuit 107 in Embodiment 1. Note that the structure of the control circuit 602 is not limited to this structure.
• the overvoltage detection circuit 611 is a circuit which operates in the case where overvoltage which exceeds normal power supply voltage is input to the input terminal 604.
• the overvoltage detection circuit 611 has a function of outputting a pulse having a high potential in the case where overvoltage is input in this embodiment.
• An output terminal of the overvoltage detection circuit 611 is connected to a gate terminal of the switch transistor 615 and the inverter 612.
• An output terminal of the inverter 612 is connected to a gate terminal of the switch transistor 614.
• the protective circuit Since the protective circuit does not frequently operate, it is not appropriate to constantly feed a large amount of current in terms of power consumption. Accordingly, it is effective in terms of reduction in power consumption that the capacitor 617 is charged with a small electric capacity. Therefore, power consumption can be reduced in such a manner that the capacitor 617 is intermittently charged by the positive power source 613 through the switch transistor 616.
• a signal in which an oscillation signal generated in the oscillator circuit 619 is divided by the divider circuit 620, and the divided signal is supplied to a gate terminal of the switch transistor 616. That is, one of output terminals of the divider circuit 620 is connected to a first input terminal of the AND circuit 622. The other of the output terminals of the divider circuit 620 is connected to a second input terminal of the AND circuit 622 through the delay circuit 621.
• a pulse which has a pulse width equivalent to delayed time of the delay circuit 621 and a cycle similar to output of the divider circuit 620 can be obtained. With the use of the pulse, the gate terminal of the switch transistor 616 can be controlled.
• the oscillator circuit 619 a general oscillator circuit such as a ring oscillator can be used, but it is not limited thereto. Further, a flip-flop can be used for the divider circuit 620. As the delay circuit 621, a circuit using an inverter, a circuit using a CR delay circuit, or the like can be used, but it is not particularly limited thereto. Furthermore, a pulse can be generated by other methods.
• FIG 16 is an example of structures of the overvoltage detection circuits 511 and 611.
• a diode chain in which transistors 701 to 705 are diode-connected, a transistor 707, a resistor 706, and an inverter 708 are included.
• the diode chain includes n transistors connected in series and the threshold voltage of a transistor is Vth, n is set so that normal operation voltage ⁇ nVth is satisfied.
• the transistors 701 to 705 are turned on when overvoltage is applied, and thus current flows through the diode chain.
• the transistor 707 is also turned on and a high potential is output from an output of the inverter 708.
• a normally-on power MOSFET which includes an oxide semiconductor layer with a wide band gap in a channel region is used as a protective element; thus, destruction of a semiconductor device due to application of overvoltage can be prevented.
• the semiconductor device which is the power device described in the above embodiments can be used, for example, for a protective circuit of a battery in an electronic device such as a computer display that can display images; and a protective circuit of a battery provided for an electromagnetic cooker or a vehicle (e.g., a bicycle) that is driven with power from a fixed power source.
• FIGS. 18A to 18C Application examples of a semiconductor device which is a power device functioning as a protective circuit will be described with reference to FIGS. 18A to 18C.
• FIG 18A illustrates an electromagnetic cooker 1000 as an application example of a semiconductor device serving as a protective circuit.
• the electromagnetic cooker 1000 heats cookware and the like by using electromagnetic induction generated by current flowing through a coil unit 1001.
• the electromagnetic cooker 1000 includes a battery 1002 for supplying current that is to flow through the coil unit 1001, a semiconductor device 1003 serving as a protective circuit, and a solar battery 1004 for charging the battery 1002.
• the solar battery 1004 is illustrated as a means to charge the battery 1002 in FIG 18A, the battery 1002 may be charged by another means.
• the semiconductor device 1003 serving as a protective circuit can reduce application of overvoltage to the battery 1002 and thus, it is possible to reduce power consumption when the protective circuit is not operated.
• FIG 18B illustrates an electric bicycle 1010 as an application example of a semiconductor device serving as a protective circuit.
• the electric bicycle 1010 obtains power when current flows through a motor unit 1011.
• the electric bicycle 1010 includes a battery 1012 for supplying current that is to flow through the motor unit 1011 and a semiconductor device 1013 serving as a protective circuit.
• a means to charge the battery 1012 is not particularly illustrated in FIG. 18B, the battery 1012 may be charged by an electric generator or the like that is additionally provided.
• the semiconductor device 1013 serving as a protective circuit can reduce application of overvoltage to the battery 1012 in charging and thus, it is possible to reduce power consumption when the protective circuit is not operated. Note that although a pedal is illustrated in FIG. 18B, the pedal is not necessarily provided.
• FIG 18C illustrates an electric car 1020 as an application example of a semiconductor device serving as a protective circuit.
• the electric car 1020 obtains power when current flows through a motor unit 1021.
• the electric car 1020 includes a battery 1022 for supplying current that is to flow through the motor unit 1021 and a semiconductor device 1023 serving as a protective circuit.
• a means to charge the battery 1022 is not particularly illustrated in FIG. 18C, the battery 1022 may be charged by an electric generator or the like that is additionally provided.
• the semiconductor device 1023 serving as a protective circuit can reduce application of overvoltage to the battery 1022 in charging and thus, it is possible to reduce power consumption when the protective circuit is not operated.

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• Thin Film Transistor (AREA)
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• Metal-Oxide And Bipolar Metal-Oxide Semiconductor Integrated Circuits (AREA)
• Electronic Switches (AREA)
• Devices For Indicating Variable Information By Combining Individual Elements (AREA)
• Shift Register Type Memory (AREA)
• Charge And Discharge Circuits For Batteries Or The Like (AREA)

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