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]
  • H10D30/6729—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
  • 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
  • H10D30/6734—Multi-gate TFTs having gate electrodes arranged on both top and bottom sides of the channel, e.g. dual-gate TFTs
• • 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]
  • H10D30/6704—Thin-film transistors [TFT] having supplementary regions or layers in the thin films or in the insulated bulk substrates for controlling properties of the device
  • H10D30/6713—Thin-film transistors [TFT] having supplementary regions or layers in the thin films or in the insulated bulk substrates for controlling properties of the device characterised by the properties of the source or drain regions, e.g. compositions or sectional shapes
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L2924/00—Indexing scheme for arrangements or methods for connecting or disconnecting semiconductor or solid-state bodies as covered by H01L24/00
  • H01L2924/10—Details of semiconductor or other solid state devices to be connected
  • H01L2924/11—Device type
  • H01L2924/13—Discrete devices, e.g. 3 terminal devices
  • H01L2924/1304—Transistor
  • H01L2924/1306—Field-effect transistor [FET]
  • H01L2924/13069—Thin film transistor [TFT]

Definitions

• This disclosure relates generally to charge storage and transfer elements, and more specifically to multi-gate thin-film transistor structures suitable for use in storing charge and transferring charge to electrical or electromechanical components such as display elements.
• Electromechanical systems include devices having electrical and mechanical elements, transducers such as actuators and sensors, optical components (including mirrors) and electronics.
• EMS can be manufactured at a variety of scales including, but not limited to, microscales and nanoscales. For example,
• MEMS devices can include structures having sizes ranging from about one micron to hundreds of microns or more.
• Nanoelectromechanical systems (NEMS) devices can include structures having sizes smaller than one micron including, for example, sizes smaller than several hundred nanometers. Electromechanical elements may be created using deposition, etching, lithography or other micromachining processes that etch away parts of substrates or deposited material layers, or that add layers to form electrical, mechanical and electromechanical devices.
• an interferometric modulator refers to a device that selectively absorbs or reflects light using the principles of optical interference.
• an IMOD may include a pair of conductive plates, one or both of which may be transparent or reflective, wholly or in part, and capable of relative motion upon application of an appropriate electrical signal.
• one plate may include a stationary layer deposited on a substrate and the other plate may include a reflective membrane separated from the stationary layer by an air gap. The position of one plate in relation to another can change the optical interference of light incident on the IMOD.
• IMOD devices have a wide range of applications, and are anticipated to be used in improving existing products and creating new products, especially those with display capabilities.
• a device includes a thin-film semiconducting layer arranged over a substrate.
• a drain is coupled to the semiconducting layer and is configured to receive an input signal.
• a source is coupled to another end of the semiconducting layer and is configured to drive an output signal.
• the device also includes three gate electrodes.
• a first gate electrode is arranged adjacent to the semiconducting layer, between the drain and the source, and is configured to receive a first control signal.
• a second gate electrode is arranged adjacent to the semiconducting layer, between the drain and the source, and is configured to receive a second control signal.
• a third gate electrode is arranged adjacent the semiconducting layer, between the drain and the source, and is configured to receive a third control signal.
• the second gate electrode is arranged between the first and third gate electrodes such that there is some overlap of the first and second gate electrodes and some overlap of the second and third gate electrodes but no overlap of the first and third gate electrodes.
• One or more dielectric layers insulate the first gate electrode from the semiconducting layer, insulate the second gate electrode from the semiconducting layer, insulate the third gate electrode from the semiconducting layer, and insulate the first, second, and third gate electrodes from one another.
• the first, second and third gate electrodes are configured such that charge is transferred in from the drain and accumulated in a potential well in a region of the semiconducting layer adjacent to the second gate electrode.
• the first, second and third gate electrodes are configured such that charge previously accumulated in the region of the semiconducting layer adjacent to the second gate electrode is transferred through the region of the semiconducting layer adjacent the third gate electrode and subsequently transferred through the source and output as an output signal.
• the third gate electrode is off; the first gate electrode receives the first control signal and is turned on in response to the first control signal; the second gate electrode receives the second control signal and is turned on in response to the second control signal; and the drain receives the input signal and transmits charge into the semiconducting layer in response to the input signal.
• the first gate electrode is then turned off after charge is accumulated along the semiconducting layer. As a result, charge accumulated along the semiconducting layer accumulates in the potential well in the region of the semiconducting layer adjacent the second gate electrode after the first gate electrode is turned off.
• the third gate electrode receives the third control signal and is turned on in response to the third control signal; the second gate electrode is turned off; and after the third gate electrode is turned on and the second gate electrode is turned off, the charge previously accumulated in the region of the semiconducting layer adjacent the second gate electrode is transferred to the source which then, in response, yields the output signal based on the transferred charge.
• the second gate electrode while in the first mode of operation, is configured to a high capacitance state in response to the second control signal to facilitate the transfer and accumulation of charge in the region of the semiconducting layer adjacent the second gate electrode. In some implementations, while in the second mode of operation, the second gate electrode is configured to a low capacitance state to facilitate the transfer of charge from the region of the
• the first gate electrode is arranged adjacent a first side of the semiconducting layer
• the second gate electrode is arranged adjacent a second side of the semiconducting layer opposite the first side
• the third gate electrode is arranged adjacent the first side.
• the second side of the semiconducting layer is adjacent the substrate such that the second gate electrode is arranged between the thin-film semiconducting layer and the substrate.
• all three gate electrodes are arranged adjacent a single side of the semiconducting layer.
• the portion of the semiconducting layer adjacent the respective gate electrode behaves as a conducting channel in accumulation mode.
• the portion of the semiconducting layer adjacent the respective gate electrode presents an energy barrier and is effectively non-conducting.
• a device includes a thin-film semiconducting means arranged over a substrate.
• a drain means is coupled to the semiconducting means and is configured to receive an input signal.
• a source means is coupled to another end of the
• the device also includes three gating means.
• a first gating means is arranged adjacent to the semiconducting means, between the drain means and the source means, and is configured to receive a first control signal.
• a second gating means is arranged adjacent to the semiconducting means, between the drain means and the source means, and is configured to receive a second control signal.
• a third gating means is arranged adjacent the semiconducting means, between the drain means and the source means, and is configured to receive a third control signal.
• the second gating means is arranged between the first and third gating means such that there is some overlap of the first and second gating means and some overlap of the second and third gating means but no overlap of the first and third gating means.
• One or more insulating means insulate the first gating means from the semiconducting means, insulate the second gating means from the semiconducting means, insulate the third gating means from the
• the first, second and third gating means are configured such that charge is transferred in from the drain means and accumulated in a potential well in a region of the semiconducting means adjacent to the second gating means.
• the first, second and third gating means are configured such that charge previously accumulated in the region of the semiconducting means adjacent to the second gating means is transferred through the region of the semiconducting means adjacent the third gating means and subsequently transferred through the source means as an output signal.
• the third gating means while in the first mode of operation, the third gating means is closed; the first gating means receives the first control signal and is opened in response to the first control signal; the second gating means receives the second control signal and is opened in response to the second control signal; and the drain means receives the input signal and transmits charge into the semiconducting means in response to the input signal.
• the first gating means is then closed after charge is accumulated along the semiconducting means. As a result, charge accumulated along the semiconducting means accumulates in the potential well in the region of the semiconducting means adjacent the second gating means after the first gating means is closed.
• the third gating means receives the third control signal and is opened in response to the third control signal; the second gating means is closed; and after the third gating means is opened and the second gating means is closed, the charge previously accumulated in the region of the semiconducting means adjacent the second gating means is transferred to the source means which then, in response, yields the output signal based on the transferred charge.
• the second gating means while in the first mode of operation, is configured to a high capacitance state in response to the second control signal to facilitate the transfer and accumulation of charge in the region of the semiconducting means adjacent the second gating means. In some implementations, while in the second mode of operation, the second gating means is configured to a low capacitance state to facilitate the transfer of charge from the region of the
• the first gating means is arranged adjacent a first side of the semiconducting means
• the second gating means is arranged adjacent a second side of the semiconducting means opposite the first side
• the third gating means is arranged adjacent the first side.
• the second side of the semiconducting means is adjacent the substrate such that the second gating means is arranged between the thin- film semiconducting means and the substrate.
• all three gating means are arranged adjacent a single side of the semiconducting means.
• the portion of the semiconducting means adjacent the respective gating means behaves as a conducting means in accumulation mode.
• the portion of the semiconducting means adjacent the respective gating means presents an energy barrier and is effectively non-conducting.
• Figure 1 shows a cross-sectional side view depiction of an example
• Figure 2 shows a cross-sectional side view depiction of an example three- gate thin-film transistor.
• Figure 3 shows a timing diagram depicting an example method of operating the three-gate thin-film transistor depicted in Figure 2.
• Figure 4 shows a plot of the capacitance between the storage region and the storage gate electrode of Figure 2 as a function of the gate voltage applied to the storage gate electrode.
• Figure 5 shows a cross-sectional side view depiction of an example three- gate thin-film transistor.
• Figure 6 shows a cross-sectional side view depiction of an example three- gate thin-film transistor.
• Figure 7 shows a flow diagram depicting an example method that can be used to form the three-gate thin- film transistor depicted in Figure 2.
• Figure 8 A is an isometric view illustration depicting two adjacent interferometric modulator (IMOD) display elements in a series or array of display elements of an IMOD display device.
• IMOD interferometric modulator
• Figure 8B is a system block diagram illustrating an electronic device incorporating an IMOD-based display including a three element by three element array of IMOD display elements.
• Figures 9A and 9B are system block diagrams illustrating a display device that includes a plurality of IMOD display elements.
• the disclosed implementations include examples of multi-gate thin-film transistors or transistor devices/structures (hereinafter all referred to as "transistors”), including a three-gate thin-film transistor (TFT).
• the three- gate TFT can be used to store charge and to transfer charge to a display element, such as an LCD pixel, an IMOD pixel, another MEMS-based device or another electrical or electromechanical element.
• a display element such as an LCD pixel, an IMOD pixel, another MEMS-based device or another electrical or electromechanical element.
• Related apparatus, systems and fabrication processes and techniques are also disclosed.
• a TFT includes a storage gate electrode that effectively functions as an "active MOS capacitor.”
• the biasing conditions are such that the storage gate electrode is configured to a high-capacitance state to facilitate the transfer and accumulation of charge in a storage region of a semiconducting layer of the TFT adjacent the storage gate electrode.
• the biasing conditions are such that the storage gate electrode is configured to a low-capacitance state to facilitate the transfer of charge from the storage region to a display element.
• a relatively low column voltage signal V column can inject a large amount of current into the storage region adjacent the storage gate electrode.
• the TFT While in the low capacitance state, when the storage region is electrically-coupled to the display element, the TFT achieves a highly efficient charge transfer to the display element.
• the ability of the TFT to switch from a high-capacitance state to write and store charge, to a low-capacitance state to transfer charge to the display element can result in significant savings in power, among other advantages.
• the capacitance switching feature also allows for accurate charge injection to the display element even though the capacitance state of the display element can vary.
• a TFT is a particular type of field-effect transistor (FET) in which a semiconducting layer as well as one or more dielectric insulating layers and metallic contacts are deposited over a substrate.
• FETs are commonly used in displays such as LCD displays.
• a TFT FET differs from a conventional FET, such as a traditional metal-oxide semiconductor (MOS) FET (MOSFET).
• MOS metal-oxide semiconductor
• MOSFET metal-oxide semiconductor
• the semiconducting layer in a MOSFET is itself part of a semiconducting substrate— typically a silicon (Si) wafer— into or over which the other electrical components including the drain and source regions, the dielectric layers, the gate, and the metallic contacts are deposited or otherwise formed.
• Figure 1 shows a cross-sectional side view depiction of an example
• MOSFET 100 The MOSFET 100 is depicted for comparison or reference purposes and includes a semiconducting substrate 102, a drain 104, a drain contact 105, a source 106, a source contact 107, a gate electrode 108 and a dielectric insulating layer 110.
• the "metal” in MOS refers to the gate electrode 108 (which may or may not actually be metallic— for example, polycrystalline Si is commonly used), the "oxide” refers to the dielectric layer 110 (which may or may not actually be an oxide— other dielectrics are frequently used having larger dielectric constants), and the "semiconductor” refers to the semiconducting substrate 102.
• the semiconducting substrate can either be /?-type (as shown) or n-type.
• the current flow is such that electrons enter the device through the source 106, are subject to the gating action of the gate electrode 108 while traveling through a channel region 112 in the semiconductor substrate 102, and leave through the drain 104. Consistent with this naming scheme, "holes" would travel the opposite direction— as with conventional current terminology, in which current is defined to flow in the same direction as positive charge carriers.
• the MOSFET 100 more particularly the semiconducting material in the channel region 112 below the gate electrode 108 and between the source 106 and the drain 104— can be considered to be in one of three physically distinct biasing states: accumulation, depletion or inversion.
• accumulation when a /?-type MOSFET 100 is biased in accumulation, majority positive charge carriers— "holes"— accumulate near the semiconductor-dielectric interface 114.
• holes When a /?-type MOSFET 100 is biased in depletion, the channel region 112 becomes depleted of the majority holes.
• a display such as an IMOD display or an LCD display, generally includes an array of display elements also referred to as pixels.
• Some such displays can include arrays of hundreds, thousands or millions of pixels arranged in hundreds or thousands of rows and hundreds or thousands of columns.
• some such displays include 1024 x 768 arrays, 1366 x 768 or 1920 x 1080 arrays, where the first number indicates the width of the display in number of columns and the second number indicates the height of the display in number of rows.
• Each pixel in turn, can include one or more sub-pixels.
• each pixel can include a red sub-pixel, a green sub-pixel and a blue sub-pixel that emit red, green and blue light, respectively.
• each red sub-pixel, green sub-pixel and blue-sub- pixel can further include an array of one or more sub-sub-pixels for discretely adjusting an intensity of each of the constituent colors— red, green and blue emitted by the pixel.
• FIG. 2 shows a cross-sectional side view depiction of an example three-gate TFT 200.
• TFT 200 includes a substrate 202, a drain 204, a source 206, a thin-film semiconducting layer 208, a first "write” gate electrode 210, a second "storage” gate electrode 212, a third "transfer” gate electrode 214, a first (or lower) dielectric layer 216 and a second (or upper) dielectric layer 218.
• no limitation is meant to be inherent or suggested by way of calling the "drain” 204 a “drain” as opposed to a “source,” or the “source” 206 a “source” as opposed to a “drain.” That is, the two terms— drain and source— are used interchangeably in this disclosure.
• electrons may enter the three-gate TFT 200 by way of the drain 204 and leave the three- gate TFT 200 by way of the source 206, or vice versa, depending on the type of semiconducting material used in the semiconducting layer 208 and on the biases applied to the gate electrodes 210, 212 and 214.
• the write gate electrode 210 and the transfer gate electrode 214 are arranged adjacent one side of the thin- film
• Figure 2 depicts an implementation in which the write gate electrode 210 and the transfer gate electrode 214 are arranged over the upper surface 220 of the thin-film semiconducting layer 208 while the storage gate electrode 212 is arranged below the lower surface 222 of the semiconducting layer 208.
• the arrangement can be reversed; that is, the storage gate electrode 212 can be arranged over the upper surface 220 while the write gate electrode 210 and the transfer gate electrode 214 can be arranged below the lower surface 222.
• Figure 5 shows a cross-sectional side view depiction of such an example three-gate TFT 500.
• TFT 500 includes a substrate 502, a drain 504, a source 506, a thin-film semiconducting layer 508, a first "write” gate electrode 510, a second “storage” gate electrode 512, a third "transfer” gate electrode 514, a first (or lower) dielectric layer 516 and a second (or upper) dielectric layer 518.
• the TFT 500 depicted in Figure 5 is arranged such that the write gate electrode 510 and the transfer gate electrode 514 are arranged adjacent one side of the thin-film semiconducting layer 508, while the storage gate electrode 512 is arranged adjacent an opposite side of the thin-film semiconducting layer 508.
• the TFT 500 depicted in Figure 5 is arranged such that the write gate electrode 510 and the transfer gate electrode 514 are arranged under the lower surface 522 of the thin-film
• all three gate electrodes—the write gate electrode 210, the storage gate electrode 212 and the transfer gate electrode 214— can be arranged on the same side of the thin-film semiconducting layer 208, below or above the thin-film semiconducting layer 208.
• such an implementation is depicted in Figure 6, described later.
• the write gate electrode 210 is insulated from the drain 204, the thin-film semiconducting layer 208 and other components by the upper dielectric layer 218.
• the upper dielectric layer 218 also insulates the transfer gate electrode 214 from the source 206, the thin- film
• the drain 204 is electrically coupled to receive a column voltage signal V column-
• a control signal is sent to each column of the display in the form of a column voltage signal and to each row of the display in the form of a row select voltage signal.
• m x n display such a scheme can require m column voltage signals and n row select voltage signals.
• Each column voltage signal V column can be applied to an entire column of display elements at one time.
• each row select voltage signal V Row seiect can be applied to an entire row of display elements at one time.
• the write gate electrode 210 is electrically coupled to receive a row select voltage signal V Row seiect-
• the storage gate electrode 212 is electrically coupled to receive a storage control voltage signal V torage
• the transfer gate electrode 214 is electrically coupled to receive a transfer control voltage signal V Transfer
• the source 206 is electrically coupled to a load, such as a pixel or pixel element 224.
• the pixel element 224 can be an LCD pixel or an IMOD pixel.
• the load to which the source 206 is coupled can be a different MEMS-based element or other electrical or electromechanical element.
• the source 206 also can be electrically coupled to receive a reset voltage signal V Reset .
• the write gate electrode 210, the storage gate electrode 212 and the transfer gate electrode 214 are configured such that electrical charge is transferred in from the drain 204 and subsequently accumulated in an electrical potential well in a region 226 of the semiconducting layer 208 adjacent the storage gate electrode 212.
• the write gate electrode 210, the storage gate electrode 212 and the transfer gate electrode 214 are configured such that the electrical charge previously accumulated in the region 226 of the semiconducting layer 208 adjacent the storage gate electrode 212 is transferred through the
• the semiconducting layer 208 through a region of the semiconducting layer 208 adjacent the transfer gate electrode 214 and subsequently transferred through to the source 206. From the source 206, the transferred charge is output to the pixel element 224.
• Figure 3 shows a timing diagram depicting an example method of operating the three-gate TFT 200 depicted in Figure 2.
• the semiconducting layer 208 is an n-type semiconducting material, although the voltages can be reversed in other implementations when a /?-type semiconducting material layer is used.
• the data to be written to the display element 224 is applied via the column voltage signal V column to the drain 204.
• the row select voltage signal VR OW s e iect is switched from low (e.g., a negative or small positive voltage) to high (e.g., a larger positive voltage than low) effectively "turning on” or "opening" the write gate electrode 210 to enable conduction through the
• the storage control voltage signal V storage is switched from low to high.
• the transfer control voltage signal V ransfer remains low during this time, causing the transfer gate electrode 214 to be in a "turned-off ' or "closed" state.
• the majority charge carriers e.g., electrons
• the majority charge carriers may accumulate below the write gate electrode 210.
• the bias Vstomge applied to the storage gate electrode 212 most of the charge flows into the region 226 adjacent the storage gate electrode 212.
• the row select voltage signal VR OW Seiect is switched from high to low. As a result, the remaining charge below the write gate electrode 210 flows into the region 226 adjacent the storage gate electrode 212.
• the column voltage Vcoiumn is switched off at t 4 .
• the charge now accumulated in the region 226 is essentially trapped in an electrical potential well in the region 226 of the semiconducting layer 208 adjacent the storage gate electrode 212.
• the storage gate electrode 210 and the region 226 of the semiconducting layer 208 serve as plates of a capacitor separated by the lower dielectric layer 216. In this way, the data to be written to the display element 224 can be efficiently stored as charge in the region 226.
• the stored charge for the entire display (or sub-array) is synchronously transferred from the three-gate TFTs 200 to the corresponding coupled pixel elements 224.
• the transfer control voltage signal V ransfer is switched from low to high effectively turning on the transfer gate electrode 214.
• the storage control voltage signal V storage is switched from high to low effectively turning off the storage gate electrode 212.
• a reset voltage V Reset is biased to reset the pixel element 224 prior to time to or prior to ts, or at another suitable time.
• the reset voltage V Reset can be used to activate or close a switch 228 (such as a MOSFET transistor) that connects the pixel element 224 to ground.
• the drain 204, the write gate electrode 210, the storage gate electrode 212, the transfer gate electrode 214 and the source 206 are arranged such that there is some "vertical" overlap of the drain 204 and the write gate electrode 210, some vertical overlap of the write gate electrode 210 and the storage gate electrode 212, some vertical overlap of the storage gate electrode 212 and the transfer gate electrode 214, and some vertical overlap of the transfer gate electrode 214 and the source 206.
• This overlap ensures high capacitive coupling and efficient transfer of charge across the semiconducting layer 208 from potential well to potential well as the biases applied to the gate electrodes are switched; that is, from the portion of the semiconducting layer 208 below a given one of the gate electrodes to the next gate electrode as the biases are switched.
• the storage gate electrode 212 effectively functions as an "active MOS capacitor.”
• the biasing conditions are such that the storage gate electrode 212 is configured to a high-capacitance state to facilitate the transfer and accumulation of charge in the region 226 of the semiconducting layer 208 adjacent the storage gate electrode 212.
• the biasing conditions are such that the storage gate electrode 212 is configured to a low-capacitance state to facilitate the transfer of charge from the region 226 through the source 206 to the pixel element 224.
• a relatively low column voltage signal V column can inject a large amount of current into the region 226 adjacent the storage gate electrode 212.
• the three-gate TFT 200 While in the low capacitance state, when the region 226 is electrically-coupled via transfer gate electrode 214 to the pixel element 224, the three-gate TFT 200 achieves a highly efficient charge transfer to the pixel element 224.
• the capacitance-switching capability of the three-gate TFT 200 plays a primary role, or is key factor, in lowering the required column voltage V column- The capacitance-switching capability also ensures that most, if not substantially all, of the charge that is stored by the storage gate electrode 212 in the region 226 of the semiconducting layer 208 can be transferred to the target load, whether a pixel element 224, another MEMS element or other electrical element or device.
• the total parasitic capacitance is the sum of transistor parasitic capacitances (also referred to as the gate to source capacitances), fringing capacitances local to the pixel, and interconnect trace to ground capacitances, such as from overlap and the fringing capacitances when integrated along the entire column drive electrode.
• transistor parasitic capacitances also referred to as the gate to source capacitances
• fringing capacitances local to the pixel and interconnect trace to ground capacitances, such as from overlap and the fringing capacitances when integrated along the entire column drive electrode.
• the total parasitic capacitance can be in the range of 10-100 pF or even higher.
• the ability of the three-gate TFT 200 to switch from a high-capacitance state to write and store charge, to a low-capacitance state to transfer charge to the pixel element 224 can result in significant savings in power, among other advantages.
• the capacitance switching feature also allows for accurate charge injection to the pixel element 224 even though the capacitance state of the pixel element 224 can vary. More specifically, the disclosed capacitance switching feature can allow for accurate charge injection by designing the three-gate TFT 200 such that the capacitance of the storage gate electrode 212 while in the low capacitance state is lower than the minimum capacitance value of the pixel element 224 by a sufficient margin. This also can have important stability implications on the control of the pixel element 224 because the charge control aspect allows for improved tuning range.
• one or more of the gate electrodes 210, 212 and 214 can each be formed from one or metals, alloys of metals, or layers of metals, alloys or other materials.
• metals that are suitable for forming the gate electrodes 210, 212 and 214 can include Al, Mo, Ti, W or Cu.
• the semiconducting layer 208 is formed of Indium Gallium Zinc Oxide (IGZO). But in some other implementations, the semiconducting layer 208 can generally be formed of any suitable oxide semiconductor, such as an amorphous oxide semiconductor.
• suitable materials that can be used in forming an amorphous oxide semiconductor layer can include In, Zn, Sn, Hf and Ga.
• oxide semiconductor layers can be formed with physical vapor deposition (PVD) techniques including, for example, one or more of pulsed laser deposition (PLD), sputter deposition, electron beam PVD (e-beam PVD) and evaporative deposition.
• PVD physical vapor deposition
• the semiconductor layer 208 has a thickness in the range of approximately 20 to 100 nm.
• the three-gate TFT 200 exhibits extremely low leakage and, when on, operates solely in accumulation mode (as opposed to an inversion mode).
• the MOS capacitor formed by the storage gate electrode 212 essentially varies from the sum of the oxide capacitance, the overlap capacitance between the storage gate electrode 212 and the write gate electrode 210, and the overlap capacitance between the storage gate electrode 212 and the transfer gate electrode 214 (high) when the MOS capacitor is in accumulation mode, to only the sum of the overlap capacitance between the storage gate electrode 212 and the write gate electrode 210 and the overlap capacitance between the storage gate electrode 212 and the transfer gate electrode 214 (small) when the MOS capacitor is turned off.
• the three-gate TFT 200 is characterized by a flat band gate voltage when the band bending due to the MOS junction physics is offset by the voltage applied to the storage gate electrode 212.
• This offset also serves as the boundary between accumulation mode— when the majority carriers migrate toward the semiconductor-dielectric interface— and the beginning of a weak depletion regime.
• IGZO is used to form an n-type semiconducting material layer 208
• the accumulation becomes stronger as the voltage V torage is increased positively above the flatband voltage, while the depletion takes effect when the voltage is driven negatively below the flatband voltage.
• Figure 4 shows a plot of the capacitance of the MOS capacitor formed by the storage region 226 and the storage gate electrode 212 as a function of the gate voltage Vstorage applied to the storage gate electrode 212.
• the capacitance-voltage (CV) profile or characteristic shown in Figure 4 is described or approximated by the equation:
• this vertical overlap of the gate electrodes 210, 212 and 214 is accomplished by fabricating the first and third gates on an opposite side of the semiconducting channel as the storage gate.
• This topology is not available for crystalline Si MOSFET transistors because the conducting channel in such transistors is produced by an inversion layer and the gate can be formed only on one side of the channel, the channel being an integral top portion of the bulk Si substrate.
• all three gate electrodes—the write gate electrode, the storage gate electrode and the transfer gate electrode— can be arranged on the same side of a thin-film semiconducting layer 208 in a TFT topology, below or above the thin-film semiconducting layer 208 in which the confinement region 626 can be formed.
• Figure 6 shows a cross-sectional side view depiction of an example three-gate TFT 600.
• the write gate electrode 610 the storage gate electrode 612 and the transfer gate electrode 614 are arranged above the thin-film semiconducting layer 608.
• the drain 604, the write gate electrode 610, the storage gate electrode 612, the transfer gate electrode 614 and the source 606 are arranged such that there is some vertical overlap of the drain 604 and the write gate electrode 610, some vertical overlap of the write gate electrode 610 and the storage gate electrode 612, some vertical overlap of the storage gate electrode 612 and the transfer gate electrode 614, and some vertical overlap of the transfer gate electrode 614 and the source 606.
• This overlap ensures high capacitive coupling and efficient transfer of charge across the semiconducting layer 608 from potential well to potential well as the biases applied to the gate electrodes are switched.
• a dielectric layer 616 insulates the gate electrodes from one another and from other components of the three-gate TFT 600.
• the three-gate TFT 600 also can be constructed on a substrate 602.
• the biasing conditions are such that the storage gate electrode 612 is configured to a high-capacitance state to facilitate the transfer and accumulation of charge in the region 626 of the semiconducting layer 608 adjacent the storage gate electrode 612.
• the biasing conditions are such that the storage gate electrode 612 is configured to a low-capacitance state to facilitate the transfer of charge from the region 626 through the source 606 to the pixel element 224 or other load.
• Figure 7 shows a flow diagram depicting an example method 700 that can be used to form the three-gate TFT 200 depicted in Figure 2.
• the method 700 begins with a substrate 202.
• substrate 202 can be a glass substrate or wafer or another dielectric substrate.
• the storage gate electrode 212 is deposited by, for example, physical vapor deposition processes (e.g., sputtering, co-sputtering, reactive sputtering or thermal evaporation) or any of the other processes described above and using any of the materials described above for forming the storage gate electrodes 210, 212 and 214.
• the storage gate electrode 212 is substantially or significantly wider or even longer than the write and transfer gate electrodes 210 and 214.
• the first lower dielectric layer 216 is deposited, grown, or otherwise arranged over the storage gate electrode 212 and other portions of the substrate 202.
• the dielectric layers 216 and 218 can be formed of an oxide such as SiO, Si0 2 or A1 2 0 3 , among other materials, and can have a thickness of approximately 200 nm (although thinner or thicker thicknesses may be appropriate or suitable in some applications).
• the thin-film semiconducting layer 208 is deposited.
• the semiconducting layer 208 has a thickness in the range of approximately 10 nanometers (nm) to approximately 100 nm and can be formed of any of the materials described above for forming the semiconducting layer 208, including IGZO.
• the drain 204 and the source 206 can be deposited.
• the drain 204 and the source 206 can be formed of a metal such as, for example, Molybdenum.
• the second upper dielectric layer 218 is then deposited in block 710.
• the write and transfer gate electrodes 210 and 214 can be deposited in block 712.
• one or more of the drain 204, the source 206 and the gate electrodes 210, 212 and 214 can be electrically coupled via traces or other conductive means (parts of which may be buried below the dielectric layers or other elements) to device contacts (not shown) that can be formed of a metal or metallic alloy (e.g., molybdenum (Mo)) and used to electrically couple the respective element to the corresponding one of the electrical signals described above.
• traces or other conductive means parts of which may be buried below the dielectric layers or other elements
• device contacts not shown
• a metal or metallic alloy e.g., molybdenum (Mo)
• the described implementations may be included in or associated with a variety of electronic devices such as, but not limited to: mobile telephones, multimedia Internet enabled cellular telephones, mobile television receivers, wireless devices, smartphones, Bluetooth® devices, personal data assistants (PDAs), wireless electronic mail receivers, hand-held or portable computers, netbooks, notebooks, smartbooks, tablets, printers, copiers, scanners, facsimile devices, global positioning system (GPS) receivers/navigators, cameras, digital media players (such as MP3 players), camcorders, game consoles, wrist watches, clocks, calculators, television monitors, flat panel displays, electronic reading devices (e.g., e-readers), computer monitors, auto displays (including odometer and speedometer displays, etc.), cockpit controls and/or displays, camera view displays (such as the display of a rear view camera in a vehicle), electronic photographs, electronic billboards or signs, projectors, architectural structures, microwaves, refrigerators, stereo systems, cassette recorders or players, DVD players
• PDAs personal data assistant
• teachings herein also can be used in non- display applications such as, but not limited to, electronic switching devices, radio frequency filters, sensors, accelerometers, gyroscopes, motion-sensing devices, magnetometers, inertial components for consumer electronics, parts of consumer electronics products, varactors, liquid crystal devices, electrophoretic devices, drive schemes, manufacturing processes and electronic test equipment.
• FIG 8 A is an isometric view illustration depicting two adjacent interferometric modulator (IMOD) display elements in a series or array of display elements of an IMOD display device.
• the IMOD display device includes one or more interferometric EMS, such as MEMS, display elements.
• the interferometric MEMS display elements can be configured in either a bright or dark state. In the bright ("relaxed,” “open” or “on,” etc.) state, the display element reflects a large portion of incident visible light. Conversely, in the dark (“actuated,” “closed” or “off,” etc.) state, the display element reflects little incident visible light.
• MEMS display elements can be configured to reflect predominantly at particular wavelengths of light allowing for a color display in addition to black and white. In some implementations, by using multiple display elements, different intensities of color primaries and shades of gray can be achieved.
• the IMOD display device can include an array of IMOD display elements which may be arranged in rows and columns.
• Each display element in the array can include at least a pair of reflective and semi-reflective layers, such as a movable reflective layer (i.e., a movable layer, also referred to as a mechanical layer) and a fixed partially reflective layer (i.e., a stationary layer), positioned at a variable and
• the movable reflective layer may be moved between at least two positions. For example, in a first position, i.e., a relaxed position, the movable reflective layer can be positioned at a distance from the fixed partially reflective layer. In a second position, i.e., an actuated position, the movable reflective layer can be positioned more closely to the partially reflective layer. Incident light that reflects from the two layers can interfere constructively and/or destructively depending on the position of the movable reflective layer and the wavelength(s) of the incident light, producing either an overall reflective or non-reflective state for each display element.
• the display element may be in a reflective state when unactuated, reflecting light within the visible spectrum, and may be in a dark state when actuated, absorbing and/or destructively interfering light within the visible range. In some other implementations, however, an IMOD display element may be in a dark state when unactuated, and in a reflective state when actuated. In some
• the introduction of an applied voltage can drive the display elements to change states.
• an applied charge can drive the display elements to change states.
• the depicted portion of the array in Figure 8 A includes two adjacent interferometric MEMS display elements in the form of IMOD display elements 12.
• the movable reflective layer 14 is illustrated in an actuated position near, adjacent or touching the optical stack 16.
• the voltage V b i as applied across the display element 12 on the right is sufficient to move and also maintain the movable reflective layer 14 in the actuated position.
• a movable reflective layer 14 is illustrated in a relaxed position at a distance (which may be predetermined based on design parameters) from an optical stack 16, which includes a partially reflective layer.
• the voltage V 0 applied across the display element 12 on the left is insufficient to cause actuation of the movable reflective layer 14 to an actuated position such as that of the display element 12 on the right.
• FIG 8 A the reflective properties of IMOD display elements 12 are generally illustrated with arrows indicating light 13 incident upon the IMOD display elements 12, and light 15 reflecting from the display element 12 on the left. Most of the light 13 incident upon the display elements 12 may be transmitted through the transparent substrate 20, toward the optical stack 16. A portion of the light incident upon the optical stack 16 may be transmitted through the partially reflective layer of the optical stack 16, and a portion will be reflected back through the transparent substrate 20. The portion of light 13 that is transmitted through the optical stack 16 may be reflected from the movable reflective layer 14, back toward (and through) the transparent substrate 20.
• the transparent substrate 20 can be a glass substrate (sometimes referred to as a glass plate or panel).
• the glass substrate may be or include, for example, a borosilicate glass, a soda lime glass, quartz, Pyrex, or other suitable glass material.
• the glass substrate may have a thickness of 0.3, 0.5 or 0.7 millimeters, although in some implementations the glass substrate can be thicker (such as tens of millimeters) or thinner (such as less than 0.3 millimeters).
• a non-glass substrate can be used, such as a polycarbonate, acrylic, polyethylene terephthalate (PET) or polyether ether ketone (PEEK) substrate. In such an implementation, the non-glass substrate will likely have a thickness of less than 0.7 millimeters, although the substrate may be thicker depending on the design considerations.
• a non-transparent substrate such as a metal foil or stainless steel-based substrate can be used.
• a reverse-IMOD-based display which includes a fixed reflective layer and a movable layer which is partially transmissive and partially reflective, may be configured to be viewed from the opposite side of a substrate as the display elements 12 of Figure 8 A and may be supported by a non-transparent substrate.
• the optical stack 16 can include a single layer or several layers.
• the layer(s) can include one or more of an electrode layer, a partially reflective and partially transmissive layer, and a transparent dielectric layer.
• the optical stack 16 is electrically conductive, partially transparent and partially reflective, and may be fabricated, for example, by depositing one or more of the above layers onto a transparent substrate 20.
• the electrode layer can be formed from a variety of materials, such as various metals, for example indium tin oxide (ITO).
• ITO indium tin oxide
• the partially reflective layer can be formed from a variety of materials that are partially reflective, such as various metals (e.g., chromium and/or molybdenum), semiconductors, and dielectrics.
• the partially reflective layer can be formed of one or more layers of materials, and each of the layers can be formed of a single material or a combination of materials.
• certain portions of the optical stack 16 can include a single semi-transparent thickness of metal or semiconductor which serves as both a partial optical absorber and electrical conductor, while different, electrically more conductive layers or portions (e.g., of the optical stack 16 or of other structures of the display element) can serve to bus signals between IMOD display elements.
• the optical stack 16 also can include one or more insulating or dielectric layers covering one or more conductive layers or an electrically conductive/partially absorptive layer.
• the layer(s) of the optical stack 16 can be patterned into parallel strips, and may form row electrodes in a display device as described further below.
• the term "patterned" is used herein to refer to masking as well as etching processes.
• a highly conductive and reflective material such as aluminum (Al) may be used for the movable reflective layer 14, and these strips may form column electrodes in a display device.
• the movable reflective layer 14 may be formed as a series of parallel strips of a deposited metal layer or layers (orthogonal to the row electrodes of the optical stack 16) to form columns deposited on top of supports, such as the illustrated posts 18, and an intervening sacrificial material located between the posts 18.
• a defined gap 19, or optical cavity can be formed between the movable reflective layer 14 and the optical stack 16.
• the spacing between posts 18 may be approximately 1-1000 ⁇ , while the gap 19 may be approximately less than 10,000 Angstroms (A).
• each IMOD display element whether in the actuated or relaxed state, can be considered as a capacitor formed by the fixed and moving reflective layers.
• the movable reflective layer 14 When no voltage is applied, the movable reflective layer 14 remains in a mechanically relaxed state, as illustrated by the display element 12 on the left in Figure 8 A, with the gap 19 between the movable reflective layer 14 and optical stack 16.
• a potential difference i.e., a voltage
• a dielectric layer (not shown) within the optical stack 16 may prevent shorting and control the separation distance between the layers 14 and 16, as illustrated by the actuated display element 12 on the right in Figure 8 A.
• the behavior can be the same regardless of the polarity of the applied potential difference.
• a series of display elements in an array may be referred to in some instances as "rows" or
• the rows can be considered columns, and the columns considered to be rows.
• the rows may be referred to as “common” lines and the columns may be referred to as “segment” lines, or vice versa.
• the display elements may be evenly arranged in orthogonal rows and columns (an “array"), or arranged in non-linear configurations, for example, having certain positional offsets with respect to one another (a "mosaic”).
• array and “mosaic” may refer to either configuration.
• the display is referred to as including an "array” or “mosaic,” the elements themselves need not be arranged orthogonally to one another, or disposed in an even distribution, in any instance, but may include arrangements having asymmetric shapes and unevenly distributed elements.
• Figure 8B is a system block diagram illustrating an electronic device incorporating an IMOD-based display including a three element by three element array of IMOD display elements.
• the electronic device includes a processor 21 that may be configured to execute one or more software modules.
• the processor 21 may be configured to execute one or more software applications, including a web browser, a telephone application, an email program, or any other software application.
• the processor 21 can be configured to communicate with an array driver 22.
• the array driver 22 can include a row driver circuit 24 and a column driver circuit 26 that provide signals to, for example a display array or panel 30.
• the cross section of the IMOD display device illustrated in Figure 8A is shown by the lines 1-1 in Figure 8B.
• Figure 8B illustrates a 3x3 array of IMOD display elements for the sake of clarity, the display array 30 may contain a very large number of IMOD display elements, and may have a different number of IMOD display elements in rows than in columns, and vice versa.
• FIGS 9A and 9B are system block diagrams illustrating a display device 40 that includes a plurality of IMOD display elements.
• the display device 40 can be, for example, a smart phone, a cellular or mobile telephone.
• the same components of the display device 40 or slight variations thereof are also illustrative of various types of display devices such as televisions, computers, tablets, e-readers, handheld devices and portable media devices.
• the display device 40 includes a housing 41, a display 30, an antenna 43, a speaker 45, an input device 48 and a microphone 46.
• the housing 41 can be formed from any of a variety of manufacturing processes, including injection molding, and vacuum forming.
• the housing 41 may be made from any of a variety of materials, including, but not limited to: plastic, metal, glass, rubber and ceramic, or a combination thereof.
• the housing 41 can include removable portions (not shown) that may be interchanged with other removable portions of different color, or containing different logos, pictures, or symbols.
• the display 30 may be any of a variety of displays, including a bi-stable or analog display, as described herein.
• the display 30 also can be configured to include a flat-panel display, such as plasma, EL, OLED, STN LCD, or TFT LCD, or a non-flat- panel display, such as a CRT or other tube device.
• the display 30 can include an IMOD-based display, as described herein.
• the components of the display device 40 are schematically illustrated in Figure 9A.
• the display device 40 includes a housing 41 and can include additional components at least partially enclosed therein.
• the display device 40 includes a network interface 27 that includes an antenna 43 which can be coupled to a transceiver 47.
• the network interface 27 may be a source for image data that could be displayed on the display device 40.
• the network interface 27 is one example of an image source module, but the processor 21 and the input device 48 also may serve as an image source module.
• the transceiver 47 is connected to a processor 21, which is connected to conditioning hardware 52.
• the conditioning hardware 52 may be configured to condition a signal (such as filter or otherwise manipulate a signal).
• the conditioning hardware 52 can be connected to a speaker 45 and a microphone 46.
• the processor 21 also can be connected to an input device 48 and a driver controller 29.
• the driver controller 29 can be coupled to a frame buffer 28, and to an array driver 22, which in turn can be coupled to a display array 30.
• One or more elements in the display device 40 including elements not specifically depicted in Figure 9A, can be configured to function as a memory device and be configured to communicate with the processor 21.
• a power supply 50 can provide power to substantially all components in the particular display device 40 design.
• the network interface 27 includes the antenna 43 and the transceiver 47 so that the display device 40 can communicate with one or more devices over a network.
• the network interface 27 also may have some processing capabilities to relieve, for example, data processing requirements of the processor 21.
• the antenna 43 can transmit and receive signals.
• the antenna 43 transmits and receives RF signals according to the IEEE 16.11 standard, including IEEE 16.11(a), (b), or (g), or the IEEE 802.11 standard, including IEEE 802.8A, b, g, n, and further implementations thereof.
• the antenna 43 transmits and receives RF signals according to the Bluetooth® standard.
• the antenna 43 can be designed to receive code division multiple access (CDMA), frequency division multiple access (FDMA), time division multiple access (TDM A), Global System for Mobile communications (GSM), GSM/General Packet Radio Service (GPRS), Enhanced Data GSM Environment (EDGE), Terrestrial Trunked Radio (TETRA), Wideband-CDMA (W-CDMA), Evolution Data Optimized (EV-DO), lxEV-DO, EV-DO Rev A, EV-DO Rev B, High Speed Packet Access (HSPA), High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access
• the transceiver 47 can pre-process the signals received from the antenna 43 so that they may be received by and further manipulated by the processor 21.
• the transceiver 47 also can process signals received from the processor 21 so that they may be transmitted from the display device 40 via the antenna 43.
• the transceiver 47 can be replaced by a receiver.
• the network interface 27 can be replaced by an image source, which can store or generate image data to be sent to the processor 21.
• the processor 21 can control the overall operation of the display device 40.
• the processor 21 receives data, such as compressed image data from the network interface 27 or an image source, and processes the data into raw image data or into a format that can be readily processed into raw image data.
• the processor 21 can send the processed data to the driver controller 29 or to the frame buffer 28 for storage.
• Raw data typically refers to the information that identifies the image characteristics at each location within an image. For example, such image characteristics can include color, saturation and gray-scale level.
• the processor 21 can include a microcontroller, CPU, or logic unit to control operation of the display device 40.
• the conditioning hardware 52 may include amplifiers and filters for transmitting signals to the speaker 45, and for receiving signals from the microphone 46.
• the conditioning hardware 52 may be discrete components within the display device 40, or may be incorporated within the processor 21 or other components.
• the driver controller 29 can take the raw image data generated by the processor 21 either directly from the processor 21 or from the frame buffer 28 and can re-format the raw image data appropriately for high speed transmission to the array driver 22. In some implementations, the driver controller 29 can re-format the raw image data into a data flow having a raster-like format, such that it has a time order suitable for scanning across the display array 30.
• driver controller 29 sends the formatted information to the array driver 22.
• a driver controller 29, such as an LCD controller is often associated with the system processor 21 as a stand-alone Integrated Circuit (IC), such controllers may be implemented in many ways.
• controllers may be embedded in the processor 21 as hardware, embedded in the processor 21 as software, or fully integrated in hardware with the array driver 22.
• the array driver 22 can receive the formatted information from the driver controller 29 and can re-format the video data into a parallel set of waveforms that are applied many times per second to the hundreds, and sometimes thousands (or more), of leads coming from the display's x-y matrix of display elements.
• the driver controller 29, the array driver 22, and the display array 30 are appropriate for any of the types of displays described herein.
• the driver controller 29 can be a conventional display controller or a bistable display controller (such as an IMOD display element controller).
• the array driver 22 can be a conventional driver or a bi-stable display driver (such as an IMOD display element driver).
• the display array 30 can be a conventional display array or a bi-stable display array (such as a display including an array of IMOD display elements).
• the driver controller 29 can be integrated with the array driver 22. Such an implementation can be useful in highly integrated systems, for example, mobile phones, portable-electronic devices, watches or small-area displays.
• the input device 48 can be configured to allow, for example, a user to control the operation of the display device 40.
• the input device 48 can include a keypad, such as a QWERTY keyboard or a telephone keypad, a button, a switch, a rocker, a touch-sensitive screen, a touch-sensitive screen integrated with the display array 30, or a pressure- or heat-sensitive membrane.
• the microphone 46 can be configured as an input device for the display device 40. In some implementations, voice commands through the microphone 46 can be used for controlling operations of the display device 40.
• the power supply 50 can include a variety of energy storage devices.
• the power supply 50 can be a rechargeable battery, such as a nickel-cadmium battery or a lithium-ion battery.
• the rechargeable battery may be chargeable using power coming from, for example, a wall socket or a photovoltaic device or array.
• the rechargeable battery can be wirelessly chargeable.
• the power supply 50 also can be a renewable energy source, a capacitor, or a solar cell, including a plastic solar cell or solar-cell paint.
• the power supply 50 also can be configured to receive power from a wall outlet.
• control programmability resides in the driver controller 29 which can be located in several places in the electronic display system. In some other implementations, control programmability resides in the array driver 22.
• the above-described optimization may be implemented in any number of hardware and/or software components and in various configurations.
• a phrase referring to "at least one of a list of items refers to any combination of those items, including single members.
• "at least one of: a, b, or c” is intended to cover: a, b, c, a-b, a-c, b-c, and a-b-c.
• the hardware and data processing apparatus used to implement the various illustrative logics, logical blocks, modules and circuits described in connection with the aspects disclosed herein may be implemented or performed with a general purpose single- or multi-chip processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein.
• a general purpose processor may be a microprocessor, or, any conventional processor, controller, microcontroller, or state machine.
• a processor also may be implemented as a combination of computing devices, such as a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration.
• a processor also may be implemented as a combination of computing devices, such as a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration.
• the claimed combination may be directed to a subcombination or variation of a subcombination.

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