JP2016029719A - Thin-film transistor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thin-film transistor high in mobility and excellent in controllability of a turn-on voltage.SOLUTION: A thin-film transistor 1 of the present invention includes a substrate 10, a gate electrode 20, a gate insulating film 30, a threshold control layer 40, active layers 50, a source electrode 60, and a drain electrode 70. The threshold control layer 40 and at least one of the active layers 50 are electrically connected between the source electrode 60 and the drain electrode 70. The threshold control layer 40 and the active layers 50 are in contact with the gate insulating film 30 and have the common gate electrode 20 separated in a vertical direction with respect to the gate insulating film 30 and connected at the same potential.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a thin film transistor.

Field effect transistors such as thin film transistors (TFTs) are widely used as unit electronic elements, high frequency signal amplifying elements, liquid crystal driving elements and the like of semiconductor memory integrated circuits, and are currently the most widely used electronic devices. . Among these, with the remarkable development of display devices in recent years, display elements in various display devices such as liquid crystal display devices (LCD), electroluminescence display devices (EL), field emission displays (FED), and MEMS displays (MEMS) As a switching element for driving a display device by applying a driving voltage to the TFT, a TFT is frequently used.
Examples of the TFT include an amorphous silicon TFT, a polycrystalline silicon TFT, and an oxide semiconductor TFT. Among these, oxide semiconductor TFTs are attracting attention because they can pass a larger current than amorphous silicon TFTs, and can be manufactured at a lower cost than polycrystalline silicon TFTs and with small variations in characteristics between elements. It came to be.
However, oxide semiconductor TFTs have drawbacks such as low field effect mobility (about 10 cm ² / V · sec) and difficulty in controlling turn-on voltage (prone to be normally on) compared to polycrystalline silicon TFTs. It was an obstacle to industrial spread. Further, when the mobility is improved, there is a problem that the on / off ratio is decreased, the leakage current is increased, and the pinch-off becomes unclear.

As an invention for solving some of these problems, for example, the active layer and the resistance layer are made of a metal oxide, and a resistance layer (more than the active layer) is provided between the active layer and at least one of the source electrode and the drain electrode. A thin film transistor having a layer having low electrical conductivity has been proposed (Patent Document 1).
The semiconductor layer is made of a metal oxide and has three regions including a first region, a second region, and a third region, and the first region is connected to the source electrode. , The third region is connected to the drain electrode, the second region is connected between the first region and the third region, and the resistivity of the three regions is equal to that of the first region. A thin film transistor having a relationship of resistivity> resistivity of the second region> resistivity of the third region has been proposed (Patent Document 2).

JP 2009-212497 A International Publication No. 2011/125940

  However, the thin film transistor described in Patent Document 1 has a problem that it is difficult to control the threshold voltage, although an improvement in on / off ratio can be expected. That is, in Patent Document 1, since the threshold voltage of the thin film transistor is not controlled, the controllability of the turn-on voltage is insufficient.

  The thin film transistor described in Patent Document 2 also has a problem that the threshold voltage is difficult to control. In Patent Document 2, the threshold voltage (Vth) in the vicinity of 0 V is an issue, but the resistivity of the three regions of the semiconductor layer is that the resistivity of the first region> the resistivity of the second region> third. The threshold voltage cannot be controlled only by the relationship of the resistivity in the region. In Patent Document 2, the total carrier amount in the region B (region sandwiched between the source electrode and the gate insulating film) and the threshold voltage are correlated with each other in FIG. Vth is estimated to depend on the total number of carriers in the channel. That is, in Patent Document 2, since the threshold voltage of the thin film transistor cannot be sufficiently controlled, the controllability of the turn-on voltage is insufficient.

  Therefore, an object of the present invention is to provide a thin film transistor having high mobility and good turn-on voltage controllability.

In order to solve the above problems, the present invention provides the following thin film transistor.
The thin film transistor of the present invention includes a substrate, a gate electrode, a gate insulating film, a threshold control layer, an active layer, a source electrode, and a drain electrode, and between the source electrode and the drain electrode, The threshold control layer and at least one or more of the active layers are electrically connected, and the threshold control layer and the active layer are in contact with the gate insulating film and separated in a direction perpendicular to the gate insulating film It has the said common gate electrode connected to the same electric potential, It is characterized by the above-mentioned.

According to the present invention, a thin film transistor having high mobility and good turn-on voltage controllability can be obtained. The reason for this is not necessarily clear, but the present inventors presume as follows.
In other words, in the present invention, the threshold control layer and the active layer are in contact with the gate insulating film, and have a common gate electrode connected to the same potential and separated in a direction perpendicular to the gate insulating film. In the case of such a structure, the present inventors have found that the turn-on voltage of the threshold control layer becomes the turn-on voltage of the thin film transistor regardless of the turn-on voltage of the active layer. For this reason, since the field effect by the gate electrode is common, for example, when the polarity of the thin film transistor is n-type, the present inventors believe that the threshold value is controlled by a threshold control layer having a high turn-on voltage in nature. presume. On the other hand, in the case of such a structure, the material of the active layer can be selected from the viewpoint of mobility and the like without considering the physical property of the turn-on voltage. You can also choose from the inside. As described above, since the room for selecting the material or process of the active layer is greatly expanded, the mobility of the active layer can be increased. In addition, the mobility of the thin film transistor can be increased by increasing the mobility of the active layer. The present inventors presume that the above-described effects of the present invention can be achieved by the operation as described above.

  According to the present invention, a thin film transistor having high mobility and good turn-on voltage controllability can be provided.

It is the schematic which shows the thin-film transistor in 1st embodiment. It is the schematic which shows the thin-film transistor in 2nd embodiment. It is the schematic which shows the thin-film transistor in 3rd embodiment. It is the schematic which shows the thin-film transistor in 4th embodiment. It is the schematic which shows the thin-film transistor in 5th embodiment. It is the schematic which shows the thin-film transistor in 6th embodiment. It is the schematic which shows the conventional thin-film transistor. 6 is a schematic view showing thin film transistors fabricated in Example 2 and Example 3. FIG. 4 is a photograph showing an optical microscope image of the thin film transistor fabricated in Example 2 and Example 3. 7 is a graph showing the relationship between current and voltage for the thin film transistors fabricated in Example 2 and Example 3. It is a graph which shows the relationship between the mobility about a thin film transistor produced in Example 2 and Example 3, and a voltage.

[First embodiment]
Hereinafter, a first embodiment which is one of preferred embodiments of the present invention will be described in detail with reference to the drawings.
The embodiment described below is a preferred specific example of the present invention, and thus various technically preferable limitations are given. However, the scope of the present invention is particularly limited in the following description. Unless otherwise stated, the present invention is not limited to these embodiments.

FIG. 1 is a schematic view showing a thin film transistor in the first embodiment of the present invention. A thin film transistor 1 illustrated in FIG. 1 includes a substrate 10, a gate electrode 20, a gate insulating film 30, a threshold control layer 40, an active layer 50, a source electrode 60, and a drain electrode 70. Between the source electrode 60 and the drain electrode 70, a threshold control layer 40 is provided in contact with the source electrode 60, and an active layer 50 is provided in contact with the threshold control layer 40 and the drain electrode 70. Connected. A partial region of the threshold control layer 40 and the active layer 50 forms a channel portion, and a current flowing between the source electrode 60 and the drain electrode 70 is controlled by a voltage applied to the gate electrode 20. To turn on / off.
Here, the channel portion is defined as a region facing the gate electrode and surrounded by a channel length (a distance between the source electrode and the drain electrode), a channel width (a width between the source electrode and the drain electrode), and a film thickness of the semiconductor layer.

The channel length is preferably from 0.1 μm to 50 μm, more preferably from 0.5 μm to 20 μm, and particularly preferably from 1 μm to 10 μm. If it exceeds 50 μm, the size of the thin film transistor becomes too large, and the degree of integration may decrease. When the thickness is less than 0.1 μm, high accuracy is required for photolithography, which may make it difficult to employ in a large area display or the like.
The channel width is preferably 0.1 μm or more and 2000 μm or less, more preferably 1 μm or more and 1000 μm or less, and particularly preferably 4 μm or more and 500 μm or less. If it exceeds 2000 μm, the thin film transistor becomes too large and the degree of integration may decrease. When the thickness is less than 0.1 μm, high accuracy is required for photolithography, which may make it difficult to employ in a large area display or the like. The reason why it has a wider value than the channel length is that, for example, when considering the use of a flat panel display, the required characteristics of the thin film transistor are different in the pixel portion, the peripheral circuit portion, and the like.

In the thin film transistor 1 of this embodiment, as shown in FIG. 1, the threshold control layer 40 and the active layer 50 are in contact with the gate insulating film, and are connected to the same potential separated in the vertical direction with respect to the gate insulating film 30. The gate electrode 20 is provided.
With such a structure, the turn-on voltage of the threshold control layer 40 is substantially the same as the turn-on voltage of the thin film transistor 1, and therefore the turn-on voltage of the thin film transistor 1 can be controlled by the threshold control layer 40.
From the viewpoint of controllability of the turn-on voltage, the threshold control layer 40 and the active layer 50 are preferably connected in series in the horizontal direction between the source electrode 60 and the drain electrode 70 as shown in FIG. . In FIG. 1, the boundary between the threshold control layer 40 and the active layer 50 is vertically linear, but this is not limitative. For example, the boundary between the threshold control layer 40 and the active layer 50 may be tapered or stepped. Furthermore, these layers may not be clearly separated at the boundary between the threshold control layer 40 and the active layer 50. For example, the boundary may be blurred because the threshold control layer 40 and the active layer 50 have a predetermined width by mutual diffusion.

(Threshold control layer and active layer)
The threshold control layer 40 is a layer that forms a channel portion together with the active layer 50. The threshold control layer 40 is not limited to an oxide semiconductor as a material capable of forming the active layer 50. However, the maximum effect can be obtained by using an oxide semiconductor.
As a material constituting the threshold control layer 40, In (indium), Zn (zinc), Sn (tin), Ga (gallium), Al (aluminum), Ti (titanium), Hf (hafnium), Zr (zirconium) And oxides such as Si (silicon) and complex oxides thereof. These may be used alone or in combination of two or more. In addition, these may be amorphous, crystalline, or an intermediate state between crystal and amorphous. More preferably, the threshold control layer 40 preferably contains at least indium.

As a material constituting the active layer 50, In (indium), Zn (zinc), Sn (tin), Ga (gallium), Al (aluminum), Ti (titanium), Hf (hafnium), Zr (zirconium), Examples thereof include oxides such as Si (silicon) and complex oxides thereof. These may be used alone or in combination of two or more. In addition, these may be amorphous, crystalline, or an intermediate state between crystal and amorphous. More preferably, the active layer 50 preferably contains at least indium.
In the present embodiment, the material constituting the active layer 50 can be selected from the viewpoint of mobility and the like without considering the physical properties of the turn-on voltage. For example, the material can be selected from materials that are normally on. From this point of view, as a material constituting the active layer 50, an oxide semiconductor having a relatively high indium ratio that has not been available so far can be used.

Examples of suitable combinations of the materials constituting the threshold control layer 40 and the active layer 50 include the following specific examples.
Specific example (i)
Materials constituting the threshold control layer 40: oxides such as gallium, aluminum, titanium, hafnium, zirconium and silicon, and materials constituting these composite oxide active layers 50: oxides such as indium, zinc and tin and these Example of complex oxide (ii)
A material constituting the threshold control layer 40: a material constituting the mixed oxide active layer 50 of an oxide such as gallium, aluminum, titanium, hafnium, zirconium and silicon and a composite oxide thereof and indium oxide: at least indium When the threshold control layer 40 and the active layer 50 contain at least indium, the total ratio of gallium, aluminum, titanium, hafnium, zirconium and silicon in the threshold control layer 40 is 0.4 in terms of metal element. The total ratio of indium, zinc and tin in the active layer is preferably 0.8 or more.
Note that the metal composition and the composition ratio constituting the threshold control layer 40 and the active layer 50 may be the same. In this case, it suffices if a difference occurs in the amount of oxygen defects and the turn-on voltage values of the threshold control layer 40 and the active layer 50 differ.

In this embodiment, when the turn-on voltage value of the threshold control layer 40 is V _on1 and the turn-on voltage value of the active layer 50 is V _on2 , the mobility is high if the following conditions are satisfied, In addition, the thin film transistor 1 with good turn-on voltage controllability can be obtained more reliably.
For example, when the polarity of the thin film transistor 1 is n-type, it satisfies at least the condition (i) among the following conditions (i) to (iii) (particularly preferably (i) to (iii) Satisfying all of the above conditions).
(I) The condition of the following formula (1n) is satisfied.
V _on1 > V _on2 (1n)
(Ii) The turn-on voltage value V _on1 of the threshold control layer 40 is 0 V or more (more preferably, 0 V or more and 5 V or less), and the turn-on voltage value V _on2 of the active layer 50 is less than 0 V (more preferably −30 V). Or more and less than 0V).
(Iii) the carrier concentration of the threshold control layer 40 is less than 1 × 10 ¹⁵ cm ⁻³ ,
The carrier concentration of the active layer 50 is 1 × 10 ¹⁵ cm ⁻³ or more.

On the other hand, when the polarity of the thin film transistor 1 is p-type, at least the condition (iv) is satisfied among the following conditions (iv) to (vi) (particularly preferably (iv) to (vi It is preferable to satisfy all of the above conditions).
(Iv) The condition of the following mathematical formula (1p) is satisfied.
V _on1 <V _on2 (1p)
(V) The turn-on voltage value V _on1 of the threshold control layer 40 is 0 V or less (more preferably −5
The turn-on voltage value V _on2 of the active layer 50 is more than 0 V (more preferably more than 0 V and not more than 30 V).
(Vi) The carrier concentration of the threshold control layer 40 is less than 1 × 10 ¹⁵ cm ⁻³ , and the carrier concentration of the active layer 50 is 1 × 10 ¹⁵ cm ⁻³ or more.

For the turn-on voltage value V _on1 of the threshold control layer 40, a sample provided with the source electrode 60 and the drain electrode 70 is prepared so that the channel portion is only the threshold control layer 40, and the turn-on voltage value of the sample is set. It can be obtained by measuring. Further, the turn-on voltage value V _on2 of the active layer 50 can be obtained by the same method.
In order to obtain the turn-on voltage value (V _on ), transfer characteristics are measured in the thin film transistor. From the transfer characteristic graph, the gate voltage Vg at which the current value Id between the source and drain electrodes is Id = 10 ⁻¹⁰ A is defined as the turn-on voltage value V _on . The transfer curve can be measured using a semiconductor parameter analyzer (Agilent B1500).

The carrier concentration in the channel portion can be obtained from the SSRM (scanning spread resistance microscopy) measurement and the hole measurement using a standard sample as described below.
SSRM measurement First, a cross section of the channel part is obtained by an underwater mechanical polishing method. Subsequently, the spreading resistance is measured by SSRM on the cross section. The carrier concentration can be calculated by comparing with a standard sample with a known carrier concentration. For example, if the carrier concentration is higher than that of the standard sample, the spreading resistance is reduced. On the other hand, if the carrier concentration is lower than that of the standard sample, the spreading resistance is reduced. At the time of comparison, it is desirable to match the element shapes at the time of SSRM measurement and to keep the state of the probe similarly.
An apparatus used for SSRM measurement and an example of the measurement conditions are shown below.
Observation apparatus: “NanoScope IVa AFM Dimension 3100” manufactured by Digital Instruments section of Bruker AXS (formerly Veeco)
Stage AFM system "+ SSRM option SSRM scan mode: contact mode and spread resistance simultaneous measurement SSRM probe (Tip): Diamond coated silicon cantilever Sample processing: After creating cross section by mechanical polishing, each layer can be short-circuited to apply bias voltage processing.
Measurement environment: Hall measurement at room temperature, in air, using standard samples It is desirable to use a substrate with the same layered structure as the thin film transistor. The semiconductor film and hole are processed in the same process as the channel layer of the thin film transistor. An electrode and a protective layer are prepared. The carrier concentration can be measured using the van der pauw method.
An example of a Hall measuring device and its measurement conditions are shown below.
Hall measuring device: “Resi Test 8310” manufactured by Toyo Technica
Measurement conditions: Room temperature (about 25 ° C.), about 0.5 [T], about 10 ⁻⁴ to 10 ⁻¹² A, AC magnetic field Hall measurement sample shape: 10 mm × 10 mm

In the present embodiment, if the photovoltage response of the threshold control layer 40 is higher than the photovoltage response of the active layer 50, the thin film transistor 1 having high mobility and good turn-on voltage controllability can be obtained more reliably. Can be obtained.
When the transfer characteristics are evaluated while covering the channel width in the horizontal direction along the channel portion located between the source and drain electrodes of the thin film transistor 1 and irradiating spot-like light in the channel direction, the spot irradiation position is When the threshold control layer 40 is covered, an increase in off current and a shift in turn-on voltage are observed. This phenomenon is called photoresponsiveness, and when observing this phenomenon, it is necessary to irradiate light having a band gap of the semiconductor constituting the threshold control layer 40. If the spot irradiation position is moved to the area of the active layer 50, the photoresponsiveness described above is smaller than that of the threshold control layer 40 area. This is because the off-state current of the transfer characteristic is borne by the region of the threshold control layer 40, so that even if the active layer 50 is irradiated with light and carriers are generated, it is blocked by the threshold control layer 40.

  The ratio of the length of the threshold control layer 40 to the channel length is preferably 50% or less and more preferably 25% or less with respect to the channel length of 100% from the viewpoint of increasing the mobility. Preferably, it is particularly preferably 10% or less.

(Method for forming threshold control layer and active layer)
Such a threshold control layer 40 and the active layer 50 can be formed by a technique known in this technical field.
Examples of film formation methods include chemical film formation methods such as spraying, dipping, and chemical vapor deposition (CVD), or sputtering, vacuum evaporation, ion plating, and pulsed laser deposition. Physical film deposition methods can be used. In addition, a spin coating method, a screen printing method, a drop cast method, a roll coating method, an ink jet method, or the like may be selected. Note that it is preferable to use a physical film formation method because the carrier density is easy to control and the film quality can be easily improved, and among these, it is more preferable to use a sputtering method because of high productivity.
More specifically, the film can be formed by DC, AC, or RF sputtering using a target composed of a predetermined material. Here, the target can be produced by a known method.
The formed film can be patterned by various etching methods.
The formed film is preferably heat-treated at, for example, 70 to 450 ° C. in an oxygen atmosphere or an inert gas atmosphere.
Further, a method may be adopted in which a part of the formed film is modified by irradiating O ₂ plasma, N ₂ O plasma or the like as an example, and a part of the active layer 50 is used as the threshold control layer 40. Good. Alternatively, the threshold control layer 40 may be formed first, and the active layer 50 may be formed by Ar plasma, He plasma, H ₂ plasma, UV irradiation, or the like. According to such a method, the process of forming the threshold control layer 40 and the active layer 50 can be simplified.

(Other components)
In the thin film transistor 1 of the present embodiment, known components can be applied to the constituent members such as the substrate 10, the gate electrode 20, the gate insulating film 30, the source electrode 60 and the drain electrode 70.
Hereinafter, examples of constituent members of the thin film transistor will be described.

(substrate)
There is no restriction | limiting in particular about the material which comprises the board | substrate 10, A well-known thing can be used in this technical field. For example, glass substrates such as alkali silicate glass, non-alkali glass and quartz glass, silicon substrates, resin substrates such as acrylic, polycarbonate and polyethylene naphthalate (PEN), polymer film bases such as polyethylene terephthalate (PET) and polyamide Materials can be used.

(Gate electrode, source electrode and drain electrode)
The material for forming each of the gate electrode 20, the source electrode 60 and the drain electrode 70 is not particularly limited, and any material generally used can be selected as long as the effects of the present invention are not lost. For example, transparent electrodes such as indium tin oxide (ITO), indium zinc oxide, ZnO, SnO ₂ , metal electrodes such as Al, Ag, Cr, Ni, Mo, Au, Ti, Ta, Cu, or these An alloy metal electrode can be used. In the case where an oxide semiconductor is used for the semiconductor layer, it can be reduced and used as an electrode.

(Gate insulation film)
The material constituting the gate insulating film 30 is not particularly limited, and any material generally used can be selected as long as the effects of the present invention are not lost. For _{example, SiO 2, SiNx, Al 2} O 3, Ta 2 O 5, TiO 2, MgO, ZrO 2, CeO 2, K 2 O, Li 2 O, Na 2 O, Rb 2 O, Sc 2 O 3, Y Oxides and nitrides such as ₂ O ₃ , Hf ₂ O ₃ , CaHfO ₃ , PbTi ₃ , BaTa ₂ O ₆ , SrTiO ₃ , and AlN can be used. Note that, as items required for the gate insulating film, it is important that the film thickness unevenness is small and that no pinholes that cause leakage exist. As a general gate insulating film, SiO ₂ , SiNx, Al ₂ O ₃ or the like is used. SiNx may contain a hydrogen element.

(Method for forming other components)
The constituent members such as the substrate 10, the gate electrode 20, the gate insulating film 30, the source electrode 60 and the drain electrode 70 can be formed by a technique known in this technical field.
Examples of film formation methods include chemical film formation methods such as spraying, dipping, and chemical vapor deposition (CVD), or sputtering, vacuum evaporation, ion plating, and pulsed laser deposition. In addition to the physical film formation method, a spin coating method, a screen printing method, a drop cast method, a roll coating method, and an ink jet method can be used. It is preferable to use a physical film forming method because the carrier density is easily controlled and the film quality can be easily improved, and among these, it is more preferable to use a sputtering method because of high productivity.
The formed film can be patterned by various etching methods. Further, the formed film may be heat-treated as necessary.

[Second Embodiment]
Next, a second embodiment of the present invention will be described based on the drawings.
Note that the constituent members of the thin film transistor 1A of the present embodiment are the same as those of the first embodiment, and therefore detailed description thereof is omitted or simplified.
FIG. 2 is a schematic view showing a thin film transistor according to the second embodiment of the present invention.
In the present embodiment, in addition to the threshold control layer 40 provided in contact with the source electrode 60, another threshold control layer 40 is provided in contact with the drain electrode 70 as compared with the first embodiment. Only the point is different.
Even when two threshold control layers 40 are provided as described above, the two threshold control layers 40 and the active layer 50 are in contact with the gate insulating film and perpendicular to the gate insulating film 30 as shown in FIG. Since the common gate electrodes 20 connected to the same potential are separated from each other, the turn-on voltages of the two threshold control layers 40 are substantially the same. Further, since the turn-on voltage of the threshold control layer 40 becomes substantially the same as the turn-on voltage of the thin film transistor 1A, the threshold control layer 40 can control the turn-on voltage of the thin film transistor 1A.

[Third embodiment]
Next, a third embodiment of the present invention will be described based on the drawings.
Note that the constituent members of the thin film transistor 1B of the present embodiment are the same as those of the first embodiment, and thus detailed description thereof is omitted or simplified.
FIG. 3 is a schematic view showing a thin film transistor in the third embodiment of the present invention.
In the present embodiment, the active layer 50 is provided in contact with the source electrode 60 between the source electrode 60 and the drain electrode 70 as compared with the first embodiment, and the threshold control layer 40 is in contact with the active layer 50. The only difference is that the active layer 50 is provided in contact with the threshold control layer 40 and the drain electrode 70.
Even when the threshold control layer 40 is not in contact with the source electrode 60 or the drain electrode 70 as described above, the turn-on voltage of the threshold control layer 40 becomes substantially the same as the turn-on voltage of the thin film transistor 1B. The turn-on voltage of 1B can be controlled.

[Fourth embodiment]
Next, a fourth embodiment of the present invention will be described based on the drawings.
Note that the constituent members of the thin film transistor 1C of this embodiment are the same as those of the first embodiment, and thus detailed description thereof is omitted or simplified.
FIG. 4 is a schematic view showing a thin film transistor in the fourth embodiment of the present invention.
This embodiment is different from the first embodiment only in that the active layer 50 includes a first active layer 51 and a different second active layer 52.
Thus, even when the active layer 50 is composed of a plurality of layers, the turn-on voltage of the threshold control layer 40 is substantially the same as the turn-on voltage of the thin film transistor 1C. Can be controlled.

[Fifth embodiment]
Next, a fifth embodiment of the present invention will be described with reference to the drawings.
Note that the constituent members of the thin film transistor 1D of the present embodiment are the same as those of the first embodiment, and thus detailed description thereof is omitted or simplified.
FIG. 5 is a schematic view showing a thin film transistor according to a fifth embodiment of the present invention.
In the present embodiment, the threshold control layer 40 is changed to a position in contact with the drain electrode 70 and the active layer 50 is different from the first active layer 51 and the second active layer 51 with respect to the first embodiment. The only difference is the layer 52.
Thus, even when the threshold control layer 40 is provided only on the drain electrode 70 side and the active layer 50 is composed of a plurality of layers, the turn-on voltage of the threshold control layer 40 is substantially the same as the turn-on voltage of the thin film transistor 1D. Therefore, the turn-on voltage of the thin film transistor 1D can be controlled by the threshold control layer 40.

[Sixth embodiment]
Next, a sixth embodiment of the present invention will be described with reference to the drawings.
Note that the constituent members of the thin film transistor 1E of this embodiment are the same as those of the first embodiment, and thus detailed description thereof is omitted or simplified.
FIG. 6 is a schematic view showing a thin film transistor in the sixth embodiment of the present invention.
A thin film transistor 1E shown in FIG. 6 has a top gate structure, whereas the thin film transistor 1 of the first embodiment has a bottom gate structure. Between the source electrode 60 and the drain electrode 70, a threshold control layer 40 is provided in contact with the source electrode 60, and an active layer 50 is provided in contact with the threshold control layer 40. The active layer 50 and the drain electrode The threshold control layer 40 is provided in contact with 70, and these are electrically connected.
Thus, even when the thin film transistor 1E has a top gate structure, the turn-on voltage of the threshold control layer 40 is substantially the same as the turn-on voltage of the thin film transistor 1E. Can be controlled.

[Modification of Embodiment]
The present invention is not limited to the above-described embodiments, and modifications, improvements, and the like within the scope that can achieve the object of the present invention are included in the present invention.
For example, the thin film transistor of the present invention can be partially used among thin film transistors mounted on a flat panel display. For example, a thin film transistor formed using only a threshold control layer can be mounted on the pixel portion, and the thin film transistor of the present invention can be used for the peripheral circuit portion.
Thus, in the thin film transistor of the present invention, the present invention may be applied to all thin film transistors, or may be partially applied.
Since the thin film transistor of the present invention includes a threshold control layer, enhancement-type (V _on > 0, n-type) characteristics can be expected. On the other hand, when a transistor is formed using only the active layer, depletion type (V _on <0, n type) characteristics can be expected. For example, in the application of a flat panel display, a thin film transistor may be used in a peripheral circuit other than the pixel portion, but the thin film transistor in the peripheral circuit portion often forms an inverter circuit. The transistor of the present invention can be used for the pixel portion, and a depletion type thin film transistor can be used for a part of the thin film transistors in the peripheral circuit portion.
There are four types of inverters: resistive load, enhancement-enhancement (E / E), enhancement-depletion (E / D), and CMOS (complementary metal oxide semiconductor). From the viewpoint, an E / D configuration or a CMOS configuration is often used. In order to operate the E / D inverter effectively, it is necessary to control the threshold voltage of the TFT and sufficiently increase the difference between the threshold voltages of the two TFTs constituting the inverter. On the other hand, the CMOS inverter requires both an n-channel TFT and a p-channel TFT, and the photolithography process is increased due to these doping processes as compared with other structures.
In addition, the thin film transistor of the present invention is not limited to a transistor for a flat panel display, but can be applied to all metal-oxide-semiconductor field effect transistors (MOS FETs). For example, an electrostatic induction transistor (Static Induction Transistor: SIT), an insulated gate bipolar transistor (Insulated Gate Bipolar Transistor: IGBT), an injection-enhanced insulated gate bipolar transistor (Injection-Enhanced Insulated Gate Transistor Bipolar Transistor, Bipolar Transistor Bipolar Transistor, Bipolar Transistor Bipolar Transistor, There is a FET (Complementary MOS FET: CMOS FET). These can be applied to both horizontal and vertical element configurations.

EXAMPLES Next, although an Example and a comparative example demonstrate this invention further in detail, this invention is not limited at all by these examples.
[Example 1]
A thin film transistor 1 having a bottom gate structure shown in FIG. 1 was produced.
A non-alkali glass substrate (substrate 10) having a diameter of 4 inches was prepared, and Cr was formed by sputtering, and then patterned into a gate wiring shape by photolithography to form a gate electrode 20. Next, this substrate was set in a PE-CVD apparatus, and SiH ₄ , N ₂ O, and N ₂ were introduced to form a gate insulating film (SiO ₂ film) 30.
Next, the substrate 10 with the gate insulating film 30 is mounted on a sputtering apparatus, and an InGaZnO film (In: Ga: Zn = 1: 1: 1 (atomic ratio)) (threshold control layer 40 is formed on the gate insulating film 30. ), And then patterned. Further, after forming an InGaO film (Ga / (In + Ga) = 0.072: atomic ratio) (active layer 50), patterning was performed to form the threshold control layer 40 and the active layer 50. Thereafter, the source electrode 60 and the drain electrode 70 were formed on the substrate 10 with the threshold control layer 40 and the active layer 50, and annealed to obtain the thin film transistor 1.
The obtained thin film transistor 1 was evaluated for field effect mobility (μ), turn-on voltage, on / off ratio, and off-current. These evaluations were carried out using a semiconductor parameter analyzer (Agilent B1500) and measuring the TFT at the center of a 4-inch glass under a dry nitrogen atmosphere at atmospheric pressure at room temperature and in a light-shielding environment. The off current was measured with a gate-source voltage (Vgs) of −30V. As a result, the field effect mobility was 45cm ² / Vs, a turn-on voltage is 0.5V, the on-off ratio is ^{10 12,} the off current was measurement limit of the apparatus ^{(about 10 -15} A). Thus, it was confirmed that the thin film transistor 1 of the present invention has high mobility and good controllability of the turn-on voltage.
In addition, a sample provided with the source electrode 60 and the drain electrode 70 so that the channel portion is only the threshold control layer 40 is manufactured, and the turn-on voltage value of the sample is measured, and the turn-on voltage value of the threshold control layer 40 is measured. V _on1 was measured. As a result, the turn-on voltage value V _on1 was 0.5V. Further, a sample provided with the source electrode 60 and the drain electrode 70 so that the channel portion is only the active layer 50 is manufactured, the turn-on voltage value of the sample is measured, and V _on2 of the turn-on voltage value of the active layer 50 is measured. Was measured. As a result, the turn-on voltage value V _on2 was −7V. Therefore, it was confirmed that the threshold control layer 40 can control the turn-on voltage of the thin film transistor 1.

[Comparative Example 1]
Instead of the structure shown in FIG. 1, a thin film transistor 1F having the structure shown in FIG. 7 was produced. The resistance layer 80 is a layer made of an InGaZnO film (In: Ga: Zn = 1: 1: 1 (atomic ratio)). The active layer 50 is made of an InGaO film (Ga / (In + Ga) = 0.072: atomic ratio).
The thin film transistor 1F obtained was evaluated for field effect mobility (μ), turn-on voltage, on / off ratio, and off-current. As a result, the field effect mobility was 55 cm ² / Vs, the turn-on voltage was −7 V, the on / off ratio was 10 ¹² , which was the measurement limit of the device (about 10 ⁻¹⁵ A).
As is clear from this result, it was found that when the resistance layer is not in contact with the gate insulating film, the turn-on voltage cannot be controlled by this resistance layer.

[Example 2]
Similarly to the thin film transistor 1 shown in FIG. 1, a thin film transistor having a bottom gate structure was manufactured. Specifically, a thin film transistor 1G illustrated in FIG. 8C was manufactured.
A low-resistance n-type crystalline silicon substrate (substrate / gate electrode) 20 on which a gate insulating film 30 made of thermally oxidized silicon is formed is mounted on a sputtering apparatus, and an InGaZnO film (In: Ga: Zn) is formed on the gate insulating film 30. = 5: 1: 4 (atomic ratio)) (active layer 50) was formed and then patterned. Further, after forming an InGaZnO film (In: Ga: Zn = 1: 1: 1 (atomic ratio)) (threshold control layer 40), patterning was performed to form the threshold control layer 40 and the active layer 50. Thereafter, the source electrode 60 and the drain electrode 70 were formed on the low resistance n-type crystalline silicon substrate 20 with the threshold control layer 40 and the active layer 50, and annealed to obtain the thin film transistor 1G.
An optical microscope image of the thin film transistor 1G is shown in FIG. As shown in FIG. 9C, the channel width W / channel length L (W / L) is 50 μm / 20 μm, and an InGaZnO film is formed on the portion from 20 μm of the channel length L to 1 μm from the source electrode side. (In: Ga: Zn = 1: 1: 1 (atomic ratio)) is formed, and an InGaZnO film (In: Ga: Zn = 5: 1: 4 (atomic ratio)) is formed on the portion from the drain electrode side to 19 μm. ) Is formed.
With respect to the obtained thin film transistor 1G, field effect mobility (μ), turn-on voltage, on / off ratio, and off-current were evaluated. These evaluations were carried out using a semiconductor parameter analyzer (Agilent B1500) and measuring the TFT at the center of a 4-inch wafer under a dry nitrogen atmosphere at atmospheric pressure at room temperature and in a light-shielding environment. The off current was measured with a gate-source voltage (Vgs) of −5V. Among the obtained results, a curve indicating the relationship between current and voltage is shown as a curve C in FIG. 10, and a curve showing the relationship between mobility and voltage is shown as a curve C in FIG.
The field effect mobility was 20 cm ² / Vs from the saturation mobility in the curve of C in FIG. From the curve of C in FIG. 10, the turn-on voltage was 2 V, the on / off ratio was 10 ¹⁰ , and the off-current was the measurement limit of the device (about 10 ⁻¹⁴ A). Thus, it was confirmed that the thin film transistor 1G of the present invention has high mobility and good controllability of the turn-on voltage.
In addition, a sample provided with the source electrode 60 and the drain electrode 70 so that the channel portion is only the threshold control layer 40 is manufactured (see FIGS. 8B and 9B). Mobility (μ), turn-on voltage, on / off ratio and off-current were evaluated. Of the obtained results, a curve indicating the relationship between current and voltage is shown as a curve B in FIG. 10, and a curve showing the relationship between mobility and voltage is shown as a curve B in FIG. From the curve of B in FIG. 10, the turn-on voltage value of this sample was measured, and V _on1 of the turn-on voltage value of the threshold control layer 40 was measured. As a result, the turn-on voltage value V _on1 was 3V.
Further, a sample provided with the source electrode 60 and the drain electrode 70 so that the channel portion is only the active layer 50 is manufactured (see FIGS. 8A and 9A). Degree (μ), turn-on voltage, on-off ratio and off-current were evaluated. Among the obtained results, a curve indicating the relationship between current and voltage is shown as a curve A in FIG. 10, and a curve showing the relationship between mobility and voltage is shown as a curve A in FIG. From the curve of A in FIG. 10, the turn-on voltage value of this sample was measured, and V _on2 of the turn-on voltage value of the active layer 50 was measured. As a result, the turn-on voltage value V _on2 was −3V. Therefore, it was confirmed that the threshold control layer 40 can control the turn-on voltage of the thin film transistor 1G.

[Example 3]
A thin film transistor having a bottom gate structure was manufactured in a manner similar to the thin film transistor 1B illustrated in FIG. Specifically, a thin film transistor 1H illustrated in FIG. 8D was manufactured.
A low-resistance n-type crystalline silicon substrate (substrate / gate electrode) 20 on which a gate insulating film 30 made of thermally oxidized silicon is formed is mounted on a sputtering apparatus, and an InGaZnO film (In: Ga: Zn) is formed on the gate insulating film 30. = 5: 1: 4 (atomic ratio)) (active layer 50) was formed and then patterned. Further, after forming an InGaZnO film (In: Ga: Zn = 1: 1: 1 (atomic ratio)) (threshold control layer 40), patterning was performed to form the threshold control layer 40 and the active layer 50. Thereafter, the source electrode 60 and the drain electrode 70 were formed on the low resistance n-type crystalline silicon substrate 20 with the threshold control layer 40 and the active layer 50, and annealed to obtain the thin film transistor 1H.
An optical microscope image of the thin film transistor 1H is shown in FIG. As shown in FIG. 9D, the channel width W / channel length L (W / L) is 50 μm / 20 μm, and the 20 μm channel length L has an InGaZnO film (In: Ga: Zn = 1: 1: 1 (atomic ratio)) is formed, and an InGaZnO film (In: Ga: Zn = 5: 1) is formed on the portion from the source electrode side to 9 μm and the portion from the drain electrode side to 9 μm. : 4 (atomic ratio)).
The obtained thin film transistor 1H was evaluated for field effect mobility (μ), turn-on voltage, on / off ratio, and off-current. These evaluations were carried out using a semiconductor parameter analyzer (Agilent B1500) and measuring the TFT at the center of a 4-inch wafer under a dry nitrogen atmosphere at atmospheric pressure at room temperature and in a light-shielding environment. The off current was measured with a gate-source voltage (Vgs) of −5V. Of the obtained results, a curve indicating the relationship between current and voltage is shown as a curve D in FIG. 10, and a curve showing the relationship between mobility and voltage is shown as a curve D in FIG.
The field effect mobility was 28 cm ² / Vs from the saturation mobility in the curve of D in FIG. From the curve of D in FIG. 10, the turn-on voltage was 2.5 V, the on / off ratio was 10 ¹⁰ , and the off-current was the measurement limit of the device (about 10 ⁻¹⁴ A). Thus, it was confirmed that the thin film transistor 1H of the present invention has high mobility and good controllability of the turn-on voltage.
Further, from the curve in FIG. 10B, the turn-on voltage value V _on1 of the threshold control layer 40 is 3V, and from the curve in FIG. 10A, the turn-on voltage value V _on2 of the active layer 50 is −3V. Therefore, it was confirmed that the threshold control layer 40 can control the turn-on voltage of the thin film transistor 1H.

DESCRIPTION OF SYMBOLS 1 ... Thin-film transistor 10 ... Substrate 20 ... Gate electrode 30 ... Gate insulating film 40 ... Threshold control layer 50 ... Active layer 60 ... Source electrode 70 ... Drain electrode

Claims (15)

A substrate, a gate electrode, a gate insulating film, a threshold control layer, an active layer, a source electrode, and a drain electrode;
The threshold control layer and at least one or more active layers are electrically connected between the source electrode and the drain electrode,
The thin film transistor characterized in that the threshold control layer and the active layer are in contact with the gate insulating film and have the common gate electrode connected to the same potential and separated in a direction perpendicular to the gate insulating film. The thin film transistor according to claim 1, wherein
The threshold control layer and the active layer are connected in series in the horizontal direction between the source electrode and the drain electrode. The thin film transistor according to claim 1 or 2,
When the turn-on voltage value of the threshold control layer is V _on1 and the turn-on voltage value of the active layer is V _on2 ,
When the polarity of the thin film transistor is n-type, the following formula (1n) is satisfied:
When the polarity of the thin film transistor is p-type, the thin film transistor is characterized by satisfying the following mathematical formula (1p).
V _on1 > V _on2 (1n)
V _on1 <V _on2 (1p) The thin film transistor according to claim 3,
When the polarity of the thin film transistor is n-type,
A turn-on voltage value V _on1 of the threshold control layer is 0 V or more;
A thin film transistor, wherein a turn-on voltage value V _on2 of the active layer is less than 0V. The thin film transistor according to claim 3,
When the polarity of the thin film transistor is p-type,
A turn-on voltage value V _on1 of the threshold control layer is 0 V or less;
A thin film transistor, wherein a turn-on voltage value V _on2 of the active layer is more than 0V. The thin film transistor according to any one of claims 1 to 5,
The thin film transistor, wherein the threshold voltage control layer has a higher photovoltage response than the active layer. The thin film transistor according to any one of claims 1 to 6,
The ratio of the length of the threshold control layer to the channel length is 50% or less with respect to 100% of the channel length. In the thin film transistor according to any one of claims 1 to 7,
The threshold control layer includes at least one selected from the group consisting of indium, zinc, tin, gallium, aluminum, titanium, hafnium, zirconium and silicon, and oxygen;
The active layer includes at least one member selected from the group consisting of indium, zinc, tin, gallium, aluminum, titanium, hafnium, zirconium, and silicon, and oxygen. In the thin film transistor according to any one of claims 1 to 7,
The threshold control layer includes oxygen and at least one selected from the group consisting of gallium, aluminum, titanium, hafnium, zirconium, and silicon;
The active layer includes at least one selected from the group consisting of indium, zinc, and tin, and oxygen. In the thin film transistor according to any one of claims 1 to 9,
The thin film transistor, wherein the threshold control layer and the active layer include at least indium. In the thin film transistor according to any one of claims 1 to 7,
The threshold control layer includes at least one selected from the group consisting of gallium, aluminum, titanium, hafnium, zirconium and silicon, indium, and oxygen;
The thin film transistor, wherein the active layer includes at least indium and oxygen. The thin film transistor according to any one of claims 8 to 11,
When the threshold control layer and the active layer include at least indium,
The total ratio of gallium, aluminum, titanium, hafnium, zirconium and silicon in the threshold control layer is 0.4 or less in terms of metal element,
The total ratio of indium, zinc and tin in the active layer is 0.8 or more. The thin film transistor according to any one of claims 1 to 12,
A thin film transistor, wherein the threshold control layer has a carrier concentration of less than 1 × 10 ¹⁵ cm ⁻³ and the active layer has a carrier concentration of 1 × 10 ¹⁵ cm ⁻³ or more. In the thin film transistor according to any one of claims 1 to 9,
The metal composition and composition ratio in the threshold control layer and the active layer are the same,
A thin film transistor, wherein the threshold control layer has a carrier concentration of less than 1 × 10 ¹⁵ cm ⁻³ and the active layer has a carrier concentration of 1 × 10 ¹⁵ cm ⁻³ or more. The thin film transistor according to any one of claims 1 to 14,
The threshold control layer and the active layer are spray method, dipping method, chemical vapor deposition method, sputtering method, vacuum deposition method, ion plating method, pulse laser deposition method, spin coating method, screen printing method, drop cast. The thin film transistor is formed by at least one method selected from the group consisting of a method, a roll coating method, and an inkjet method.