JPH0229633A - Thin film transistor - Google Patents

Abstract

PURPOSE:To allow the control of a threshold voltage by addition of a small amt. of an impurity such as boron by using the two-layered structure consisting of an i a-Si:H layer and microcrystal Si added with the impurity as an active layer. CONSTITUTION:The active layer 26 of the thin film transistor is made of the two-layered structure laminated with the i a-Si:H 24 which is positioned on a gate electrode 22 side and has 10-500Angstrom thickness and the microcrystal Si layer 25 which is positioned on a source electrode 29 and drain electrode 30 side and has 50-1,000Angstrom thickness. Namely, the microcrystal Si is the intermediate product of amorphous silicon and polycrystalline Si and has the crystal grains of the Si smaller than the crystal grains of the polycrystalline Si. The activation rate of the boron added into the microcrystal Si is higher than the activation rate of the boron added to the a-Si:H. The desired threshold voltage is controllable with the amt. of said boron to be added smaller than the amt. of the boron to be added to the a-Si:H.

Description

[Detailed Description of the Invention] [Summary] Regarding thin film transistors using amorphous silicon suitable for active matrix display devices, etc., the control width of the gate voltage for driving the thin film transistor is wide (the operating margin of the transistor is In a thin film transistor in which a gate electrode is disposed on one of two opposing main surfaces of the active layer with a gate insulating film interposed therebetween, and a source electrode and a drain electrode are disposed on the other side, the active layer has a large thickness. , 10 Å located on the gate insulating film side
It is formed of a laminated film of an intrinsic amorphous silicon layer with a thickness of 500 Å and a microcrystalline silicon layer with a thickness of 50 Å to 1000 Å and doped with impurity atoms located on the source and drain electrode sides. do.

(Industrial Application Field) The present invention relates to a thin film transistor using amorphous silicon.

With the spread of office automation equipment, there is a demand for compact, flat, high-quality, large-sized liquid crystal display panels.

Such liquid crystal display panels include a simple matrix structure in which liquid crystal is sandwiched between opposing electrodes, and an active matrix drive liquid crystal display panel in which each pixel is provided with a switching element such as a thin film transistor to drive the pixel. The former type of liquid crystal display panel has a high yield and can be manufactured at a low cost due to a simple process, but it is difficult to form a full-color panel, and the latter type of liquid crystal display panel can achieve full color, but the entire surface of a large-area panel is difficult to form. It is difficult to form a thin film transistor with uniform characteristics over the entire area, and therefore a thin film transistor with uniform characteristics with good yield is desired.

[Conventional technology]

As an active matrix type liquid crystal panel using such a thin film transistor, the present applicant has previously published Japanese Patent Application Laid-Open No. 63-6
In Japanese Patent No. 8818, a liquid crystal display panel as shown in FIGS. 4 and 5 is proposed.

FIG. 4 is an exploded view of this liquid crystal display panel, and FIG. 5 is an equivalent circuit diagram of the panel shown in FIG.

As shown in FIGS. 4 and 5, a first glass substrate 1
A thin film transistor 2, one electrode 4 of a liquid crystal display element 3, and a scan path line 5 are formed on the second glass substrate 6, and a striped data bus line 7 is formed as the other electrode of the display element 3 on a second glass substrate 6. Then, the gate electrode 8 of the thin film transistor 2 is connected to the scan path line 5, and a liquid crystal is sealed between the first glass substrate l and the second glass substrate 6 to form a liquid crystal display panel with an active matrix structure. There is.

By the way, the characteristics of a thin film transistor used in a liquid crystal display panel having such a structure are such that the off-state current value of the drain current is small when a voltage is applied to the gate electrode, so that the switching characteristics of the element are good. Thin film transistors are required to have characteristics that maintain a large allowable dark value voltage on the positive voltage side.

Therefore, the structure of a thin film transistor that satisfies the above characteristics is as shown in FIG.
silicon nitride (5iN) is formed on the gate electrode 12.
) such as an intrinsic hydrogen-doped amorphous silicon layer (hereinafter referred to as the a-5t:H layer) and an a-St layer doped with boron (B).
: A contact layer consisting of an N''a-3isH layer on which an active layer 16 with a two-layer structure of an H layer 15 is formed and N-type impurity atoms such as phosphorus (P) or arsenic (As) are added thereto. A thin film transistor is formed of a source electrode 19 and a drain electrode 20, which are formed by dividing a laminated structure of an ohmic electrode 18 made of 17 and titanium (Ti) or chromium (Cr) into a predetermined pattern.

[Problem to be solved by the invention]

By the way, the second layer of the active layer 16 is a-
The activation rate of boron in the 3tsH layer 15 (the efficiency with which the added boron controls the amount of charge fixed in the space charge region 31 of the active layer shown in FIG. 3) is poor, and a large amount of boron is added more than necessary. There are issues that require this.

However, a-5i:
When added to H, defects such as dangling bonds occur in the a-5isH layer, and these defects cause a small change in drain current (dI) with respect to a small change in gate voltage of the thin film transistor.
*/dV*), resulting in deterioration of the switching characteristics of the device.

The present invention solves the above problems and improves the dID/dV of the device.
*The operation margin of the device can be increased by controlling the dark value of the thin film transistor to a desired value on the positive voltage side by adding impurities without causing a decrease in the value of (that is, without causing a decrease in the switching characteristics of the device). The purpose is to provide a thin film transistor that can be made large.

[Means to solve the problem]

In the present invention, as shown in FIG. 1, as the active layer 26 of the thin film transistor, an ia-Si: 8 layer 24 with a thickness of 10 to 500 layers located on the gate electrode 22 side, a source electrode 29 and a drain electrode 30 are used. Thickness located on the side is 50~
A two-layer structure is adopted in which a microcrystalline Si layer 25 doped with impurities such as 1,000 yen of boron is laminated.

[For production]

In the present invention, as described above, the active layer of the thin film transistor is formed with a two-layer structure of an intrinsic 1a-5isH layer to which no impurity atoms are added and microcrystalline Si to which boron is added.

This microcrystalline Si is an intermediate between amorphous silicon and polycrystalline Si, and has Si crystal grains smaller than the crystal grains of polycrystalline Si, and the activation rate of boron added to this microcrystalline Si is a- It is possible to control the desired threshold voltage with an addition amount that is larger than the activation rate of boron added to Si=H (, and smaller than the addition amount of boron added to a-St:H. Therefore, as in the conventional case, a-5i:
By doping a large amount of boron into the 8th layer, a-St:
This eliminates the problem of defects occurring in the l'1 layer, which would cause a decrease in the values of dl, /dV of the thin film transistor, and provides a thin film transistor with good switching characteristics.

The reason why the dark voltage is shifted by boron doping into the microcrystals 5i25 of the active layer 26 is due to the third
This is because, in the energy band diagram of FIG. 3A, acceptors in the space charge region 31 on the active layer side are ionized and become fixed charges.

Therefore, at least the channel forming region of the interface between the gate insulating film 23 and the active layer 26 can be
By using microcrystal St formed using a-Si:8 layer 24 and doped with boron in the remaining space charge region 31, the dark value voltage can be controlled without reducing the mobility of the active layer. .

FIG. 4(c) is an energy band diagram of a thin film transistor in which such an ia-5t:8 layer 24 and boron-doped microcrystals 5i 25 are laminated.

31 in the figure is a space charge region, and the interface with the gate insulating film 23 is formed using an ia-Si:8 layer 24 to prevent a decrease in mobility, and a doped microcrystalline Si layer. The darkness value is controlled by forming 25 on the ia-3i:11 layer.

The hatched area in the figure is the microcrystalline Si layer 25 doped with boron, and as shown in this figure, the i a-5i:8 layer 24 is bent by the microcrystalline Si layer 25 to control the dark value. becomes possible.

The thickness of the space charge region 31 of the thin film transistor is approximately 3
000 people, but if you take into account the generation of photocurrent, it will be 1000 people.
It is desirable that the microcrystalline Si layer 25 doped with boron has a thickness of Å or less, and in order for the boron-doped microcrystalline Si layer 25 to contribute to threshold control, it must exist within approximately 1000 nm from the interface of the gate insulating film 23. Therefore, the thickness of the microcrystalline Si layer 25 doped with boron is at most 1000 Å.
The following must be true.

On the other hand, if the formed thin film is too thin, it will be difficult to form a -like film, and the practical lower limit is about 50 people.

As described above, according to the structure of the present invention, the gate insulating film 2
Since the interface between 3 and the active layer 26 is an ia-3i:8 layer 24, the mobility does not decrease, and the boron-doped microcrystalline Si layer 25 does not touch the gate insulating film 23, so it affects the mobility. not give. Therefore doped microcrystal SiJ! The doping concentration of 25 can be arbitrarily selected, and the dark value voltage can be selected as desired.

〔Example〕

FIG. 1 is a sectional view of a main part showing the structure of an inverted staggered thin film transistor according to an embodiment of the present invention.

In the figure, reference numeral 21 denotes a transparent glass substrate, on which a gate electrode 22 made of titanium (Ti) with a thickness of, for example, about 700 mm is formed. A gate insulating film 23 made of silicon nitride (5iN) film is formed. Further, on this gate oxide film 23, an ia-Si:8 layer 24 is formed to a thickness of about 100 layers by plasma CVD, and on top of this, a microcrystal Si layer 24 doped with boron is formed to a thickness of about 200 layers.
Layer 25 is formed by plasma CVD using silane (SiHa) gas as a reactive gas.

This method of adding boron is, for example, diborane (BJh
) can be easily formed by plasma CVD by adding doping gas such as ) to silane (5iH4) gas at a predetermined concentration.

In this way, the active layer 26 has a two-layer structure of the i a-St:1 layer 24 and the boron-doped microcrystal S1 layer 25.
is formed. Then, on the boron-doped microcrystal S1 layer 25, a contact layer 27 made of an N''a-5isH layer doped with an N-type impurity of phosphorus or arsenic is formed by a plasma CVD method, and is made of a titanium or chromium film. The ohmic electrode 28 is formed by sputtering or vapor deposition, and the contact layer 27 and the ohmic electrode 28 are formed in a predetermined pattern to form the source electrode 2.
9 and a drain electrode 30 are formed.

The gate voltage versus drain current characteristics of the thin film transistor of this example formed in this manner will be explained with reference to FIG. 2 while comparing and examining the characteristics of a conventional thin film transistor.

Curve 41 in the figure shows the active layer with a thickness of 300 people i a-3ts
Curve 42, a characteristic curve of a conventional thin film transistor formed with an H layer, shows the active layer having a thickness of 100 mm i a-3t:H
layer and thickness 200 people boron doped ia-3tzH
Curve 43, which is a characteristic curve of a conventional thin film transistor formed with a two-layer structure, shows that the active layer has a thickness of 100 nm.
1 is a characteristic curve of a thin film transistor of the present invention formed with a two-layer structure of an sH layer and a microcrystalline SiN doped with boron of 200 nm thick.

As shown by curve 41 in the figure, the active layer is i a-5isH
In a conventional thin film transistor formed with a single layer structure, if the threshold voltage Vth is a current value of la=10-', then V=-2v.

Further, as shown by curve 42 in the figure, the active layer is ia-Si
: H layer and boron i a-3i: 40 ppm relative to H
Added i a-Si:Hli (in case of D two-layer structure,
The value of vtk is IV, and the active layer is i a-5ts
It can be seen that by forming the H layer and the boron-doped ia-Si=H layer into a two-layer structure, the gate voltage was shifted to the positive side by about 3V.

However, in order to further shift the value of Vth of the conventional thin film transistor exhibiting the characteristic of this curve 42 to the positive side, i
If the doping amount of boron to a-3t:H is increased and 80 ppm of boron is added to ia-Si:H, V = 3V, but this will cause a decrease in the value of dI D / dV. , the switching characteristics of the device deteriorate.

Curve 43 shows the active layer i a-5tsH layer and boron 40
In the characteristic curve of the thin film transistor of the present invention, which has a two-layer structure of microcrystalline Si layers doped with ppm, when boron is added to the microcrystals, i a-3
Since the activation rate of boron is better than when boron is added to t:H, Vth
=3V. Even when shifting the Vtk value to the positive side of 3V, the amount of boron added to Si is less than 20 pp-, and there is no need to add 40 pp- to Si as in the past. When Vtk is set to about 3V, there is no decrease in the value of dl/dV*, and the switching characteristics of the device are improved compared to conventional devices.

In the above example, the i6-St:H layer is of N type, so the 21st layer is explained using an example of a thin film transistor doped with boron (B), but the i6-St:H layer of the first layer is
When the a-3isH layer is of P type, an N-type impurity such as arsenic or phosphorus may be added to the microcrystal Si, and the contact layer may be formed of an a-3i:H layer doped with boron.

Further, in this embodiment, the a-3i:1 layer 24 and the microcrystalline Si layer 25 doped with boron are used as the gate insulating film 23.
Although an example in which they are formed over the entire surface has been described, it is sufficient that both of the above two layers are provided in a region directly above the gate electrode 22 where a channel is formed.

Further, in this embodiment, an example in which an inverted staggered thin film transistor is formed has been described, but the present invention can also be applied to a staggered thin film transistor.

〔Effect of the invention〕

As is clear from the above description, according to the present invention, by using a two-layer structure of an ia-5i:H layer and microcrystalline Si doped with impurities such as boron in the active layer, dark It becomes possible to control the value voltage, and therefore, crystal defects that occur in the active layer due to diffusion of impurities into t a-3i:H, etc., do not occur.
The dark value voltage can be shifted to a desired value without causing a decrease in the value of dl, /dV.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of a thin film transistor of the present invention, FIG. 2 is a characteristic diagram of a thin film transistor of the present invention, FIG. 3 is an explanatory diagram of energy bands of a thin film transistor, and FIG. 4 is an exploded view of an active matrix liquid crystal display panel. FIG. 5 is an equivalent circuit diagram of an active matrix liquid crystal display panel, and FIG. 6 is a configuration diagram of a conventional thin film transistor. In the figure, 21 is a glass substrate, 22 is a gate electrode, 23 is a gate insulating film, 24 is an ia-5i:H layer, 25 is microcrystalline Si doped with boron, 26 is an active layer, 27 is a contact layer, 28 29 is an ohmic electrode, 29 is a source electrode, 30 is a drain electrode, 31 is a space charge region, 41.
42 and 43 show characteristic curves of thin film transistors. Agent: Patent Attorney Sada Iji - Npl Tra> Dista ψ Engineering Machine F's Circumcision 1 Changen Figure 3 Gate electrician, 4 in the θth position, 1, 1, 2, 7, 1, 1, 2, 7, 7, 91, an exploded view of the main cell, Figure 4, Aku knife matrix C (displayed in a roaring manner). Ki J Shino Ime ω Work 45] Figure 5

Claims (1)

[Claims] A gate electrode (22) is provided on one of the two opposing main surfaces of the active layer (26) via a gate insulating film (23), and a source electrode (29) and a drain electrode (30) are provided on the other side. In the thin film transistor, the active layer (26) includes an intrinsic amorphous silicon layer (24) with a thickness of 10 Å to 500 Å located on the gate insulating film (23) side, the source electrode (29), 5 doped with impurity atoms located on the drain electrode (30) side
A thin film transistor characterized in that it is formed of a laminated film including a microcrystalline silicon layer (25) with a thickness of 0 Å to 1000 Å.