JPH04360583A - Thin film transistor - Google Patents

Abstract

PURPOSE:To prevent an increase in an OFF current due to back light at the time of using a thin film transistor having electrodes of a comblike structure, as a pixel switch of a liquid crystal display without complicated structure. CONSTITUTION:In a thin film transistor having a gate electrode 2 formed on an insulating board 1, a gate insulating film 3, a semiconductor layer 4, and source, drain electrodes made of ohmic layer 5 and metal layer 6 of a comblike structure, the comb teeth of the source/drain electrode do not cross the electrode 2, and the teeth edges are disposed on the electrode 2. Even if an electric resistance of the semiconductor layer except the gate electrode is reduced due to back light, if the edges of the teeth of the source/drain electrodes are disposed on the gate electrode, a leakage current at the time of OFF is not increased.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to thin film transistors (hereinafter referred to as T) used in active matrix liquid crystal display devices.
Abbreviated as FT. ).

0002

[Prior Art] In recent years, so-called active matrix liquid crystals have been developed in which thin film transistors (hereinafter abbreviated as TFTs) are formed on insulating substrates such as glass, and are used to drive liquid crystals by controlling the voltage applied to each pixel. Research and development regarding displays (hereinafter abbreviated as LCD) are actively being conducted.

FIG. 2 shows a cross-sectional structure of an inverted staggered TFT that is most commonly applied to active matrix LCDs. In FIG. 2, 11 is an insulating substrate such as glass;
2 is Cr or Ta, or Ta or A with anodized surface
13 is a gate insulating film layer, 14 is a channel layer made of a semiconductor such as amorphous silicon, 15 is an ohmic contact layer, 16 is a source or drain electrode, and 17 is a pixel electrode. It is a transparent conductive film. In this inverted staggered TFT, the gate insulating layer 13 and the channel layer 14 made of a semiconductor can be successively deposited, and a relatively high-quality interface can be easily obtained, and as shown in FIG. Since the numeral 12 serves as a light-shielding layer for the backlight, there is an advantage that there is no need to form a light-shielding layer separately. For this reason, an inverted staggered TF
T is known as the most common TFT structure. However, when applied to an LCD, this inverted staggered TFT has a disadvantage in that tolerance for misalignment is small and it is difficult to produce a large-area LCD with a high yield. In addition, in order to increase the alignment tolerance, there is a method of increasing the overlap between the source/drain electrode and the gate electrode, but in this case, the TF
Since the parasitic capacitance of T becomes large, it is still difficult to realize a high-definition, large-area LCD.

On the other hand, a comb-shaped electrode TF is used as a technology to provide redundancy against misalignment without increasing parasitic capacitance and to fabricate TFTs with uniformly high characteristics on a large-area substrate.
There is T (Japanese Unexamined Patent Publication No. 2-27567). Figure 3(a)
shows a top view of this comb-shaped electrode TFT, and FIG. 3(b) shows its cross-sectional structure. In Fig. 3, 21 is an insulating substrate such as glass, and 22 is Cr, Ta, or T whose surface is anodized.
23 is a gate insulating film, 24 is a channel layer made of semiconductor, 25 is an ohmic contact layer, and 26 is a source or drain electrode.

[0005]

[Problems to be Solved by the Invention] However, in this comb-shaped electrode TFT, since the source/drain electrode layer 26 completely crosses the gate electrode 22, the portion of the gate electrode 22 on the same side where the gate electrode 22 is not , since the source electrode and the drain electrode are in contact with the semiconductor layer stacked directly on the substrate 21, when applied to an LCD and backlight is incident from the gate electrode layer side, the source electrode and the drain electrode The drawback is that light hits the semiconductor layer between them, increasing the amount of current when they are off. Further, in order to solve this problem, there are measures such as providing a light shielding layer separately, but this results in a problem that the process becomes complicated and the yield decreases.

[0006] The present invention takes advantage of the advantages of the comb-shaped electrode TFT, which has high alignment redundancy and can create a TFT with uniform characteristics over a large area, and even when applied to an LCD, it is possible to avoid light incident from the gate electrode layer side. The present invention provides a TFT that does not require a light shielding layer for a backlight.

[0007]

[Means for Solving the Problems] The present invention provides an inverse structure in which source/drain electrodes consisting of a gate electrode layer, a gate insulating film, a semiconductor layer, a pair of ohmic layers, and a metal electrode layer are sequentially laminated on an insulating substrate. In the staggered stacked structure thin film transistor, each of the pair of source/drain electrodes has a comb-shaped structure having a plurality of teeth, the teeth are combined on the semiconductor in a non-contact state, and the tips of the teeth are It is characterized by being located on the semiconductor layer above the gate electrode. The present invention also provides a staggered laminated structure thin film transistor in which a pair of source/drain electrodes, a semiconductor layer, a gate insulating film, and a gate electrode layer, each consisting of a metal electrode layer and an ohmic layer, are laminated in this order on an insulating substrate. Each of the source/drain electrode layers has a comb-shaped structure having a plurality of teeth, and the teeth are combined on the semiconductor in a non-contact state, and the tips of the teeth are placed under the semiconductor layer below the gate electrode. It is characterized by certain things.

As the semiconductor layer of the present invention, it is particularly effective to use amorphous silicon, which has high photosensitivity, as a semiconductor thin film that can be formed on a transparent insulating substrate by thin film deposition technology. Naturally, this is also effective when using polycrystalline silicon with a small .

The present invention is also applicable to thin film transistors in which the semiconductor layer is an amorphous silicon layer doped with impurities such as boron or phosphorus to control electrical conduction, or in which carbon is added to control the band gap. Of course, this method is also effective in the case of a thin film transistor which is an amorphous silicon layer mixed with any impurity selected from the group consisting of nitrogen, oxygen, and germanium.

0010

[Operation] In the thin film transistor of the present invention, each of the source and drain electrodes composed of an ohmic electrode layer and a metal layer has a comb-shaped structure having a plurality of teeth,
The teeth are assembled on the semiconductor layer in a non-contact state, and the tips of the teeth are on the semiconductor layer above the gate electrode. In the thin film transistor of the present invention, the backlight is arranged so that it enters from the gate electrode layer side. If the tips of the comb teeth of the source/drain comb-shaped electrodes are placed on the semiconductor layer above the gate electrode, even if the backlight that enters from the gate electrode layer side reduces the electrical resistance of the part of the semiconductor layer that is illuminated by the light, Unlike ordinary comb-shaped electrode thin film transistors, leakage current does not flow between the source and drain electrodes due to photoconduction in the semiconductor layer, and the off-state characteristics of the transistor do not deteriorate. Therefore, there is no need to provide a special light-shielding layer on the gate electrode layer side, and the process can be simplified while taking advantage of the advantages of the comb-shaped electrode thin film transistor.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention will be explained below with reference to illustrated embodiments. FIG. 1 shows an example in which the present invention is applied to an inverted staggered comb-shaped electrode TFT. Figure 1(a) shows the TFT of the present invention.
FIG. 1(b) is a top view of the structure of FIG. In FIG. 1, 1 is a Corning 7059 glass substrate, 2 is a gate electrode layer formed by wet etching a Cr film formed by electron beam evaporation, 3 is a gate insulating film made of a silicon nitride film made by plasma CVD method, 4 5 is a semiconductor channel layer made of a-Si:H also created by plasma CVD, and 5 is an ohmic contact layer made of n + type a-Si:H also created by plasma CVD.
Reference numeral 6 denotes source and drain electrode layers formed by electron beam evaporation and wet etching in the same manner as 2. Film formation by plasma CVD from 3 to 5 in FIG. 1 was performed continuously using a multi-chamber plasma CVD apparatus without breaking the vacuum in order to prevent deterioration of the interface. In this example, the width of the comb teeth and the interval between the comb teeth are both 10 μm, and the number of comb teeth is 10 on each side, totaling 20.
I made it into a book.

FIGS. 4(a) and 4(b) show a comb-shaped electrode TFT according to the present invention and a conventional comb-shaped electrode TFT described in ``JP-A-2-27567'' as a backlight from the gate electrode layer side. ] I-V when the light is incident
Show characteristics. In the ordinary comb-shaped electrode TFT shown in FIG. 4(b),
The backlight increases the off-state current and reduces the on-off ratio to 1.
However, in the comb-shaped electrode TFT of the present invention, the characteristics are insufficient as a TFT for LCD.
It can be seen that there is almost no influence from the backlight, and the on-to-off ratio is more than 5 digits, indicating that it has sufficient characteristics as a TFT for LCD.

[0013]

Effects of the Invention The present invention utilizes the advantages of the comb-shaped electrode TFT described in ``Japanese Unexamined Patent Publication No. 2-275672'' in which the source and drain electrodes in the TFT have a comb-shaped structure, while making the comb teeth cross the gate electrode. This prevents deterioration of off-characteristics due to backlight, which is a problem when comb-shaped TFTs are applied to LCDs, without providing a special light-shielding layer. This makes it possible to simplify the manufacturing process of an active matrix type liquid crystal display device and further improve the yield.

[Brief explanation of drawings]

FIG. 1 shows the structure of the new comb-shaped electrode TFT of the present invention, (a)
is its top view, and (b) is its cross-sectional view.

FIG. 2 is a cross-sectional view of a normal inverted staggered TFT when applied to an LCD.

[Figure 3] Ordinary comb-shaped electrode TFT (Japanese Patent Application Laid-Open No. 2-27567
(a) is a top view thereof, and (b) is a sectional view thereof.

FIG. 4(a) shows the IV characteristics of the comb-shaped electrode TFT of the present invention when illuminated with a backlight of 750 [lx];
b) is the IV characteristic of a normal comb-shaped electrode TFT when illuminated with a backlight of 750 [lx].

[Explanation of symbols]

1 Glass substrate 2 Gate electrode layer 3 Gate insulating film 4 Channel layer 5 Ohmic contact layer

Claims (1)

[Claims] 1. A semiconductor layer made of amorphous silicon, a pair of source/drain electrode layers connected to the surface of the semiconductor layer across an ohmic layer, a gate insulating layer formed in contact with the surface of the semiconductor layer, and a gate electrode layer formed in contact with the surface of the gate insulating layer opposite to the surface in contact with the semiconductor layer and maintained insulated from the semiconductor layer by the gate insulating layer, which is stacked on an insulating substrate. In the insulated gate field effect thin film transistor, the pair of sources and
Each of the drain electrode layers has a comb-shaped structure having a plurality of teeth, and the comb teeth of the pair of source/drain electrode layers are combined on the semiconductor in a non-contact state, and the comb teeth are connected to the gate electrode. placed perpendicular to
A thin film transistor characterized in that the tips of the teeth are located on a semiconductor layer above a gate electrode layer.