Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Patent
1994-12-23
1996-10-08
Wojciechowicz, Edward
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
257 52, 257 57, 257294, 257347, 257384, 257388, 257401, 257412, 257659, 359 59, H01C 31062, G02F 11343
Patent
active
055634320
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a thin-film transistor ("TFT") and a display device such as an active matrix type liquid crystal display device, using the same.
BACKGROUND ART
TFTs are widely used as switching elements in drive circuits of various types of devices. For example, in an active matrix type liquid crystal display device, each display pixel is selected by a TFT.
Generally, a TFT has a source electrode and a drain electrode formed to be separated from each other, a semiconductor film formed to electrically contact these electrodes, a gate insulating film formed on the semiconductor film, and a gate electrode formed on the gate insulating film. TFTs of this type can be classified into a staggered type TFT in which a semiconductor film, a gate insulating film, and a gate electrode are sequentially formed on the source and drain electrodes, and an inverted staggered type TFT in which a gate insulating film, a semiconductor film, and source and drain electrodes are sequentially formed on a gate electrode.
In recent years, an increase in ON/OFF current ratio is requested in order to render a TFT operable in a higher frequency range. To achieve an increase in the ON/OFF current ratio, the length of a channel corresponding to the semiconductor film portion between the source and drain electrodes has been decreased. When the semiconductor film of the TFT is made of a silicide semiconductor represented by amorphous silicon (a-Si:H), photocarriers are generated in the semiconductor film upon incidence of light. Although a decrease in channel length can increase an ON current Ion, it facilitates movement of the photocarriers, thereby undesirably increasing an OFF current, i.e., a photoleakage current Ioff. As a result, the ON/OFF current ratio is not improved as expected.
Especially, in an active matrix type liquid crystal display device, generation of the photoleakage current Ioff directly causes variations in potential of the pixel electrodes, leading to a degradation in visual quality of the display image.
Therefore, conventionally, countermeasures have been taken such as formation of a light-shielding layer made of a metal material, e.g., chromium (Cr), on a counter substrate which is arranged to oppose an array substrate on which a plurality of pixel electrodes are formed. However, this cannot prevent the photoleakage current Ioff which is caused by the photocarrier generated in the semiconductor layer of the TFT upon incidence of light reflected by the surface of the light-shielding layer.
According to another countermeasure, for example, a light-shielding layer adjacent to the upper or lower portion of the TFT may be provided on the array substrate. However, this countermeasure cannot essentially decrease the photoleakage current Ioff sufficiently.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above situations, and has as its object to provide a TFT which can reduce the photoleakage current Ioff upon incidence of light.
It is another object of the present invention to provide a display device which does not cause defective display due to the photoleakage current Ioff of the TFT.
It is still another object of the present invention to provide a TFT in which the photoleakage current Ioff can be reduced and a parasitic capacitance Cgs generated in the gate-source path or parasitic capacitance Cgd generated in the gate-drain path of the TFT can be reduced.
It is still another object of the present invention to provide a display device which can reduce a potential drop .DELTA.Vp in pixel electrode potential at an instance during which the TFT is changed from an ON state to an OFF state due to the parasitic capacitance Cgs of the TFT and also reduce the optical leakage current Ioff, so that display defect, e.g., flicker, will not be caused.
According to the present invention, there is provided a thin-film transistor comprising a gate electrode arranged on an insulating substrate, and source and drain electrodes formed on the gate electrode through at lea
REFERENCES:
Patent Abstracts of Japan, vol. 16, No. 316 (E-1231) Jul. 10, 1992 & JP-A-04 088 641 (Toshiba Corp) Mar. 23, 1992.
Patent Abstracts of Japan, vol. 13, No. 268 (E-775) Jun. 20, 1989 & JP-A-01 059 863 (Matsushita Electric Ind Co. Ltd) Mar. 7, 1989.
Akiyama et al, "An a-Si TFT With a New Light-Shield Structure And Its Application to Active-Matrix Liquid Crystal Displays", International Electron Devices Meeting 1988, San Francisco, CA, Dec. 11-14, 1988, pp. 268-271.
Miura Yasunori
Seiki Masahiro
Shibusawa Makoto
Sugahara Atsushi
Kabushiki Kaisha Toshiba
Wojciechowicz Edward
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