Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
1999-05-27
2001-02-06
Dudek, James A. (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S043000, C349S141000, C349S143000
Reexamination Certificate
active
06184946
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an active matrix type liquid crystal display which has a wide range of viewing angles, and that the power consumption is low.
BACKGROUND ART
An active matrix type liquid crystal display using switching elements such as thin film transistors (TFTS) begins to spread widely as a display terminal of an office automation apparatus, etc. The display type of such a liquid crystal display is roughly divided into the following two kinds. One is a method in which the liquid crystal is sandwiched by two substrates on which transparent electrodes are formed, and it is operated by applying a voltage to the transparent electrodes, and the display is performed by modulating the light penetrated the transparent electrodes and launched into the liquid crystal. The other is a method in which the liquid crystal is sandwiched by two substrates on which transparent electrodes are formed, and it is operated by applying an electric field substantially parallel to one substrate and between two electrodes (a pixel electrode and an opposed electrode) formed on the substrate, and the display is performed by modulating the light incident from the space of one electrode into the liquid crystal. Because this method has features such as a wide range of viewing angles and low load capacitance, etc, it is a promising technology concerning the active matrix type liquid crystal display. Hereinafter, the latter method is called a horizontal electric field method.
While the horizontal electric field method has the above-mentioned features, there is a problem that it is necessary to use a bright back light of which electric power consumption is large, because opaque electrodes are formed like comb teeth. It is because the frontage area through which light can be penetrated is small and thus display screen is dark.
On the other hand, the same applicant has proposed one method in the Japanese Patent Application No. 6-199247, in which the role of the common electrode wiring of supplying an external voltage to the common electrode is given to the scanning electrode wiring, the common electrode wiring is eliminated, and thus the frontage area in the horizontal electric field method is enlarged. Hereinafter, the above-mentioned method technology will be called horizontal electric field method having no common-electrode-wiring (common-less horizontal electric field method).
In the common-less horizontal electric field method, it is required that the thin film transistor or the switching element indicates fully the enhancement type switching characteristic that the threshold voltage is higher than the maximum voltage of the liquid crystal operating voltage required to modulate optically the liquid crystal. As a method of making the thin film transistor which shows the enhancement type switching characteristic, a method of making a thin film of the amorphous silicon semiconductor layer or a method of tLr controlling the voltage of the back electrode provided at a position opposed to the gate scanning electrode of the thin film transistor has been described in the above Japanese Patent Application No. 6-199247.
However, in these methods, it is necessary to reduce enough the difference of threshold voltages of the THIN FILM TRANSISTOR. There is a problem that the display quality of the active matrix type liquid crystal display having no common-electrode-wiring is low.
An object of the present invention is to provide an active matrix type liquid crystal display having no opposed-electrode-wiring of the horizontal electric field method, in which the difference of threshold voltages are reduced, thin film transistors with simple structure are used, and the picture resolution is improved.
DISCLOSURE OF INVENTION
The thin film transistors used as the switching elements of the active matrix type liquid crystal display of the present invention is of an enhancement type, and has a silicon nitride formed on a scanning electrode, an insulating layer formed on the silicon nitride film, and a semiconductor layer having a source area and a drain area each formed on the insulating layer.
The threshold voltage of this thin film transistor is larger than the maximum value of the liquid crystal operating voltage. In a preferred embodiment, the threshold voltage is 10V or more.
As the concrete example of the insulating layer formed on the silicon nitride film, there is an silicon oxide film, and as for the thickness of the insulating layer, 30 Å or more is desirable.
A semiconductor layer where a source electrode and a drain electrode were connected through the electric contact layer is formed on the insulating layer.
In the present invention, the horizontal electric field method is adopted, and an electric field parallel to the substrate is generated in the liquid crystal layer by the voltage applied to an opposed electrode corresponding to a pixel electrode.
The transistor structure used in the present invention is a thin film memory transistor structure which is called MNOS (Metal Nitride Oxide Semiconductor) structure.
In the present invention, the threshold voltage of the thin film transistor is controlled as follows. A positive threshold control voltage very higher than the liquid crystal operating voltage (before and behind ±10V), is applied to the gate electrode, with the drain electrode, or the drain electrode and the source electrode grounded. Further, when the thin film transistor of the present invention is arranged like a matrix at the intersection of the scanning (gate) electrode wiring and signal (drain) electrode wiring in the active matrix type liquid crystal display, the above-mentioned threshold control voltage is individually applied between the gate/drain electrodes of each thin film transistor, for example, by using the line-sequential drive method so that the threshold voltage of each thin film transistor may become equal. In this case, the homogeneity of the display quality is secured by controlling while monitoring the brightness distribution of the liquid crystal display in order to decrease the differences of threshold voltages.
When the threshold control voltage is applied as mentioned above, the electrons in the semiconductor layer pierces through the silicon oxide film and are trapped in the silicon nitride film because the gate electrode becomes a positive voltage with respect to the drain electrode. The threshold voltage of the thin film transistor shifts to the plus side by the action of the electrons trapped in the silicon nitride film. Therefore, the thin film transistor comes to show the characteristic of the enhancement type. An amount of the electrons trapped in the silicon nitride film increases if the threshold control voltage is increased, and threshold voltage increases. Therefore, the threshold voltage of each thin film transistor can be shifted to the same value by individually adjusting the threshold control voltage applied to each thin film transistor. As a result, the differences of threshold voltages can be greatly decreased.
Further, since the silicon oxide film thickness is 30A in the embodiment of the thin film transistor of the present invention, the threshold value is not changed during the operation of display when using the thin film transistor for a horizontal electric field type switching element having no common-electrode-wiring. Therefore, the characteristic of the stable enhancement type can be provided. This operation will be explained in detail hereinafter.
In general, the electrons in the silicon nitride film pierced through the silicon oxide film is discharged in the semiconductor layer, or the positive hole in the semiconductor layer pierced through the silicon oxide film is trapped in the silicon nitride film when the voltage reversed in its polarity to the above-mentioned is applied between the gate/drain electrodes of the thin film transistor of the MNOS structure so that the gate electrode may become negative. Therefore, the threshold voltage is shifted to a minus side. The voltage with both polarities and approx
Ando Masahiko
Fukaya Ritsuo
Wakagi Masatoshi
Antonelli Terry Stout & Kraus LLP
Dudek James A.
Hitachi , Ltd.
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