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A BIPOLAR ADDRESSING SCHEME FOR A THIN FILM TRANSISTOR LIQUID CRYSTAL DISPLAY

IP.com Disclosure Number: IPCOM000024907D
Original Publication Date: 1982-Oct-31
Included in the Prior Art Database: 2004-Apr-04
Document File: 2 page(s) / 108K

Publishing Venue

Xerox Disclosure Journal

Abstract

The provision of a thin film transistor (TFT) at each pixel in a liquid crystal display (LCD) is an attractive and viable flat panel scheme. In the simplest form, the TFT functions as an on-off switch to control the electrooptic effect of the liquid crystal medium. In this arrangement, whenever the gate is positively biased with respect to the source, the TFT will act as a closed switch and whenever that same bias is negative the TFT functions essentially as an open switch. Therefore, if the gate-source bias is positive, the maximum voltage across the liquid crystal cell will be equal to the source voltage.

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Page 1 of 2

XEROX DISCLOSURE JOURNAL

POLARITY LOGIC Vg

A BIPOLAR ADDRESSING SCHEME FOR A THIN FILM TRANSISTOR LIQUID CRYSTAL DISPLAY Int. C1. G09f 9/00

Proposed Classification
U.S. Cl. 340/784

ELECTRO-OPTIC EFFECT

K. F. Nelson

                I I POSITIVE POLARITY NEGATIVE POLARITY

I

V. -2 +18v"&-lJ-&K

VS

VC

+'5v*'*

0 v.

IIIIIIIII

Volume 7 Number 5 September/October 1982 309

- Vs TFT STATE V, - Vc

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A BIPOLAR ADDRESSING SCHEME FOR A THIN FILM TRANSISTOR LIQUID CRYSTAL DISPLAY (Cont'd)

The provision of a thin film transistor (TFT) at each pixel in a liquid crystal display (LCD) is an attractive and viable flat panel scheme. In the simplest form, the TFT functions as an on-off switch to control the electrooptic effect of the liquid crystal medium. In this arrangement, whenever the gate is positively biased with respect to the source, the TFT will act as a closed switch and whenever that same bias is negative the TFT functions essentially as an open switch. Therefore, if the gate- source bias is positive, the maximum voltage across the liquid crystal cell will be equal to the source voltage.

At this juncture, the detrimental effects of DC-driven liquid crystal displays are well known. The present practice is, therefore, to periodically reverse the polarity of the voltage applied to the cell. It has heretofore been proposed to accomplish this by merely reversing the polarity of the source voltage in the TFT-LCD once every other frame. Since, with this approach, the liquid crystal cell is actually turned on once every other frame, there is a fifty percent duty factor which, although not dramatically observed, results in a partial llonll condition in some vertical rows of pixels.

Th...