Liquid crystal display threshold devices

Fig. S. Matrix-addressed liquid crystal display using an FET as an extrinsic threshold device. The capacitor (dashed) is optional.

LIQUID CRYSTALS are widely used in devices for flat-panel electronic displays and other applications. The most usual form of device is one in which a thin film, 5-10 /um thick, of liquid crystal is confined between glass plates bearing transparent electrodes that are used to induce an optical change in selected portions of the film by application of an above-threshold voltage. Liquid crystal displays with color and video display capabilities equal to those of the cathode ray tube are now being developed. 

The simplest way to overcome the scanning limitations encountered in passive matrix liquid crystal display panels is to provide one or more nonlinear circuit elements at the intersection of the gate and data electrodes, thus producing sharper threshold characteristics for each pixel. Presently, diodes and intrinsic nonlinear devices are used for these nonlinear circuit elements. 

The double threshold addressed matrix liquid crystal display panel uses a two-terminal device that accomplished both set and reset functions for each pixel with appropriate signal pulses from the gate electrode drivers. To realize this panel, an amorphous silicon (a-Si) PIN thin film diode (TFD) matrix construction is used. The circuit diagram of a matrix panel addressed by these diodes is shown in Fig. 1 [4], Each pixel is composed of a pair of diodes and one liquid crystal cell connected in series between a gate electrode bus and a data electrode bus. The diode pair is referred to as a diode ring, because the connection is a ring. 

Liquid-crystal displays have also been constructed on a matrix of single-crystal silicon MOS FET s. Pictorial-gray-scale images have been created on a 1 inch display although line defects were present. Though large arrays of TFT s should be much more economical than silicon MOS FET s, TFT s have suffered in the past from stability and reliability problems. More experimentation is necessary to prove their capabilities as the threshold devices in a liquid-crystal panel. 

The Fredericks transition plays a fundamental role in the operation of many liquid crystal displays because the director orientation can be controlled easily by applying an external magnetic or electric field. One striking feature is that the magnitude of the Fredericks threshold voltage is essential to the operation of many liquid-crystal devices. If the Fredericks threshold voltage could be reduced reliably, this would make it possible for the manufacture of very low-power liquid crystal displays and other liquid-crystalline devices. The Fredericks transition can also be employed to achieve tunable all-optical switching and diode operation in the photonic devices (Miroshnichenko et al. 2006). 

We first discuss the classical Freedericksz transitions and critical thresholds for a nematic. The understanding of these phenomena is crucial to the basic traditional idea of switching liquid crystal cells by fields having magnitudes above the critical threshold. The commercial exploitation of these results in liquid crystal display devices, especially the twisted nematic display to be discussed in Section 3.7 below, has greatly increased the general interest in theoretical and experimental aspects of Freedericksz transitions, and vice-versa. 

Multiplexed twisted nematic liquid crystal devices (TN-LCDs) are the mainstay of low power, medium information content displays. At present, the voltage threshold and the nonlinearity of the electrooptic response intrinsic to the twisted nematic structure are used to achieve multiplexing. The multiplexing level achievable using this technology is limited by the sharpness of the electrooptic response and by the variations of this response with viewing angle. ... 

The simplest display application of a nematic polymer liquid crystal, using polarized light, involves the splay Freedericksz transition shown in Fig. 5(a) for a material with positive Ac and planar surface alignment. The device is constructed using two polarizers crossed with each other, placed above and below the cell, with their polarization direction at 45° to the director of the planar texture. Above a threshold voltage given by... 

Figure 17. Electrooptic response curve of a typical liquid crystal device, showing the relationship of applied voltage to the threshold voltage at high temperature and oblique viewing angle (A) and the saturation voltage at low temperature and normal angle of view (B) required to achieve high contrast over the operating envelope of the display.

Because of the combined threshold-voltage and rise-time requirements, none of the approaches described here are capable of matrix addressing a high-resolution, high-speed display. Lechner, Marlowe, Nester, and Tults have investigated the application of liquid-crystal matrix displays to television and have concluded that a nonlinear threshold or isolation device, such as a diode or transistor, must be inserted in series with the liquid-crystal element at each matrix intersection to obtain the required speed and legibility for line-at-a-time addressing. 

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