Back-to-Back Amorphous Silicon Diodes for Driving LC Displays
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BACK-TO-BACK AMORPHOUS SILICON DIODES FOR DRIVING LC DISPLAYS JOSEPH DRESNER RCA Laboratories Princeton, NJ 08540 ABSTRACT This paper describes the preparation and electrical characteristics of a-Si:H p-i-n-i-p and n-i-p-i-n thin film diodes suitable for driving monochrome liquid crystal displays with more than 500 lines. The symmetrical current-voltage curves in the reverse breakdown regime can ge described by i=i exp (E/Eo) where E !xlO V/cm. In the range 20-125 C, the current is tAermally activated with an energy of 0.25eV. The response time to applied voltage pulses is < 10 us. The stabilýty of the electrical characteristics is adequate for at least 10 hours of operation in an liquid crystal display. Electrical characteristics indicate that the reverse breakdown current is a tunneling current injected into the i-layer and that electrons are likely to be dominant. INTRODUCTION This paper describes the fabrication and electrical performance of amorphous hydrogenated silicon (a-Si:H) back-to-back p-i-n-i-p and n-i-p-i-n diodes deposited on metal-coated glass substrates. The need for a non-linear two or three-terminal device in conjunction with liquid crystal (LC) light valves arises from limitations of the direct multiplexing method for addressing arrays; the maximum number of lines which can be addressed is a strong function of the steepness of the electro-optic response curve of the LC cell.[1] The use of a two-terminal non-linear symmetrical element in series with the LC cell is a well-known idea; the electrical characteristics which are needed to drive arrays of several hundred lines have been discussed by several authors.[2,3,4] Further details of circuit analysis, which are beyond the scope of this paper, yield some requirements on the performance of these diodes which can be summarized as follows for a monochrome display in which the elements are either fully on or fully off. For an array of 500 lines operated at TV frame rates (60 Hz refresh), the ratio of currents through the diode I /Iff must be at least 10 , Iff must be low enough to prevent dis2Bar• of the LC cell during ong frame time, and the capacitance of the diode must not exceed 10% of that of the LC cell plus its associated stray capacitances. It is also required that the operating voltage V be properly chosen to meet the voltage requirements of the LC cell. Fifilly, if one requires that the ratio of diode area to LC cell area not exceed 2%, one finds that the diode must be capable of delivering current transients j>O.2 A/cm . The a-Si back-to-back diodes described here, which have the configuration p-i-n-i-p meet all the above requirements. Multilayer a-Si:H diodes of various doping configurations have been studied by others[5,6] with particular attention to the magnitude of the forward current and of the rectification ratio. The reverse breakdown characteristic was found to be "soft" compared to single crystal devices; from the negative temperature dependence of the reverse breakdown voltage it was inferred that the current was supplied by Zen
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