Novel a-Si:H Thin Film High Voltage Transistor
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NOVEL a-Si:H THIN FILM HIGH VOLTAGE TRANSISTOR HSING C. TUAN Xerox Palo Alto Research Center, 3333 Coyote Hill Rd., Palo Alto, CA 94304 ABSTRACT A new novel thin film high voltage transistor in a-Si:H is described in this paper. This new structure extends the operation of a-Si:H TFT to 500 volts or more. The fabrication process of this new high voltage transistor is simple and compatible with that of the conventional low voltage TFT. The high voltage TFT can be switched by low voltage signals and is demonstrated to be capable of switching several hundreds of volts. INTRODUCTION Many important input/output devices consist of a large number of transducers which need to be individually addressed. Examples of these devices include flat-panel displays,[1-2] page-wide document scanners,[31 high-speed printer heads, etc. For reliability and low manufacturing cost reasons, it is highly desirable that switches needed to address the transducer elements are fabricated integrally on the same substrate as the transducer array. In the past few years, amorphous silicon thin film transistor (a-Si:H TFT) has become a leading candidate as switching elements for these types of applications. The major advantages of a-Si:H TFT over other types of TFTs, such as CdSe TFT and poly-Si TFT, are its more mature state of development and its low-temperature fabrication process, making possible the use of inexpensive substrate materials, such as glass. Up until now, however, the application of a-Si:H TFT has been limited to low-voltage devices, requiring voltage drives of the order of 0-20 volts. In this paper, a new novel amorphous silicon high voltage thin film transistor (a-Si:H HVTFT) which extends the operation of a-Si:H TFT to much higher voltage levels is described. In order that this a-Si:H HVTFT is useful for large-area electronics applications, it is desirable that it has the following features: (i) high voltage operation, up to 500 volts or more (ii) it can be switched by low-voltage signals (0-15 volts) (iii) its fabrication process is simple and compatible with that of the low-voltage a-Si:H TFT (LVTFT). This is important because both HVTFT and LVTFT can then be fabricated simultaneously on the same substrate. On the transducer array the LVTFTs can perform the necessary data transfer and manipulation and the HVTFTs can be used purely as the output drivers which switch the high voltage transducer elements. (iv) its device geometry should be consistent with large-area fabrication, i.e., the critical dimension should be of the order of 10 microns. It should be pointed out that most high voltage physical phenomena are voltage-controlled rather than current-controlled. There is usually a well-defined threshold voltage involved which has to be reached before the physical phenomenon can occur, but once the threshold voltage is reached the current requirement, particularly on a per-transducer basis, is frequently quite low. For many types of applications, this current may range from 10- 9 amp to 10- 5 amp per transducer element. This match
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