Fabrication and Characterization of Hydrogenated Amorphous Silicon Bipolar Thin Film Transistor (B-TFT)
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Fabrication and Characterization of Hydrogenated Amorphous Silicon Bipolar Thin Film Transistor (B-TFT) Yue Kuo, Yu Lei, and Helinda Nominanda Thin Film Nano & Microelectronics Research Laboratory, Texas A&M University, College Station, TX, U.S.A. ABSTRACT The conventional a-Si:H thin film transistor (TFT) is a field effect transistor (FET), which has disadvantages of a low operation speed and a small current driving capability. To achieve a higher speed and larger current driving capability, a potential solution is to fabricate the a-Si:Hbased bipolar thin film transistor (B-TFT). In this study, a-Si:H p-i-n junctions were prepared and studied in order to determine the proper layer thickness for good junction behaviors. B-TFTs composed of a stacked structure of n+/i/p/i/n+ were then fabricated. The complete B-TFT was made using plasma enhanced chemical vapor deposition (PECVD) to deposit all doped and undoped a-Si:H layers and SiNx dielectrics at 250oC. Reactive ion etching (RIE) and wet etching methods were used to define base and emitter regions and contacts. The I-V characteristics of the complete B-TFT were investigated. The common-emitter current gain is 3~6, which is larger than the literature report of 2~3. In addition, a collector current larger than the literature value was obtained. A significant current noise was observed, which may be contributed to the high series resistance of the base layer and defective junction interfaces. In this paper, process and structure influences on the a-Si:H junction and B-TFT performances are discussed.
I. INTRODUCTION Recently, a-Si:H TFT arrays were successfully fabricated on large-area, low-temperature glass substrates for active matrix liquid crystal displays. The a-Si:H TFT is an FET that can be fabricated by the low-temperature PECVD method on a wide variety of materials with few limits on substrate sizes and materials instead of the single crystal silicon [1]. The conventional a-Si:H TFT has a major disadvantage of a low field effect mobility, which limits its current driving capability and switching speed. It is well known that the bipolar junction transistor (BJT) can drive a larger current and be operated at a higher speed than the FET. If a functional BJT can be fabricated from a-Si:H films, the above problem can probably be solved. However, the a-Si:H film contains a high concentration of defects that results in a lower charge carrier mobility and a very short minority carrier diffusion length. In addition, the lateral resistance of a thin a-Si:H film can be extremely large [2]. Therefore, it is a challenge to fabricate a functional bipolar junction TFT (B-TFT) based on a-Si:H films. The first complete a-Si:H B-TFT, which was made by a sophisticated process, was reported in 1984 [3]. The finished B-TFT displayed nonsaturated common-emitter output characteristics and a current gain of 2~3 in the non-saturation region. There was no other report on this kind of devices since then. In this research, authors investigated the feasibility of fabricating func
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