Hot-electron Phototransistors in Hydrogenated Amorphous Silicon

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Hot-electron Phototransistors in Hydrogenated Amorphous Silicon J. M. Shannon and E. G. Gerstner School of Electronic Engineering, IT and Mathematics, University of Surrey, Guildford, GU2 5XH, United Kingdom. Email: [email protected] ABSTRACT It has been shown that useful current gains can be obtained in hot-electron device structures containing very thin chromium disilicide layers of nanometer dimensions in hydrogenated amorphous silicon [1]. The a-Si:H/a-CrSi2/a-Si:H device structure made using PECVD and sputtering techniques naturally forms a hot-electron transistor device where the electrons are emitted across a high potential barrier on one side of the silicide and are collected over a low barrier on the other. Recent results [2] have shown that current gains can be in excess of 40 in structures having a-CrSi2 bases ~1 nm thick. Here we outline the relatively simple technology used to make these devices and examine their performance as phototransistors in which the photo-current is amplified by hot-electron transistor action. The speed of response can be maximised by operating the phototransistor with high electric field across the collector since it is the transit time of the photo-induced carriers that determines the response time. We show that these devices provide a useful new active element for large area amorphous silicon electronics. INTRODUCTION The concept of the hot-electron transistor was proposed many years ago [3-4] and many attempts have been made to make devices in crystalline semiconductors with useful current gain and high frequency performance. The basic idea is to generate hot electrons at the emitter of a transistor and use a metal-like base region with a thickness small compared to the ballistic mean free path of the hot electrons so that most are able to traverse the base and be collected by a collector barrier. Such transistor structures should be unipolar, and operate at high speeds. In practice, however, current gains in these structures were disappointingly low mainly due to quantum mechanical reflections at the interface between the base and collector materials [5-6]. Improvements to current gain were made using monolithic structures in crystalline materials in which the emitter barrier was much higher than the collector barrier [7] thus allowing an electron to lose some energy and still be collected with minimal quantum reflections. Current gains of ~15 were obtained in these structures but they were extremely difficult to make and control. We have reported that a thin layer of chromium disilicide sandwiched between two layers of undoped amorphous silicon naturally forms a hot-electron structure. The amorphous chromium disilicide forms a very thin metal-like base region of nanometre dimensions which is bounded by a large potential barrier on one side and a small barrier on the other. The large barrier occurs when depositing chromium on top of a-Si:H whilst the low barrier forms when depositing a-Si:H on the silicide. The structures have been shown to provide current gain [1]

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