Prevention of InP/InGaAs/InP Double Heterojunction Bipolar Transistors from Current Gain Reduction during Passivation
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Prevention of InP/InGaAs/InP Double Heterojunction Bipolar Transistors from Current Gain Reduction during Passivation Byoung-Gue Min, Jong-Min Lee, Seong-Il Kim, Chul-Won Ju, Kyung-Ho Lee InP IC Team, Basic Research Laboratory, Electronics and Telecommunications Research Institute, Daejeon 305-350, KOREA ABSTRACT A significant degradation of current gain of InP/InGaAs/InP double heterojunction bipolar transistors was observed after passivation. The amount of degradation depended on the degree of surface exposure of the p-type InGaAs base layer according to the epi-structure and device structure. The deposition conditions such as deposition temperature, kinds of materials (silicon oxide, silicon nitride and aluminum oxide) and film thickness were not major variables to affect the device performance. The gain reduction was prevented by the BOE treatment before the passivation. A possible explanation of this behavior is that unstable non-stoichiometric surface states produced by excess In, Ga, or As after mesa etching are eliminated by BOE treatment and reduce the surface recombination sites. INTRODUCTION Heterojunction bipolar transistors (HBTs) of compound semiconductors have emerged as a competitive technology for wireless and fiber optic communications markets [1]. When the device geometry is scaled down to improve the performance, the current gain of an HBT is strongly influenced by the surface recombination effect. It is well known that highly doped ptype base layer has a high surface recombination velocity and that the surface recombination on the extrinsic base region around the emitter perimeter significantly degrades the current gain of the device. The dc characteristics of InP heterojunction bipolar transistors were reported to significantly depend on the kind of passivation film and the deposition method used. It was also known that the surface recombination current, induced by surface passivation film formation, caused the I-V characteristics of InP-HBTs to deteriorate [2]. The degradation of electrical characteristics of InP/InGaAs HBTs by silicon nitride passivation was reported due to the preferential etching of phosphorus during silicon-nitride deposition [3]. The effect of the surface Fermi level position on dc characteristics of InP-based HBT has been reported [4]. HBT passivated with silicon nitride film showed excellent dc characteristics with very small excess base current. Meanwhile, the Fermi level of an InP surface covered with silicon oxide was located at an energy close to the conduction band minimum of InP, which showed large excess base current and poor current gain. A study of the suitability of various insulators in combination with various deposition techniques for the passivation of InP surfaces showed that among the techniques investigated, the best results were obtained with the remote plasma-enhanced chemical vapor deposition (PECVD) technique [5]. As mentioned above, the purpose of passivation is to reduce the surface recombination current by suppressing oxidation in atmosph
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