Effects of Electrical Stress and High-Energy Electron Irradiation on the InGaP/GaAs Heterojunction Phototransistor
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Effects of Electrical Stress and High-Energy Electron Irradiation on the InGaP/GaAs Heterojunction Phototransistor Phuc Hong Than1, Kazuo Uchida1, Takahiro Makino2, Takeshi Ohshima2 and Shinji Nozaki1,* 1 Graduate School of Informatics and Engineering, The University of Electro-Communications, Chofu-shi, Tokyo 182-8585, Japan 2 Japan Atomic Energy Agency, 1233 Watanuki, Takasaki, Gunma 370-1292, Japan * Phone/Fax: +81-42-489-4486, Email: [email protected] ABSTRACT In this paper, we discuss the characteristics of the InGaP/GaAs heterojunction phototransistors (HPTs) before and after the electrical stress at room temperature and assess the effectiveness of the emitter-ledge passivation. Although an electrical stress given to the phototransistors by keeping a collector current density of 37 A/cm2 for 1 hour at room temperature was too small to affect the room-temperature common-emitter current gain and photocurrent of both HPTs with and without the emitter-ledge passivation, they showed a significant decrease at 420 K due to the room-temperature electrical stress. Nevertheless, the room-temperature common-emitter current gain and photocurrent of the HPT with the emitterledge passivation were still higher than those of the HPT without the emitter-ledge passivation. The effectiveness of the emitter-ledge passivation against the electrical stress was more significant than that on the current gain in the dark. In addition to the electrical stress experiment, for a potential application of the InGaP/GaAs HPTs in space, we will irradiate the HPTs with 1MeV electrons at the Japan Atomic Energy Agency. Both current gain and photocurrent decreased significantly after the electron irradiation. In contrast to the electrical stress, the damage due to the high-energy electron irradiation is a bulk-related phenomenon, and the emitter-ledge passivation does not seem to suppress the degradation. INTRODUCTION A heterojunction phototransistor (HPT) is very attractive compared with photodiodes because of its good compatibility with a heterojunction bipolar transistor (HBT), high photoresponse even at low bias voltage and immunity from avalanche noise [1] - [3]. Since InGaP has an advantage over AlGaAs in the material properties and fabrication process, the InGaP emitter has been actively employed to replace the AlGaAs in the AlGaAs/GaAs HPT [4]. The emitter-ledge passivation of the external base surface has found to increase the current gain of the GaAs-based HBT and suppress the electrical stress-caused degradation [5] – [13]. However, there is little or no study on the electrically-stressed InGaP/GaAs HPTs. In this paper, we compare the performances of the InGaP/GaAs HPTs with and without the emitter-ledge passivation and assess the effect of the emitter-ledge passivation on the stressed HPTs. Furthermore, InGaP/GaAs HPTs are expected to be potential applications in the space-based communication systems. Therefore, reliability of HPTs when exposed to high-energy radiation is of major concern. Although the radiation effects on HBTs ha
