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 . 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. Although there have been many reports on the study of electrically-stressed InGaP/GaAs HBTs , there is a little or none on the electrically-stressed InGaP/GaAs HPTs. In this paper, we report the effects of electrical stress on the characteristics of InGaP/GaAs HPTs in detail. We gave an electrical stress, which was much smaller than that usually given to the HBTs, at room temperature and an elevated temperature to the InGaP/GaAs HPTs with and without the emitter-ledge passivation, which has demonstrated the improved performance of HBTs. The HPT epilayers were grown on S.I. GaAs (100) substrate by metal-organic chemical vapor deposition (MOCVD). Although the structure and design are similar to the HBTs, the emitter area is 160,800 ?m2, much larger than those of typical HBTs in order to increase the collector photocurrent. The electrical stress was given to the phototransistors by keeping a collector current of 60 mA (corresponding to a current density of 37 A/cm2) for 1 hour at room temperature. The electrical stress was found to be too small to affect the common-emitter current gain and photocurrent of both HPTs without and with the InGaP emitter-ledge passivation at room temperature. However, both current gain and photoresponse at 420 K decreased significantly in the electrically-stressed HPTs, but those of the HPT with the emitter-ledge passivation were still higher than those of the HPT without the emitter-ledge passivation. The effect of the electrical stress was more significant if it was given at 420 K even for the period reduced to 15 min. There was a significant decrease in the current gain and photoresponse even at room temperature in the HPT without the emitter-ledge passivation, while a little decrease was observed in the HPT with the emitter-ledge passivation. The effect of the electrical stress on the photoresponse was also much more significant than that on the current gain, and the emitter-ledge passivation was more effective in suppressing the degradation of the HPTs. Furthermore, for a potential application of the InGaP/GaAs HPTs in space, we will irradiate the HPTs with high-energy electrons at the Japan Atomic Energy Agency and characterize their degradation as a function of the energy and fluence. References  S. Chandrasekhar et al., IEEE Photon. Technol. Lett 5, 1316 (1993).  W. Liu et al., IEEE Electron Device Lett 14, 301 (1993).
The University of Electro-Communications
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