In this study, an InGaP/GaAs heterojunction phototransistor (HPT) and a GaAs solar cell were monolithically integrated into an HPT epitaxial wafer, and the battery-free operation of the HPT was demonstrated for energy harvesting. Although the thickness and doping condition of the layers were optimized for the HPT performance, but not for the solar cell performance, the obtained short-circuit current was high enough to operate the InGaP/GaAs HPT in a two-terminal (2T) configuration. A collector photocurrent of 0.63 mA was obtained when the energy-harvesting InGaP/GaAs 2T-HPT was exposed to white light with a power density of 35 mW/cm2, and it linearly increased with the power density. For a potential application of the energy-harvesting InGaP/GaAs HPT as a photosensor in space, the device was irradiated with electrons of 1 MeV energy and 1015 cm−2 fluence. No significant degradation of the fabricated energy-harvesting 2T-HPT after the high-energy electron irradiation guarantees its battery-free operation in space.
A heterojunction phototransistor (HPT) is more attractive as a photosensor than a photodiode because of its high photoresponse even at low bias voltage and immunity from avalanche noise.1–3) In particular, the GaAs-based HPT with an AlGaAs emitter demonstrated a high performance. Recently, the InGaP emitter has replaced the AlGaAs emitter in the AlGaAs/GaAs HPT owing to its superior material properties.4) The photosensor may be widely used in space, where it needs to be operated without a battery. An HPT has a process compatibility with a heterojunction bipolar transistor (HBT) for the fabrication of monolithically integrated photoreceivers.5) The InGaP/GaAs HPT also has good compatibility with the GaAs heteroface solar cell for a battery-free operation. Solar cells made of III–V compound semiconductors have been developed and used in space owing to their high conversion efficiency, lower temperature coefficient, and superior radiation resistance.6–9) The significant potential of high-efficiency GaAs heteroface solar cells for space applications has been extensively investigated by many researchers.10–17) Compared with Si, which has been widely used as a material of terrestrial solar cells, III–V compound semiconductors have a superior radiation resistance for the same electron energy and fluence. In particular, the InGaP solar cells demonstrated a radiation resistance superior to that of GaAs solar cells. Since the migration energy of radiation-induced defects and the activation energy of defect annealing in InGaP are lower than those in GaAs, InGaP has a higher radiation resistance than GaAs.18) In this study, radiation resistant InGaP was used as a window layer in a GaAs heteroface solar cell.
In space, high-energy electron or particle irradiation often induces a significant degradation of the performance of semiconductor devices. Since the battery-free operation of an InGaP/GaAs HPT monolithically integrated with a GaAs solar cell is also proposed for use in space in this paper, the effects of high-energy electron irradiation on the fabricated energy-harvesting HPTs were studied by 1 MeV electron irradiation.
- GaAs/Si ; AlGaAs/GaAs HPT
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