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A highly sensitive InGaAs/InP heterojunction phototransistor (HPT) has been proposed and characterized using front-side optical injection. We also present optimized epitaxial structures to increase the optical gain and discuss the device operation mechanism and experimental results. The experimental results show that our device has an optical responsivity of 122.5 A/W, which is significantly higher than that of a conventional PIN photodetector with the same light absorbing area. This optical responsivity is extremely high and seems to exceed that of the APD. This high value of responsivity originates from the optical gain-enhanced device structure. The typical optical gain is approximately 188, which is much higher than that of any HPTs that have been reported. This means HPTs are 188 times more sensitive than PIN photodetectors. The proposed device can be easily fabricated using conventional HBT processing without additional steps. With these InGaAs/InP HPTs, we can construct a highly sensitive near-IR imaging system to detect very weak optical signals.

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