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Temperature-dependent dark and optical characteristics of the InGaP/GaAs heterojunction phototransistors ͑HPTs͒ with and without sulfur treatment are studied. As compared to the HPT without ͑NH 4 ͒ 2 S treatment ͑HPT A͒, treatment at 50°C for 20 min leads to a reduced p-i-n dark current ͑I dark ͒ and a reduced collector dark current ͑I Cdark ͒ for the HPT ͑HPT D͒ in the emitter-floated and base-floated configurations, respectively. Moreover, the effective reduction of the surface defects also induces an enhanced p-i-n photocurrent ͑I ph ͒. The enhanced I ph combined with the promoted dc current gain results in an enhanced optical gain ͑G͒ and signal-to-noise ratio ͑SNR͒. For HPT A ͑D͒ under P in = 107.6 nW at 298 K, the G is 1.42 ͑20.3͒ while the SNR is 42 ͑94͒ dB. Experimental results indicate that the treated HPTs, compared to the untreated one, are more sensitive to low-power illumination.

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We report on the InGaP/GaAs dual-emitter heterojunction phototransistors (DEPTs) with an emitter biased using a voltage for comparison to heterojunction phototransistors (HPTs) with a floating base operated in the p-i-n and transistor modes and to the HPTs with a base biased using a voltage. The optical gain of the DEPT is presented to be tunable with both of the voltage applied to the emitter and the incident optical power. On the contrary, a conventional HPT's configuration does not simultaneously exhibit both. The power-tunable optical gain is available when the base of the HPT is floated. Otherwise, the voltage-tunable optical is expected for the HPT with a voltage biased base. Experimental results show that (1) the HPT with a voltage biased base exhibits a gain-tuning efficiency of 4.4 V −1 and (2) the DEPT with an emitter biased using a voltage exhibits a gain-tuning efficiency of 43.4 V −1 .