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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