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Temperature-dependent dark and optical characteristics of the InGaP/GaAs heterojunction phototransistors (HPTs) with and without sulfur treatment are studied. As compared to the HPT without (NH4)2S treatment (HPT A), treatment at 50°C for 20 mm leads to a reduced p-i-n dark current (Idark) and a reduced collector dark current (ICdark) for the HPT (HPT D) 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 (Igb). The enhanced Igb combined with the promoted dc current gain results in an enhanced optical gain (G) and signal-to-noise ratio (SNR). For HPT A (D) under Pin = 107.6 nW at 298 K, the G is 1.42 (20.3) while the SNR is 42 (94) dB. Experimental results indicate that the treated HPTs, compared to the untreated one, are more sensitive to low-power illumination. [ABSTRACT FROM AUTHOR]