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)(2)S treatment (HPT A), treatment at 50 degrees 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 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 (I-ph). The enhanced I-ph 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 P-in=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. (C) 2007 The Electrochemical Society.