This paper presents the effect of surface treatment on the electrical properties of an AlGaAs/GaAs heterojunction bipolar transistor (HBT). The extrinsic base surface of the HBT was treated in sequence with a (NH4)(2)S-x:H2O = 1:1 solution, H-2 plasma, and NH3 plasma. The treated HBT had a lower surface recombination current, base resistance, and low-frequency base current noise than the untreated HBT. These values decreased by 32%, 42%, and similar to 3 dB, respectively. Because of the reduced base resistance, the maximum frequency of oscillation f(max), which was 40.3 GHz without surface treatment, improved to 57.8 GHz. (c) 2005 Elsevier Ltd. All rights reserved.