This paper presents a method of surface treatment for improving the current gain of an AlGaAs/GaAs heterojunction bipolar transistor (HBT). Several recipes for GaAs surface treatment, including each and several combinations of the H, plasma, NH3 plasma, and (NH4)(2)S-x chemical treatments are examined. The lowest SiO2/GaAs interface state density of 1.23 x 10(10) cm(-2) eV(-1) is obtained when the GaAs surface is treated in sequence with (NH4)(2)S-x:H2O = 1:1 solution, H-2 plasma, and NH3 plasma. The current gain of an AlGaAs/GaAs HBT, whose extrinsic base surface is treated with this recipe, is enhanced at a low collector current by a factor of 2.5 from that without surface treatment. The maximum current gain is also enhanced by a factor of 1.37. (C) 2004 Elsevier Ltd. All rights reserved.