This article describes a novel preepitaxial cleaning treatment of GaAs, using a (NH4)(2)S-x solution. This cleaning treatment is characterized by low carbon and oxygen surface contamination. The treated-surface presents a low temperature (2X1) reconstruction (<500 degrees C), which allows selective chemical beam epitaxy processes at low temperature. Applying this process to the selective regrowth of the base contact layer of heterojunction bipolar transistors (HBTs), high de current gains are measured, similar to those obtained with HBTs processed in a conventional way.