Stable passivation of GaAs surfaces and AlGaAs/GaAs heterojunction bipolar transistors has been achieved using sulfide solutions and SiNxoverlayers. The SiNxlayers are deposited at ∼200 °C using electron cyclotron resonance plasma‐enhanced chemical vapor deposition technique. The capacitance–voltage measurements indicate a substantial reduction in the density of electronic defects at the SiNx/S–GaAs interface as a result of annealing in N2ambient. The base current of a 36×36 μm2AlGaAs/GaAs heterojunction bipolar transistor is reduced by approximately two orders of magnitude in the low collector current regime by using sulfide treatment, SiNxdeposition, and anneal. Both the Al/SiNx/S–GaAs capacitors and transistors are stable for several months with no noticeable degradation in their electrical characteristics.