The over‐passivations of sulfur treated GaAs were investigated by the photoluminescence measurement and device characteristics of AlGaAs/GaAs heterojunction bipolar transistors. The two chemical vapor deposition systems of the conventional plasma enhanced process and electron cyclotron resonance process were employed. The surface recombination current density calculated from the current gain and the base current noise was reduced to 25% by sulfur treatment and over‐passivation process using the electron cyclotron resonance deposition system, whereas over‐passivation by the plasma enhanced deposition system gave some damage to the sulfur treated surface.