We investigate how the mobility and carrier density of AlGaAs/GaAs two‐dimensional electron gas are influenced by the fabrication process with plasma‐excited chemical vapor deposition (plasma‐CVD) SiN film. These properties are greatly reduced by annealing with plasma‐CVD SiN film as a cap but are restored by reannealing after removing the SiN film. We further use capacitance‐voltage measurements to investigate the influence of this same process on a more simplified structure, Si‐doped GaAs layer. Annealing with a plasma‐CVD SiN film changes the defect density of Si‐doped GaAs in two, temperature dependent ways: annealing below 380 °C reduces deposition damage, and annealing above 300 °C produces new defects, which might be caused by the film stress. These new defects can be reduced by reannealing after removing the SiN film. ©1995 American Institute of Physics.