Plasma processing is an essential part for the fabrication of GaAs ICs.It was found that the exposure of sulfur doped n-type GaAs layers to a plasma of helium, oxygen, or nitrogen changed their electrical characteristics without introducing crystalline damage, as observed by electron diffraction measurments or etching.Exposure to a plasma depletes the surface carrier concentration but the mobility remains unchanged.Compared to O2 and N2 the helium plasma has the largest effect on the GaAs surface.Exposure of S-doped GaAs layers to a He plasma at 350°C produces two new deep levels at 840-nm and in the region between 863 and 872-nm.These levels were not observed for a He plasma exposure at room temperature or for O2 plasma exposure at 350°C.