Fluorine atoms intruded into the channel layer in P-HEMT and degraded its carrier density and electron mobility during the SiO₂ RIE process. Thermal annealing at 300°C for 10 minutes was rather effective for recovering from this plasma damage. Our SIMS investigation revealed that the mechanism of this recovery was the removal of fluorine atoms from the channel layer and their accumulation in the δ-doped Si layer. Ionized impurities in the channel layer were removed, and electron mobility was significantly recovered. However, the carrier density was recovered to a lesser degree than that of the electron mobility because of the neutralization of Si donors in the δ-doped layer caused by the fluorine accumulation.