The thermal stability of Ti/Pt/Au Schottky gates on high-low doped GaAs metal–semiconductor field-effect transistors (MESFETs) was investigated in the temperature range of 300–500 °C, using current–voltage and capacitance–voltage measurements and cross-sectional transmission electron microscopy with energy dispersive x-ray spectroscopy. At annealing temperatures>350 °C,the interfacial reactions cause the formation of a layered structure of Ti/Ti–Ga/Ti–As/GaAs. The depth distribution of electron concentration moves toward the Ti/GaAs interface as the annealing temperature increases. This is due to the growth of Ti–As, followed by the reduction of the channel thickness. The activation energy for the growth of Ti–As is determined to be 1.74 eV. The electron concentration in the channel layer decreases with the increase of the annealing temperature. This is due to the out-diffusion of Ga to the Ti film, resulting in the production of acceptor-type Ga vacancies and thereby the decrease of electron concentration in the channel. The activation energy for the reduction of electron concentration, equal to the formation energy of the acceptor-type Ga vacancies, is determined to be 1.42 eV. This is closed to the activation energy of 1.3 eV for the device failure obtained at annealing temperatures ranged from 300 to 350 °C. From these observations, it is proposed that the thermal degradation of GaAs MESFETs at temperatures<350 °C,mainly proceeds by the electron compensation with acceptor-type Ga vacancies in the channel, whereas it occurs by the growth of Ti–As below the original Ti/GaAs interface at temperatures>350 °C.©1997 American Institute of Physics.