Abstract Electrical characteristics of GaAs metal–oxide–semiconductor field effect transistor with atomic layer deposition deposited Al 2 O 3 gate dielectric have been investigated. The IV characteristics were studied after various constant voltage stress (CVS) has been applied. A power law dependence of the gate leakage current ( I g ) on the gate voltage ( V g ) was found to fit the CVS data of the low positive V g range. The percolation model well explains the degradation of I g after a high positive V g stress. A positive threshold voltage ( V th ) shift for both +1.5 V and +2 V CVS was observed. Our data indicated that positive mobile charges may be first removed from the Al 2 O 3 layer during the initial CVS, while the trapping of electrons by existing traps in the Al 2 O 3 layer is responsible for the V th shift during the subsequent CVS.