Abstract Forward AC stress and relaxation have been performed on GaN-on-Si Schottky diodes to study the shift of diode characteristics such as threshold voltage. Influence of frequency and length of first anode field plate are studied. PBTI measurement performed on HEMTs with gate similar to the first anode field plate rules out the degradation of the dielectric under the anode field plate and tends to prove that Schottky contact itself could be the cause of forward stress degradation.