Abstract This work shows results of temperature-dependent (from −40 °C to 140 °C) on-state breakdown characterization of commercial PHEMTs from two vendors. Our data shows that in both samples impact ionization in the channel dominates the breakdown phenomenon and that a classical and simple analytical model can describe it with good accuracy. We have observed negligible temperature dependence of impact ionization in one of the samples, while the other shows moderate reduction of impact ionization with increasing temperature.