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Gated transmission line model pattern measurements of the transient current–voltage characteristics of AlGaN/GaN heterostructurefield-effect transistors(HFETs) and metal–oxide–semiconductor HFETs were made to develop a phenomenological model for current collapse. Our measurements show that, under pulsed gate bias, the current collapse results from increased source–gate and gate–drain resistances but not from the channel resistance under the gate. We propose a model linking this increase in series resistances (and, therefore, the current collapse) to a decrease in piezoelectriccharge resulting from the gate bias-induced nonuniform strain in the AlGaN barrier layer.