We　present　calculation　of　critical　voltage　for　AlGaN/GaN　high　electron　mobility　transistors　(HEMTs)　with　GaN　cap　layer.　The　calculation　includes　mechanical　stress　and　relaxable　energy　in　the　GaN/AlGaN　barrier　layer.　Under　high　voltage　conditions,　the　high　electric　field　results　in　an　increase　in　stored　relaxable　energy.　If　this　exceeds　a　critical　value,　crystallographic　defects　are　formed.　This　degradation　mechanism　is　voltage　driven　and　characterized　by　a　critical　voltage　beyond　which　non-reversible　degradation　takes　place.　The　dependence　of　the　GaN　cap　layer＇s　thickness　on　the　critical　voltage　has　been　discussed.　The　calculated　results　indicate　that　thicker　GaN　cap　layer　results　in　higher　critical　voltage.