Gallium　Nitride　(GaN)　transistors　are　emerging　as　promising　candidates　for　making　high-frequency,　low-loss　and　small-size　power　converters.　To　realize　normally-off,　p-GaN　gate　technique　is　widely　adopted　in　commercially　available　GaN-based　power　devices.　However,　owing　to　the　distinctions　in　device　structure,　the　intrinsic　capacitances　with　regard　to　gate　region　vary　from　those　of　Si　MOSFET.　Besides,　with　drain-bias　rising,　the　variation　of　gate　region’s　net　charge　could　make　the　threshold　voltage　of　GaN　transistor　unstable.　Thus,　the　switching　transient　waveforms　of　GaN　transistor　could　be　significantly　influenced　by　the　aforementioned　factors,　and　the　commonly　used　analysis　method　for　Si　MOSFET　would　not　be　sufficient.　In　this　work,　the　threshold　voltage　instability　is　firstly　analyzed,　which　is　related　to　drain-to-gate　voltage　stress.　Due　to　the　difficulties　in　directly　measuring　the　gate-related　capacitances　and　their　dynamic　behaviors,　a　hybrid　physical-behavioral　modeling　method　is　proposed,　which　is　capable　of　extracting　the　relationship　between　the　gate-related　capacitances　and　their　bias　from　the　static　measurements.　The　proposed　analysis　methods　are　then　implemented　on　a　GaN-based　phase-leg　circuit.　Through　the　comparison　with　the　experimental　results　and　the　simulated　waveforms　of　the　most　advanced　analysis,　the　proposed　analysis　approach　exhibits　outstanding　performance.　©　2017　IEEE.