The mechanism of a positive Vth drift of trench and planar SiC power MOSFETs under bipolar AC gate stress was investigated by measuring electrical characteristics and detecting light emission from the channel. The positive Vth drift in bipolar AC stress can be explained by a photon-assisted electron injection model. In this model, electrons in the inversion layer are excited by receiving photon energy from recombination of holes and electrons at interface states, and subsequently injected into the gate oxide. Trench MOSFETs showed a stronger light emission above 2.7 eV and a larger positive Vth drift than planar MOSFETs. This can be attributed to a higher interface state density near Ev and a smaller conduction band offset.