According to example embodiments, a HEMT includes a channel supply layer on a channel layer, a p-type semiconductor structure on the channel supply layer, a gate electrode on the p-type semiconductor structure, and source and drain electrodes spaced apart from two sides of the gate electrode respectively. The channel supply layer may have a higher energy bandgap than the channel layer. The p-type semiconductor structure may have an energy bandgap that is different than the channel supply layer. The p-type semiconductor structure may include a hole injection layer (HIL) on the channel supply layer and be configured to inject holes into at least one of the channel layer and the channel supply in an on state. The p-type semiconductor structure may include a depletion forming layer on part of the HIL. The depletion forming layer may have a dopant concentration that is different than the dopant concentration of the HIL.