According to example embodiments, a normally-off high electron mobility transistor (HEMT) includes: a channel layer having a first nitride semiconductor, a channel supply layer on the channel layer, a source electrode and a drain electrode at sides of the channel supply layer, a depletion-forming layer on the channel supply layer, a gate insulating layer on the depletion-forming layer, and a gate electrode on the gate insulation layer. The channel supply layer includes a second nitride semiconductor and is configured to induce a two-dimensional electron gas (2DEG) in the channel layer. The depletion-forming layer is configured has at least two thicknesses and is configured to form a depletion region in at least a partial region of the 2DEG. The gate electrode contacts the depletion-forming layer.