A high electron mobility transistor (HEMT) device structure is provided. The HEMT device structure includes a channel layer formed over a substrate and an active layer formed over the channel layer. The HEMT device structure also includes a gate structure formed over the active layer, and the gate structure includes: a p-doped gallium nitride (p-GaN) layer or a p-doped aluminum gallium nitride (p-GaN) layer formed over the active layer, and a portion of the p-GaN layer or p-AlGaN layer has a stepwise or gradient doping concentration. The HEMT device structure also includes a gate electrode over the p-GaN layer or p-AlGaN layer.