An embodiment high electron mobility transistor (HEMT) includes a gate electrode over a semiconductor substrate and a multi-layer semiconductor cap over the semiconductor substrate and adjacent the gate electrode. The multi-layer semiconductor cap includes a first semiconductor layer and a second semiconductor layer comprising a different material than the first semiconductor layer. The first semiconductor layer is laterally spaced apart from the gate electrode by a first spacing, and the second semiconductor layer is spaced apart from the gate electrode by a second spacing greater than the first spacing.