A high electron mobility transistor (HEMT) includes a buffer layer on a substrate, ridges extending along a first direction on the buffer layer, gaps extending along the first direction between the ridges, a p-type semiconductor layer extending along a second direction on the ridges and inserted into the gaps, and a source electrode and a drain electrode adjacent to two sides of the p-type semiconductor layer. Preferably, the source electrode and the drain electrode are extending along the second direction and directly on top of the ridges.