A high electron mobility transistor (HEMT) device includes a substrate, a channel layer, a source, a drain, a buffer layer, and a plurality of amorphous regions. The channel layer is located above the substrate. The source is located on the channel layer. The drain is located on the channel layer. The buffer layer is located between the substrate and the channel layer. The plurality of amorphous regions are located in the buffer layer below the source and the drain.