Various embodiments of the disclosure include the formation of enhancement-mode (e-mode) gate injection high electron mobility transistors (HEMT). Embodiments can include GaN, AlGaN, and InAlN based HEMTs. Embodiments also can include self-aligned P-type gate and field plate structures. The gates can be self-aligned to the source and drain, which can allow for precise control over the gate-source and gate-drain spacing. Additional embodiments include the addition of a GaN cap structure, an AlGaN buffer layer, AlN, recess etching, and/or using a thin oxidized AlN layer. In manufacturing the HEMTs according to present teachings, selective epitaxial growth (SEG) and epitaxial lateral overgrowth (ELO) can both be utilized to form gates.