Consistent with an example embodiment, a GaN heterojunction structure has a three-layer dielectric structure. The lowermost and middle portions of the gate electrode together define the gate foot, and this is associated with two dielectric layers. A thinner first dielectric layer is adjacent the gate edge at the bottom of the gate electrode. The second dielectriclayer corresponds to the layer in the conventional structure, and it is level with the main portion of the gate foot.