One example provides an enhancement-mode High Electron Mobility Transistor (HEMT) includes a substrate, a Group IIIA-N active layer over the substrate, a Group IIIA-N barrier layer over the active layer, and at least one isolation region through the barrier layer to provide an isolated active area having the barrier layer on the active layer. A gate stack is located between source and drain contacts to the active layer. A tunnel diode in the gate stack includes an n-GaN layer on an InGaN layer on a p-GaN layer located on the barrier layer.