The present disclosure provides a semiconductor structure. The semiconductor structure includes a buffer layer on a substrate, an graded aluminum gallium nitride (AlGaN) layer disposed on the buffer layer, a gallium nitride (GaN) layer disposed on the graded AlGaN layer, a second AlGaN layer disposed on the GaN layer and a gate stack disposed on the second AlGaN layer. The gate stack includes one or more of a III-V compound p-doped layer, a III-V compound n-doped layer, an aluminum nitride (AlN) layer between the III-V compound p-doped and n-doped layers, and a metal layer formed over the p-doped, AlN, and n-doped layers. A dielectric layer can also underlie the metal layer.