In some embodiments, a metal insulator semiconductor heterostructure field effect transistor (MISHFET) is disclosed that has a source, a drain, an insulation layer, a gate dielectric, and a gate. The source and drain are on opposing sides of a channel region of a channel layer. The channel region is an upper portion of the channel layer. The channel layer comprises gallium nitride. The insulation layer is over the channel layer and has a first portion and a second portion. The first portion is nearer the drain than the source and has a first thickness. The second portion is nearer the source than drain and has the first thickness. The insulation layer has an opening through the insulation layer. The opening is between the first portion and the second portion.