Referring to FIG. 1, a gate is formed on the donor-supply layer in operation 107. The gate includes a number of layers shown as layer 209 in FIG. 3. In some embodiments, an “epi wafer” is provided without any gate stack material. In other embodiments, the “epi wafer” is provided with a p-type doped gallium nitride (p-GaN) layer. The p-GaN layer may be about 300 angstroms to about 3000 angstroms thick. The p-type doping may occur by adding a dopant during the epitaxial growth process at a high temperature of between about 800 degrees Celsius and about 1200 degrees Celsius. P-type dopant candidates include carbon, iron, magnesium, calcium, beryllium, and zinc. The p-type doping may also occur by other processes such as ion implant; however, care must be taken not to incorporate the dopant in underlying layers, which may adversely affect the electrical properties of the transistor. The dopant concentration may be about 1E16/cm3 and 1E18/cm3. The different embodiments result in HEMTs having different properties referred to as depletion mode (d-mode) or enhanced mode (e-mode). The various aspects of the present disclosure apply to both types of the HEMTs.
