High electron mobility transistors are provided that include a non-uniform
aluminum concentration AlGaN based cap layer having a high aluminum
concentration adjacent a surface of the cap layer that is remote from the
barrier layer on which the cap layer is provided. High electron mobility
transistors are provided that include a cap layer having a doped region
adjacent a surface of the cap layer that is remote from the barrier layer
on which the cap layer is provided. Graphitic BN passivation structures
for wide bandgap semiconductor devices are provided. SiC passivation
structures for Group III-nitride semiconductor devices are provided.
Oxygen anneals of passivation structures are also provided. Ohmic
contacts without a recess are also provided.