N-Polar Deep Recess MISHEMTs With Record 2.9 W/mm at 94 GHz

W-band power performance is reported on an N-polar GaN HEMT for the first time, resulting in a record output power density for any GaN device on a sapphire substrate. This result is achieved using an N-polar GaN deep recess MISHEMT structure grown by metal-organic chemical vapor deposition on the sapphire substrates. The key component in this device design is the addition of an in situ unintentionally doped GaN epitaxial passivation layer in the access regions of the transistor. This GaN layer functions both to control DC-to-RF dispersion as well as to increase the conductivity in the access regions of the HEMT. Devices with very low dispersion and a simultaneous f(max)/f(t) combination of 276/149 GHz are demonstrated. Load pull measurements at 94 GHz give a peak power added efficiency (PAE) of 20% with an associated output power density of 1.73 W/mm at V-DS = 8 V. A record 2.9-W/mm maximum output power density with an associated 15.5% PAE at V-DS = 10 V is achieved despite the low thermal conductivity of the sample's sapphire substrate.