N-polar GaN Cap MISHEMT with record 6.7 W/mm at 94 GHz

The high critical electric fields and large charge densities associated with III-N based high electron mobility transistors (HEMTs) make this technology an attractive candidate for solid state power amplification at W-band. To date, reports on W-band GaN HEMTs and MMICs have primarily featured devices fabricated in the Ga-polar orientation [1, 2]. In this work, the advantages of the N-polar orientation are exploited to produce a MISHEMT exhibiting a record high 6.7 W/mm peak output power (Pout) at 94 GHz. The key component of this device design is an in situ GaN epitaxial passivation layer in the access regions of the device. In combination with the inherent polarization fields of N-polar, this GaN cap acts to both eliminate surface state dispersion and improve the conductivity in the access regions of the transistor. The device results presented here build upon the work reported in [3] and [4] by transitioning from a sapphire to a SiC substrate to reduce self-heating. This has resulted in over a 2x increase in power density compared to [4].