mm-Wave N-polar GaN MISHEMT with a self-aligned recessed gate exhibiting record 4.2 W/mm at 94 GHz on Sapphire

GaN based high electron mobility transistors have emerged as a leading technology for mm-wave solid state power amplification at W-band. To date, reports on W-band GaN HEMTs and MMICs have primarily featured devices grown in the Ga-polar orientation [1, 2]. In this work, the advantages of the N-polar orientation are exploited to produce a MISHEMT exhibiting record high 4.2 W/mm peak output power (Pout) at 94 GHz. The key enabling advantage of N-polar GaN devices are their inverted polarization fields. These fields create a natural, charge-inducing back-barrier that decouples the tradeoff between aspect ratio and channel electron density. Further, the reversed field in an AlGaN cap above the GaN channel opposes gate leakage and improves breakdown voltage [3]. Additionally, to mitigate surface-state induced dispersion and enhance the conductivity of the access regions, a GaN cap layer is added in the access regions through which the gate is recessed [4]. The fabrication process reported in this paper extends that of [4, 5] to have the foot gate metal deposited in a self-aligned fashion to the GaN cap recess etch.