UC Santa Barbara

Nitrogen-polar gallium nitride (N-polar GaN) based deep recess high electron mobility transistor (HEMT) technology has emerged as a better alternative to the traditional Ga-polar devices. This technology has demonstrated superior output power densities and efficiencies at W-band in part due to its excellent dispersion control. Although the performance has been outstanding so far, the technology needs to be further explored for low-cost and reliable fabrication techniques, optimization of the per transistor power with multi-finger devices, maximization of gain and efficiency, and innovative characterization techniques beyond W-band. This dissertation addresses all these points in detail and thereby helps illuminate a path for future development.Novel fabrication methods with low-cost and high reliability have been explored for N-polar GaN HEMTs. With these techniques, high performance N-polar GaN transistors have been demonstrated. For the maximization of the power per transistor while keeping the gain and efficiency high, air bridged multi-finger devices were explored for the N-polar technology for the first time. An air bridge process, which does not degrade the delicate T-gate structures, was developed and tested on the passive-only structures. The parasitics of the airbridges and their impact on the device gain were modeled and measured. For large periphery devices, the limitations of the passive load pull were identified at 94 GHz. An active load pull system was verified against passive load pull and it was used to characterize >100 μm periphery devices. The measurement results demonstrated record transistor level output powers at W-band: 712 mW with 31.7% PAE from a 4x25 μm device, 1 W with 21.4% PAE from a 4x37.5 μm device. To improve the gain and power-added efficiency (PAE), vertical gate scaling was explored. The optimization of the gate characteristics yielded a very high extrinsic transconductance. The improvement in the aspect ratio resulted in a high linear gain and PAE at 94 GHz. Characterization at W-band and below have been primarily carried out by load pull systems for N-polar GaN technology. For bands beyond W-band, such as D-band, there is no commercial load pull system available today. For this reason, on-wafer pre-matching networks were designed, fabricated, measured, and verified against an innovative modeling technique that does not rely on a large signal model. The fabricated fully pre-matched transistors were then characterized at D-band. The devices demonstrated a high power density and PAE compared to the Ga-polar MMICs at this band. The first demonstration of D-band operation of N-polar GaN with excellent large signal performance underlines this technology’s superiority beyond 100 GHz as well.