ABSTRACT: The millimeter-wave frequency spectrum is in increasing demand for wireless applications including communication and radar. Nitrogen-polar GaN devices at UCSB have now shown transformative performance with record-breaking power density of 8 W/mm and excellent efficiency in the W-band (75-110 GHz) frequency range. This talk will discuss some of UCSB's work on deep recess N-polar GaN MISHEMTs which has enabled this performance. The focus will be on this researcher's contribution to UCSB's N-polar GaN device program which has been to develop new methods to characterize and model these devices in a way which provides quick and insightful feedback for designing improved devices. This is achieved by providing unambiguous evaluations of large-signal performance by load pull at extremely high frequency, and using accurately extracted equivalent circuit models to identify which specific aspects of the device can be adjusted to realize improvements in performance. These methods led to improvement in the efficiency at 94 GHz from 17% to 28%, while the power density was increased to 8 W/mm. New characterization and modeling techniques are underway to understand and improve the linearity of the device, because distortion in amplifiers is of increasing importance for complex digital modulation schemes employed in modern communication systems.