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N-Polar GaN Devices for Efficiency and Linearity
N-polar deep-recess GaN broke traditional mm-wave transistor power-density and efficiency limits. The device structure enables high drain current with high breakdown-voltage, high cutoff-frequency, and dispersion management. Improvements to N-polar material, design, and fabrication increased the 94GHz gain to over 9dB, enabling 42% PAE at 4.2W/mm, a record combined efficiency and power, on a lower thermal conductivity sapphire substrate. Non-linear distortion often limits wireless system performance. OIP3/Pdc is one transistor linearity metric, traditionally well below 10dB at mm-wave. N-polar GaN showed exceptional 20dB peak OIP3/Pdc at 30GHz, but was very sensitive to the bias point. Adapting circuit-level derivative cancellation to the transistor-level, by varying Vt in sections along the gate to cancel non-linearities, reduced the bias sensitivity by half. These innovative results show the possibility of N-polar GaN to provide high power, efficiency, and linearity, even on low-cost sapphire substrates, which has the potential to greatly reduce cost of GaN devices.