UC Santa Barbara

Gallium Nitride is widely employed for microwave and mm-wave applications. Though GaN HEMTs have primarily been used for power amplification, they are also well suited for receiver applications. In the receiver front-end, the linearity of an RF transistor is an important requisite. In a crowded wireless spectrum with large in band interferers, non-linearities can mask or distort a weak desired signal. At mm-wave frequencies and high data rates, circuit-level linearization techniques increase system complexity. Therefore, this study focuses on linearization at the device-level.

Traditionally, GaN is grown and fabricated in the Ga-polar orientation. In the N-polar orientation, the inverse polarization fields enable the implementation of device structures that provide excellent mm-wave performance. The devices presented in this work are based on N polar GaN MIS-HEMT technology and are designed to achieve high gain and high linearity performance, simultaneously. This dissertation reports the device architecture with a detailed description of the fabrication process, the challenges involved in device fabrication and the solutions explored to overcome them.

Device results including linearity characterization at 30 GHz are discussed. The linearity performance is described through OIP3 and OIP3/Pdc. An OIP3 of 32 dBm, OIP3/Pdc of 15 dB and 12.7 dB transducer gain at 30 GHz were obtained at a bias specific for high linearity. A strategy to achieve high linearity over a wide input bias range is to use derivative superposition at the device-level with a dual-threshold voltage device design. In this approach, the device transconductance (gm) is engineered to achieve a flat gm profile over a wide range of gate-bias. The results of the first demonstration of a dual-threshold voltage N-polar GaN transistor show 10.8 dB OIP3/Pdc, 34 dBm OIP3 and 11 dB transducer gain at 30 GHz.