UC Santa Barbara

GaN-based high electron mobility transistors (HEMTs) have emerged to be a leading technology for RF millimeter-wave application. While GaN technologies utilizing the Ga-polar (0001) orientation have shown good performance in W-band, its performance is saturated due to the DC-RF dispersion and the limit on device gate to channel distance. As an alternative, the N-polar (000-1) GaN deep recess HEMTs can overcome those disadvantages and outperform Ga-polar GaN devices with excellent output power and efficiency at 94 GHz. However, the ability to enhance efficiency of GaN-based RF transistor simultaneously with high output power density is still limited by the gain of the device, making high gain a critical need at W-Band at mm-wave frequencies.Works in this dissertation focus on improving gain and efficiency of N-polar GaN deep recess HEMTs. A predictive physics-based model on transport has been proposed to inspect the performance N-polar GaN RF transistor and understand electron transport in devices. The modelling results show good agreement with experimental data on device DC transfer characteristics and RF cut-off frequency. After understanding the predictive behaving manner of the devices, effects on fringing capacitance from ex-situ SiN passivation and GaN cap layer have been studied to evaluate electrostatics of N-polar GaN deep recess devices. With knowledge acquired on transport and device electrostatics, thin GaN cap layer (20 nm) and Atomic Layer Deposition (ALD) Ru gate have been implemented with great commercial N-polar GaN-on-sapphire epi to improve the device gain. The fabricated GaN-on-Sapphire devices demonstrated record 94GHz large signal performance with high linear transducer gain of 9.65 dB, enabling excellent performance at 12 V with record 42% power-added efficiency (PAE) with associated 4.4 W/mm of output power density. Furthermore, at 8 V the device demonstrated even higher PAE of 44% with associated 2.6 W/mm of output power density. After 20 nm PECVD SiN passivation, devices show very high output power density of 5.83 W/mm with a high PAE of 38.5% at 94 GHz. The excellent results demonstrate the great potential of N-polar GaN-on-sapphire technology for mm-wave application with simultaneous high efficiency and power density. Strain engineering on GaN has also been explored for improving the device gain. Both relaxed InGaN channel and strain GaN channel were proposed for electron velocity enhancement. The GaN/InGaN HEMT with a relaxed InGaN channel has been fabricated utilizing a porous GaN buffer achieved by the selective and controlled electrochemical etch of GaN. The results show ~70% of InGaN relaxation relative to GaN and ~10% 2DEG mobility enhancement with respect to the strained InGaN channel. For strained GaN channel, electron effective mass in GaN under the biaxial tensile strain was calculated using first principal DFT calculation. The effect of biaxial tensile strain on device performance of GaN HEMT are also investigated. The results demonstrated the enhanced electron velocity over 15% with 4% compressive biaxial strain and improvement in both DC and RF performance of the transistor with the improved electron velocity.