Jul 19 (Tue) @ 12:00pm: "Improving Gain and Efficiency of N-polar GaN Deep Recess HEMT for mm-Wave Applications," Weiyi Li, ECE PhD Defense

Date and Time
Engineering Science Building (ESB), Rm. 1001


GaN-based high electron mobility transistors (HEMTs) have been demonstrated 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, the N-polar (000-1) GaN deep recess HEMTs have outperformed them with excellent output power and efficiency at 94GHz. However, the ability of GaN HEMT to produce high efficiency with high output power density (Po) is still limited by the gain of the device. Higher gain is needed for GaN HEMT to improve its performance at W-band and achieve good efficiency and output power at higher frequency.

Works in this dissertation focus on improving gain and efficiency of N-polar GaN deep recess HEMTs. To get deeper understanding of electron transport in short-gate-length N-polar GaN transistor, a physics-based model on transport has been proposed and shows good agreement on device DC transfer characteristics and RF cut-off frequency with experimental results. The fringing capacitance in the N-polar GaN deep recess HEMTs was also extracted to check the effects from different ex-situ passivation and in-situ GaN cap passivation. After doing the electrostatics study, thin GaN cap layer (20nm) and Atomic Layer Deposition (ALD) Ru gate have been implemented with great commercial N-polar GaN-on-sapphire epi for device processing. The fabricated GaN-on-Sapphire devices demonstrated record 94GHz large signal performance with record 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. The 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 with lower effective mass and strain GaN channel were proposed to provide higher electron velocity. The first GaN/InGaN high electron mobility transistor with a relaxed InGaN channel has been fabricated utilizing a porous GaN buffer obtained 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 and its effect on performance of GaN HEMT are 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 enhanced electron velocity.


Weiyi Li is a Ph.D. candidate in the Department of Electrical & Computer Engineering at the University of California, Santa Barbara. His research focuses on III-nitride semiconductors for high-speed and high-power electronics, under the supervision of Prof. Umesh Mishra. He obtained his Bachelor’s degree from Zhejiang University, China in 2014, and Master’s degree from University of Chinese Academy of Science, China in 2017. His research attempts to understand the device physics, material and processing for achieving high-speed and high-power electronics


Hosted by: Professor Umesh Mishra

Submitted by:  Weiyi Li <weiyili@ucsb.edu>