May 14 (Thu) 9:00am: "Advances in MOCVD-Grown N-Polar GaN HEMT Heterostructures: InAlGaN Back-Barrier Design, p-GaN Gate Integration, and High-Temperature Operation," Robert Hamwey, ECE PhD Defense

Location: Engineering Science Building (ESB), Room 1001
Zoom Link: https://ucsb.zoom.us/j/89508326386

Abstract

GaN-based HEMTs have played an increasingly important role in high-power and high-frequency applications. Traditionally, these devices have been realized in the Ga-polar orientation, yet recent efforts have shifted focus toward the N-polar orientation, which offers improved two-dimensional electron gas (2DEG) confinement, lower contact resistance, and superior electrostatics for scaling. While significant progress has been made, the full potential of N-polar GaN HEMTs remains to be realized. In this work, novel MOCVD-grown N-polar GaN HEMTs are demonstrated, spanning high-2DEG density back-barriers, Mg-doped p-GaN gate integration, and semi-insulating buffer design for high-performance and high-temperature applications.

The first reported N-polar InAlGaN back-barrier HEMT is demonstrated, achieving an ultra-low 2DEG sheet density of 2.85 × 1013 cm-2 and associated sheet resistance of 179 Ω/□. A fabricated device with a 0.6 μm gate length achieves a peak drain current of 1.92 A/mm and transconductance of 212 mS/mm.

The development of N-polar p-GaN gate HEMTs with AlGaN back barriers is then presented, with emphasis on MOCVD epitaxial engineering. SIMS, Hall, and CV measurements reveal the critical roles of p-GaN growth temperature and Al(Ga)N spacer layers in mitigating Mg back-diffusion and preserving 2DEG integrity. A selective p-GaN etch process is developed to enable HEMT fabrication. High-temperature characterization of fabricated HEMTs reveals stable depletion-mode operation at high temperature, with devices achieving ION/IOFF > 1000 at 800°C. This necessitated the development and integration of C-doped semi-insulating GaN buffers, motivating a comparison of Fe- and C-doped buffer leakage at elevated temperatures — establishing C-doped buffers as the superior platform for high-temperature operation.

Broader epitaxial engineering of the N-polar p-GaN gate platform — initially developed for depletion-mode operation — ultimately enabled enhancement-mode HEMTs with a threshold voltage of +0.68 V at room temperature, expanding the versatility of N-polar GaN HEMTs for next-generation power and RF applications.

Bio

Robert Hamwey is a PhD candidate in the Department of Electrical and Computer Engineering at UC Santa Barbara, advised by Professor Umesh K. Mishra. He received his B.S. in Chemical Engineering from UC Berkeley in 2019 and previously worked as a Process Engineer at Lumentum. His research focuses on MOCVD growth of novel N-polar GaN HEMTs.

Hosted by: Professor Umesh Mishra

Submitted by: Robert Hamwey <rhamwey@ucsb.edu>