PhD Defense: "N-polar GaN-based self-aligned MIS-HEMTs for ultra-high speed applications"


July 25th (Monday), 1:00pm
Elings Hall (CNSI Building) 1601

AlGaN/GaN based high-electron-mobility transistors have been of interest to the semiconductor community because of their high breakdown voltage, high sheet carrier density, and the high saturation velocity of GaN. However, most established GaN electronic devices are fabricated on the Ga-polar orientation of GaN. Recently, N-polar GaN based devices are being explored for high frequency applications due their advantages over Ga-face, such as lower contact resistance and better electron confinement. This project focuses on the development of N-polar GaN transistors from the 15-20 GHz range to the 170-210 GHz range. To improve the high frequency performance of N-polar GaN transistors, the device physics principles were revisited and scaling behavior of GaN transistors was explored to develop appropriate device designs for high frequency applications. Parasitic resistances have been found to be important parameters limiting the device delay. To reduce parasitic access resistances, self-aligned regrowth of polarization-doped graded InGaN/InN layers was developed. This method helped reduce the contact resistance to the GaN 2DEG from the commonly reported values of 200 ohm-micron down to 6-20 ohm-micron, which is comparable to the contact resistance values reported for low-bandgap semiconductor FETs. GaN/AlGaN-based self-aligned devices fabricated using these ohmic contact regrowth technique show excellent scaling behavior and demonstrated state-of-the-art values of ft.Lg product of 19 GHz-micron up to Lg = 150 nm. InAlN-back-barriers were developed as a part of the dissertation to enable vertical scaling of transistors to get a record transconductance of 1100 mS/mm. Further analysis of InAlN devices was done to understand trap-related effects observed in the rf-performance of these devices. As a result of the dissertation, the N-polar GaN-based MIS-HEMTs are now considered valid candidates for future of high frequency GaN electronics and it has opened a new area of exploration and device engineering to design future devices.

Hosted by: Prof. Umesh K. Mishra