Optimization of the High Frequency Performance of N-polar Nitride-Based Transistors

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 since the 2DEG is contacted through a lower bandgap material and better electron confinement due to a natural back-barrier provided by the charge-inducing barrier. This project focuses on the development of the high-frequency operation of N-polar GaN transistors from the 15-20 GHz range to the 170-220 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 100-200 O-mum down to 6 O-mum, which is comparable or even better than the contact resistance values reported for low-bandgap semiconductor FETs. This was a ground-breaking result for GaN electronics, because it destroyed the myth that it is not possible to make good ohmic contacts to wide band-gap semiconductors. 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 = 120 nm. These devices were further analyzed for improvements in performance and InAlN-back-barriers were developed as a part of the dissertation to enable vertical scaling of transistors. These devices showed record transconductance of 1100 mS/mm; again breaking the 1S/mm barrier for GaN devices and approaching the numbers reported by the low band-gap semiconductors. 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.