Characterization of Wide Bandgap and Ultrawide Bandgap III-N RF Devices for High Speed and Extreme Environment Applications

Abstract

This thesis presents an in-depth characterization and modeling study of wide and ultra-wide bandgap (WBG/UWBG) high electron mobility transistors (HEMTs), targeting key performance metrics such as intrinsic electron velocity, high-temperature reliability, and small-signal parasitic analysis. The work is divided into three core components, each addressing a critical aspect of device behavior relevant to advanced RF applications. In the first part, high-Al content AlGaN channel HEMTs grown on sapphire were analyzed using a delay-based approach to extract effective intrinsic electron velocity. Small-signal S- parameter measurements, combined with parasitic de-embedding and capacitance scaling, yielded a peak velocity of 1.15 × 107 cm/s for a device with a 245 nm gate length. This velocity, after accounting for fringing fields and parasitic delays, ranks among the highest experimental values reported for AlGaN-channel HEMTs and highlights their potential for high Johnson’s figure of merit (JFOM) RF designs. In the second part, a small-signal equivalent circuit model was constructed in Keysight ADS to validate the analytical delay framework and S-parameter extractions. Complementary TCAD simulations and calculations estimated the electric field under the gate, confirming velocity saturation. The close agreement between simulated and calculated fields (~0.65–0.67 MV/cm) supports the accuracy of the extraction and the reliability of the modeling approach. These methods were further analyzed for their potential to derive velocity Vs Electric field curves highlighting their underlying assumptions, benefits, and limitations and examine the distinctions between experimental and simulation based approaches.

The final part investigates the high-temperature performance of a short-channel GaN HEMT with a 150 nm Schottky Ni/Au gate. DC and RF measurements were conducted from room temperature up to 150 °C. The device maintained a peak cutoff frequency (ft) of 56 GHz at 25 °C, degrading by 27 % at 150 °C due to phonon-limited mobility and increased parasitics. Intrinsic velocity dropped from 1.14 × 107 cm/s to 5.9 × 106 cm/s, and dispersion increased from 8 % to 29 %, driven by thermally activated traps. Despite this, gate leakage and threshold voltage shift remained minimal, underscoring thermal robustness. Together, these results offer a unified framework for understanding transport physics, thermal degradation, and parasitic-limited frequency behavior in GaN and AlGaN HEMTs. The insights developed serve as tools for future scaling, reliability, and performance optimization in WBG/UWBG RF technologies.