Optimization and Characterization of GaN-Based High Electron Mobility Transistors

Because of their outstanding combination of physical properties, GaN-based high-electron-mobility transistors (HEMTs) are very attractive components for highpower, high-speed, and high-temperature applications. This dissertation focuses on highly scaled GaN-based HEMTs for millimeter-wave frequencies.

A device fabrication process for fully-passivated GaN HEMTs was developed. The two principal isolation technologies of mesa dry-etching and ion implantation were implemented and compared. Low resistance Ohmic contacts were achieved. The T-shaped gate process was implemented for GaN HEMTs, enabling the gate lengths shorter than 0.1 µm to be realized.

High-performance AlGaN/GaN HEMTs grown on high-resistivity silicon (HR-Si) substrates were implemented. Multiple records were set for the switching speed of AlGaN/GaN HEMTs on silicon substrates: from 75 GHz, 90 GHz, and 114 GHz to a new record now of 130 GHz. These are to be contrasted to the previous published record of 27 GHz for a 0.1 µm GaN HEMT on Silicon: a near 5-fold improvement was achieved, proving the interest of GaN-on-Si for millimeter-wave applications. Our devices also feature a noise figure Fmin = 0.65 (1.2-1.3) dB and associated gain GA = 12 (9.3) dB at 10 (20) GHz. A comparison of the microwave small-signal performance of 0.1 µm HEMTs on HR-Si and on insulating sapphire was performed. The extracted small-signal equivalent circuit models revealed no differences attributable to the different substrates.

This thesis has also defined the state-of-the-art performance on AlInN/GaN
HEMTs. A record fT of 143 GHz and fMAX of 176 GHz were demonstrated for GaNon-Si
HEMTs. An even higher fT of 205 GHz was achieved on SiC substrate, the highest ever reported for AlInN/GaN HEMTs to date. This is also the first demonstration of fT beyond 200 GHz on any GaN–based HEMTs. The microwave noise performance of AlInN/GaN HEMTs was also characterized for the first time. At 10 (20) GHz, our devices feature a noise figure Fmin = 0.62 (1.5) dB and
associated gain GA = 15.4 (13.3) dB. The Fmin values are among the lowest reported
in nitride HEMTs and the GA values are the best so far found in the literature.