PhD Defense: "N-Polar GaN MIS-HEMTs with SiNx Passivation for mm-Wave Applications"

Xun Zheng

December 1st (Friday), 11:00am
Elings, Rm 1601

GaN-based high-electron-mobility transistors (HEMTs) have emerged as a leading technology for high frequency RF power applications. While commercial GaN HEMTs are fabricated in the Ga-polar orientation, N-polar HEMTs have demonstrated advantages to achieving both high gain and high voltage operation simultaneously, including low contact resistance, strong electron confinement, improved gate-channel distance scalability, and reduced field near the gate.

Outstanding 103/248 GHz current/power cutoff frequency (fT/fmax) and 114 V off-state three-terminal breakdown voltage (BVDS) were simultaneously achieved with PECVD SiNx passivated N-polar planar MIS-HEMTs on sapphire grown by MOCVD. The record fmax∙ BVDS 28.4 THz∙V exhibits great promise of N-polar HEMTs for mm-wave power amplification. With enhanced film quality of PECVD SiNx passivation layer, DC-RF dispersion control of planar HEMTs was significantly improved. The current collapse and ON-resistance increase were mitigated from 42% and 61% to 25% and 13% respectively at quiescent drain bias (VDS,Q) of 15 V. 10 GHz load-pull measurements verify such dispersion mitigation by demonstrating an enhanced dPout/dVDS,Q from 0.16 W/(mm·V) to 0.23 W/(mm·V) and a reduced drain efficiency drop from 15% to 2%.

To further enhance the power gain at high VDS,Q, MOCVD SiNx passivated HEMTs with a trench-gate design were fabricated and yielded a 318 GHz at VDS,Q = 30 V. The record 9.5 THz·V fmax∙ VDS,Q outperforms other reported GaN HEMT technologies. The small-signal behavior of the trench-gate devices in comparison with planar devices were studied with VDS,Q up to 30 V for the first time.
N-polar HEMT design space of both material and device process were investigated. 1) In comparison with mesa isolation, ion implant isolation technology not only improves the T-gate yield, but also reduces the buffer leakage and enhances the buffer breakdown. 2) Gate leakage is significantly suppressed by increasing the Al mole fraction of AlGaN cap on top of the channel. 3) The enhancement of low field mobility and average electron velocity under the gate of InGaN/GaN composite channel in comparison with pure GaN channel are verified by gated-transfer-length measurements and small-signal performance.

Hosted by: Professor Umesh K. Mishra