Performance of Strained AlInN/AlN/GaN HEMTs with Si3N4 and Ultra-Thin Al203 Passivation

K.D. Chabak, D.E. Walker Jr., A. Crespo, M. Trejo, M. Kossler, J.K. Gillespie, R. Gilbert, B. Poling, G.D. Via
Sensors Directorate, Air Force Research Laboratory
Wright-Patterson Air Force Base, OH 45433
J. Yang, R. Gaska
Sensor Electronic Technology
Columbia, SC 29209

Keywords: piezoelectric stress, high frequency, strained AlInN, GaN, HEMT

Abstract

We report on the dc/RF characteristics of AlInN/AlN/GaN HEMTs on SiC substrates grown by MOCVD. The novel barrier layer contains a lower Indium concentration (15%) which induces a lattice strain-induced piezoelectric electric field to increase the two-dimensional electron gas density. HEMTs were fabricated with a 2x150-μm gate periphery, T-gate length LG~ 100 nm and source-drain spacing LSD~ 1.5 μm. The total barrier thickness was 6-nm AlInN with 1-nm interlayer as shown in Fig. 1. The active region sheet resistance was ~240 ohm/sq, and the contact resistance was ~0.45 ohm-mm measured by TLM structures. Two types of passivation layers were compared—100 nm PECVD Si3N4 versus an ultra-thin 5 nm Al2O3 film deposited by thermal atomic layer deposition. Interestingly, the ALD-deposited sample showed higher small signal gain which is depicted in Fig. 2. We show analytically this is likely a result of a decreased gate capacitance from a thin dielectric layer. While the small signal performance is as good or higher with the ALD passivation the pulsed I-V characteristics remain inferior to PECVD Si3N4. We also attempt to reflect the dc-to-RF dispersion with a comparison of Ka-band power performance.

Paper 7b.3.pdf