High performance of AlGaN/GaN HEMT with AlN cap layer

In this study, GaN high electron mobility transistors (HEMTs) with a 2 nm AlN cap layer (AC-HEMTs) were fabricated on SiC substrate. The AlN/AlGaN/AlN/GaN heterostructures feature smooth surface and low sheet resistivity. Compared to GaN HEMTs with a 2 nm GaN cap layer (GC-HEMTs), higher two-dimensional electron gas (2DEG) density was observed in AC-HEMTs due to the passivation and enhanced polarization effects from AlN cap layer. The introduction of AlN cap layer results in fewer additional polarization charge and higher 2DEG density, both of which weaken polarization Coulomb field (PCF) scattering intensity. Thus, AC-HEMTs exhibit higher electron mobility and better DC electrical characteristics including higher drain current, lower on-resistance, larger on/off current ratio and higher transconductance. Moreover, AC-HEMTs show prominent improvement of breakdown voltage from 625 V (GaN cap) to 855 V (AlN cap).