Record-high electron mobility at near pinch-off in N-polar GaN HEMT structures grown on on-axis N-polar GaN substrates by plasma-assisted molecular beam epitaxy

In power amplifiers, achieving maximum drain efficiency requires deeper class-AB operation. Class-AB devices are biased near pinch-off, where the charge density is low. N-polar gallium nitride (GaN) high electron mobility transistors (HEMTs) have demonstrated exceptional performance, but they face a challenge of reduced electron mobility at low charge densities near pinch-off. This contrasts with conventional Ga-polar GaN HEMTs, where electron mobility increases as charge density decreases. In this work, we present an increase in electron mobility as charge density was reduced by applying a higher gate reverse bias in N-polar HEMT structures. The N-polar GaN HEMT structures presented here were grown on low-dislocation density on-axis N-polar GaN substrate using plasma-assisted molecular beam epitaxy (PAMBE). Epi-structures with different barrier materials, including InAlN and AlN/GaN digital alloy, were investigated. Electron mobility as a function of 2-dimensional electron gas (2DEG) density was extracted using a combination of the gated transfer length method and capacitance–voltage measurements. A record high mobility of 2000 and 6000 cm2/(V s) was achieved, at room temperature and 85 K, respectively, near pinch-off at a carrier density of 2.4 × 1012 cm−2. This study shows the potential of growth on on-axis substrates by PAMBE to improve efficiency in N-polar HEMTs.