Interface charge engineering in down-scaled AlGaN (<6 nm)/GaN heterostructure for fabrication of GaN-based power HEMTs and MIS-HEMTs

The physical mechanism for recovery of 2D electron gas (2DEG) in down-scaled AlGaN/GaN heterostructures with SiNx layers grown by low-pressure chemical vapor deposition (LPCVD) was investigated by means of Hall-effect characterization, scanning Kelvin probe microscopy (SKPM), and self-consistent Poisson–Schrödinger calculations. Observations using SKPM show that the surface potential of the AlGaN/GaN heterostructure remained nearly unchanged (∼1.08 eV) as the thickness of the AlGaN barrier was reduced from 18.5 to 5.5 nm and likely originated from the surface pinning effect. This led to a significant depletion of 2DEG from 9.60 × 1012 to 1.53 × 1012 cm−2, as determined by Hall measurements, toward a normally OFF 2DEG channel. Based on a consistent solution of the Schrödinger–Poisson equations and analytical simulations, approximately 3.50 × 1013 cm−2 of positive fixed charges were confirmed to be induced by a 20-nm LPCVD-SiNx passivation over the AlGaN/GaN heterostructures. The interface charge exerted a strong modulation of band bending in the down-scaled AlGaN/GaN heterostructure, contributing to the efficient recovery of 2DEG charge density (∼1.63 × 1013 cm−2). E-mode ultrathin-barrier AlGaN/GaN metal–insulator–semiconductor high-electron-mobility transistors with a low ON-resistance (RON), high ON/OFF current ratio, and steep subthreshold slope were implemented using LPCVD-SiNx passivation.