Monolithic Integrated AlGaN/GaN Power Converter Topologies on High-Voltage AlN/GaN Superlattice Buffer

Abstract

A high-voltage AlN/GaN superlattice (SL) buffer for monolithic AlGaN/GaN power circuits is experimentally compared with a step-graded AlGaN/GaN buffer. The SL as part of a 5.1 μm epitaxial stack withstands over 1.3 kV. Although the step-graded buffer is sufficient for low-side circuits, the operation voltage of monolithic topologies such as a half-bridge is limited: static negative back gating at −200 V depletes the lateral channel completely. Asymmetrical buffer leakage at a positive substrate voltage of +250 V limits the operation voltage further. The SL buffer mitigates both effects: a negative substrate voltage of −200 V reduced the lateral channel current only by 25%. However, this condition is not required for half-bridge operation on the SL, because low symmetrical vertical buffer leakage at substrate voltages of ±500 V allows operation of power topologies with positive substrate bias. High-electron-mobility transistors (HEMTs) on the graded buffer show excessive threshold voltage shift at negative substrate bias. On the SL buffer, the threshold voltage is shifted only +1 V from negative substrate biases, which allows monolithic high-voltage power topology operation. 98.8% efficient operation of a 6 × 4 mm2 GaN-on-Si power integrated circuit with a monolithic half bridge, freewheeling diodes, and drivers is demonstrated on the SL.