Instabilities by Parasitic Substrate-Loop of GaN-on-Si HEMTs in Half-Bridges

The stability of GaN-on-Si HEMTs with substrate-to-source termination is analyzed in a high-voltage half-bridge. The work exposes that external substrate termination creates a parasitic substrate loop, which leads to unstable switching behavior under certain conditions. Stability analysis reveals that parasitic inductance in the substrate-loop alone is sufficient for instabilities, even with zero parasitic inductance in the gate- and power-loops. A systematic analytical stability analysis is carried out based on a small-signal equivalent circuit. The theory is verified by measurements using a PCB-embedded 600 V GaN HEMT with integrated gate driver. Adequate damping of the substrate loop resonance enables stable operation of the half-bridge module.