Asymmetrical Substrate-Biasing Effects at up to 350V Operation of Symmetrical Monolithic Normally-Off GaN-on-Si Half-Bridges

The work shows that the efficiency of symmetrical monolithic normally-off GaN half-bridges on semi-floating Si-substrate depends on load current polarity, unlike discrete half-bridges. Discrete and monolithic half-bridges with intrinsic reverse diodes were characterized up to 350V in buck and floating-buck (or boost) converters, varying load current polarity but maintaining voltage and power. Asymmetric effects from negative substrate-to-source back-gate voltage during high-side conduction are observed, which reduce the channel current depending on its polarity: In buck converters, 1st quadrant high-side conduction is degraded by significantly reduced channel current below the gate. In a floating-buck converter, 3rd quadrant high-side conduction current bypasses the degraded gate region through an intrinsic reverse diode, flowing only through the drain access region, which showed lower degradation from negative back-gating. A duty-cycle independent substrate termination network is proposed to shift the average value of the floating substrate voltage towards positive values, avoiding static back-gating of the high-side transistor. It is shown how vertical buffer leakage at positive substrate voltages, which is not relevant for discrete transistors, limits the monolithic half-bridge's operation voltage. Operation of the monolithic normally-off GaN-on-Si half-bridge with semi-floating substrate in a floating-buck (instead of a buck) converter showed efficiencies above 98% at 200W and 200V.