A turn-on time of 1.4ns is measured in a normally-off GaN synchronous boost converter switching 400V. The high-speed performance is achieved by significantly improving the GaN switches, packaging and gate drive. A recently developed normally-off, AlN-based insulating-gate, AlGaN/GaN-on-Si HFET operates with a high gate voltage of 6V . The higher gate voltage increases gate current for faster switching. A Multi-Chip-Module (MCM) allows paralleling GaN switch up to 20Arms with low parasitic inductance of ∼ 3.6nH in the power loop. The gate drive uses 50m bare MOSFETs integrated onto the MCM to significantly reduce gate driver inductance to 1nH. The very fast switching results in large drain undershoot of 200V, and gate overshoot of more than 6V. Increasing the gate turn-on resistance to 1.4 eliminates gate voltage overshoot and reduces drain voltage overshoot to ∼20V, at the cost of an increased turn-on time of 3ns.