This paper presents a fully-integrated half-bridge GaN driver for bidirectional power transfer. Three-level/bipolar gate driver is introduced for the power switches to improve reliability at high dv/dt. On-chip bootstrap circuit with charge sharing reduces chip area and protects high-side switch. The designed GaN driver is fabricated with a 0.18µm BCD process. Measurement results show that at 5MHz operating frequency, the highest input voltage reaches 40V in buck mode and the highest output voltage ups to 36V in boost mode. The peak efficiencies of the design are 93.21% for the 20V-to-12V conversion and 91.17% for the 12V-to-18V conversion. The design delivers a maximum power of 21.15W in buck mode.