This work presents a driver and controller integrated circuit (IC) for depletion-mode gallium nitride (GaN) high-electron-mobility transistors (HEMTs). The dual-mode driver can be configured for cascode-drive (CD) or HEMT-drive (HD) mode. In the CD mode, a cascode low-voltage DMOS is driven to achieve high-speed normally OFF operation. An active clamping circuit is proposed for the DMOS breakdown protection. In the HD mode, an HEMT gate driver with negative drive-voltage capability and programmable slope control is presented. A digital peak current-mode controller is also integrated with the dual-mode driver. The IC was implemented in a 140-nm automotive bipolar-CMOS-DMOS silicon-on-insulator process. The driver/controller IC is copackaged with an optimized 600-V GaN HEMT fabricated in a GaN-on-Si process. The solution was verified to operate at up to 1 MHz in a 35-W boost converter prototype and achieves a programmable switching-node dv/dt of up to 20 V/ns. To the best of the author's knowledge, this is the first monolithic integration of a cascode MOSFET, device driver, and digital current-mode controller that is designed specifically for high-voltage GaN devices.