An integrated circuit structure includes a semiconductor substrate and a high-voltage metal-oxide-semiconductor (HVMOS) device, which includes a first high-voltage well (HVW) region of a first conductivity type in the semiconductor substrate; a drain region of a second conductivity type opposite the first conductivity type in the semiconductor substrate and spaced apart from the first HVW region; a gate dielectric with at least a portion directly over the first HVW region; and a gate electrode over the gate dielectric. The gate dielectric includes a bottom gate oxide region; and a silicon nitride region over the bottom gate oxide region.