The present disclosure provides a controller for controlling a GaN-based semiconductor device. The controller is configured to receive a current sensing signal VCS which is indicative of a drain-to-source current of the GaN-based semiconductor device and generate a control driving signal VDRV to the GaN-based semiconductor device such that a gate-to-source voltage VGS applied to the GaN-based semiconductor device for switching on the GaN-based semiconductor device is stabilized to a voltage value equal to a reference voltage Vref over an on-time duration. Impact of the change in the voltage drop across the current sensing resistor to the operation of the GaN-based semiconductor device is eliminated.