A method for operating a semiconductor device is described, the semiconductor device including a high-voltage device and a control circuit coupled to each other on a single chip and the high-voltage device including a source, a drain and a gate. This method applies a drain voltage of about 20V or higher to the drain while the gate and the source are floated, such that the high-voltage device self-turns on to produce a current from the drain to the source charging up the source and forming a source voltage. The source voltage serves as a power source of the control circuit, and the control circuit is driven when the source voltage is higher than the threshold voltage thereof.