Abstract: A power circuit is applicable to a Direct Current (DC) to DC converter. The power circuit includes a gate driver circuit and a High Electron Mobility Transistor (HEMT). The gate driver circuit functions as a Sigmoid (S) function and controls a gate and a source of the HEMT with a cross voltage of the sigmoid (S) type function. Accordingly, an overall characteristic curve of the HEMT and the gate driver circuit is like a characteristic curve of a single rectifier diode, so as to achieve a rectifying, freewheeling, or reversing effect. In addition, since an energy loss is low when the HEMT is conducted, the energy loss of the whole power circuit is much less than that of a conventional diode.