This application provides a field effect transistor, a method for preparing the field effect transistor, and a switch circuit. The field effect transistor includes a channel layer, a source, a drain, a gate structure, and a gate metal layer; and the gate structure includes a P-type gallium nitride layer and an N-type gallium nitride layer that are disposed in a stacking manner, so that a gate metal/pGaN Schottky diode is replaced with an nGaN/pGaN reverse bias diode, to improve a gate voltage-withstand capability of the field effect transistor, thereby improving a breakdown capability of the field effect transistor. A doping density of the P-type gallium nitride layer is between 1 × 10 18 cm -3 and 1 x 10 19 cm -3 , so that a charge storage effect during operation of a device can be reduced, carriers at the pGaN layer can be exhausted as much as possible, and redundant-charge storage is avoided, thereby improving operating threshold voltage stability of the device. The gate metal layer is in ohmic contact with the gate structure, so that reliability of a connection between the gate metal layer and the gate structure can be improved, thereby improving reliability of the field effect transistor.