A steep-slope field-effect transistor and a fabrication method thereof are disclosed. The steep-slope field-effect transistor according to an embodiment of the inventive concept includes a source, a channel region, and a drain formed on a substrate; a gate insulating film formed on an upper portion of the channel region; a floating gate formed on an upper portion of the gate insulating film; a transition layer formed on an upper portion of the floating gate; and a control gate formed on an upper portion of the transition layer. The steep-slope field-effect transistor applies a reference potential or more to the control gate to discharge or bring in at least one charge stored in the floating gate.