A semiconductor element according to the present invention can perform both a transistor operation and a diode operation via its channel layer. If the potential Vgs of its gate electrode 165 with respect to that of its source electrode 150 is 0 volts, then a depletion layer with a thickness Dc, which has been depleted entirely in the thickness direction, is formed in at least a part of the channel layer 150 due to the presence of a pn junction between a portion of its body region 130 and the channel layer 150, and another depletion layer that has a thickness Db as measured from the junction surface of the pn junction is formed in that portion of the body region 130. If the dielectric constant of the wide bandgap semiconductor is identified by ∈s, the dielectric constant and the thickness of the insulating film 160 are identified by ∈i and Di, respectively, the sum of Dc and Db is identified by Ds, and the absolute value of the turn-on voltage of the diode is identified by Vf0, then Ds