an emitter comprising a wide bandgap semiconductor having at 300 K a bandgap of 2.2 eV or wider, the wide bandgap semiconductor including a region in which both acceptor impurity atoms and donor impurity atoms are doped, an activation energy of the donor impurity atoms, which is obtained by subtracting a value of an energy level of the donor impurity atoms from an energy level of conduction band edge, being larger than the activation energy of the acceptor impurity atoms, which is obtained by subtracting an energy of a valence band edge from a value of an energy level of the acceptor impurity atoms; and

current supply terminals configured to supply electric current to the emitter for resistively heating the emitter so as to establish the thermionic emission.