PURPOSE: To eliminate the need for strictly controlling dope quantity and film thickness of a charge supply layer and to make charge capacity larger.

CONSTITUTION: An AlxGa1-xAs epitaxial layer (the third epitaxial layer) on a non-dope GaAs epitaxial layer 3, to be the second epitaxial layer, is to be non-dope, so that it does not work as a charge supply layer, and instead, an n-type impurities area 10 provided so as to reach the second, third and fourth epitaxial layers 3, 14, and 5, respectively, under an input diode 7, is made to work as a charge supply layer.