A semiconductor device comprises a semiconductor substrate of a low impurity concentration, a channel region formed on the substrate and having a low impurity concentration, a source region formed on the channel region and having a high impurity concentration of a conductive type opposite to that of the substrate, and a drain region formed on the channel region and having a high impurity concentration of a conductive type opposite to that of the substrate. The source region and the drain region are arranged along a predetermined direction along the substrate. The semiconductor device further includes an accumulating gate region of a conductive type same as that of the substrate, so formed as to surround either one of the source region and drain region, leaving a part of said channel region sandwiched between the source region and the drain region. A current flows from either to the other of the source region and the drain region through the part of the channel region sandwiched between the source region and the region. A charge is accumulated in the accumulating gate region according to the intensity of radiation incident on the device. The potential of the accumulating gate region is variable according to the accumulated charge. The current is variable according to the potential of the accumulating gate region.