Patent US 4,841,349

A semiconductor photodetector device comprises an insulating gate field effect transistor having a gate in which a PN junction (J) is formed on an insulating layer. The gate is formed of a gate electrode (14) of P.sup.+ -type single crystalline silicon and a gate extension portion (17) of N.sup.+ -type single crystalline silicon. Electric charges generated by a light falling on an area including the PN junction are accummulated in the gate electrode (14). A signal of the accumulated electric charge is amplified by the transistor to obtain an output signal (V.sub.out) for detection.