A conductivity modulation type field effect transistor comprises an n.sup.- type low concentration impurity layer of high resistance formed on an n.sup.+ type silicon substrate, a first channel region of a given width formed on the low concentration impurity layer, a pair of p type gates oppositely formed with the first channel region therebetween, an n.sup.- type low concentration impurity layer formed on the first channel region including the p.sup.+ gate, a p channel layer including two channel regions formed on the n.sup.- type low concentration impurity layer, and a pair of n.sup.+ type sources formed on the second channel region with their center aligned with a center of the first gate means, in which, after the formation of the n.sup.+ type source, a groove is formed at each side of a respective semiconductor device, first gate electrodes are provided on the bottom surface and side wall of the groove with a gate oxide film therebetween whereby the transistor has the blocking capability of achieving a normally OFF state.