PURPOSE: To inhibit the lowering of punch-through breakdown strength, the decrease of electric-field drive and the lowering of current driving force by reducing the size of a channel region to an extent that depletion layers extending from a channel surface when gate potential is applied so as to invert the channel surface are brought into contact mutually before an inversion layer is formed.

CONSTITUTION: A channel region 3 in width (w) is shaped between source-drain sections 1, and a gate insulating film 4 is applied so as to coat the channel region and a gate region 2 further onto the gate insulating film 4, thus forming a gate section. That is (w) is brought to not more than size where depletion layers extending into the channel region when voltage is applied to a gate electrode are brought into contact mutually before an inversion layer is shaped onto the surface of the channel region. Consequently, the inversion layer is formed effectively into the channel region 3 by applying voltage to the gate electrode 2, thus increasing the mobility of electrons in the region, then elevating current driving force. Punch-through currents are prevented in such a field-effect type semiconductor device.