A wiring formed on a substrate is oxidized and the oxide is used as a mask for forming source and drain impurity regions of a transistor, or as a material for insulating wirings from each other, or as a dielectric of a capacitor. Thickness of the oxide is determined depending on purpose of the oxide. In a transistor adapted to be used in an active-matrix liquid-crystal display, the channel length, or the distance between the source region and the drain region, is made larger than the length of the gate electrode taken in the longitudinal direction of the channel. Offset regions are formed in the channel region on the sides of the source and drain regions. No or very weak electric field is applied to these offset regions from the gate electrode.