Abstract: An MIS transistor has a channel portion of a first conduction type of first semiconductor material formed on an insulating substrate, second conduction type source and drain regions sandwiching said channel portion therebetween, and a gate electrode formed on a main surface of the channel portion with an insulating film therebetween, wherein the source region is made of the first semiconductor material and a second semiconductor material having an energy band gap smaller than that of the first semiconductor material and a heterojunction between the first and second semiconductor materials is provided outside of a depletion layer region formed in the junction between the source and channel portions, and inside a diffusion length L.sub.d from a depletion edge.