Abstract: A thin film semiconductor device and a method for producing it are described. In the thin film layer of semiconductor of the device, a plurality of large size single-crystalline grains of semiconductor are formed in a regulated configuration, and each of single crystalline grains is equipped with one unit of electric circuit having a gate electrode, a source electrode and drain electrode. Such regulated arrangement of large size single-crystalline grains in the semiconductor layer is realized by a process including a step of irradiating the layer of amorphous or polycrystalline semiconductor with energy beam such as excimer laser so that maximum irradiation intensity points and minimum irradiation intensity points are arranged regulatedly. The device can have a high mobility such as about 500 cm2/V sec..