An active layer of an NTFT includes a channel forming region, at least a first impurity region, at least a second impurity region and at least a third impurity region therein. Concentrations of an impurity in each of the first, second and third impurity regions increase as distances from the channel forming region become longer. The first impurity region is formed to be overlapped with a side wall. A gate overlapping structure can be realized with the side wall functioning as an electrode.