temperature polysilicon semiconductor layer may also be referred to simply as a polysilicon semiconductor layer fig1 shows schematic cross sectional views for a major part illustrating a fabrication process of a thin film transistor substrate according to embodiment 1 of the invention on a glass substrate sub shown in fig1 a an amorphous silicon semiconductor layer as is deposited fig1 b this amorphous silicon semiconductor layer as is annealed by being irradiated with a laser preferably an excimer laser so that the amorphous silicon semiconductor layer is crystallized to form a polysilicon semiconductor layer ps fig1 c the polysilicon semiconductor layer ps is subjected to a photolithography process so that a polysilicon semiconductor layer island psi is left in the location where a thin film transistor form is to be formed a gate insulating layer gi is deposited so as to cover this island fig1 d a gate electrode gt is patterned at the center portion of the island psi on the gate insulating layer gi using the gate electrode gt as a mask an ldd ion implantation is performed fig1 e next a mask msk is formed so as to cover the gate electrode gt fig1 f this mask msk is formed so as to cover the gate electrode gt and also to avoid source and drain regions in the island psi of the polysilicon semiconductor layer by performing ion implantation in this state ions are implanted in the source and drain regions in the island psi it is desirable to implant the second element of the invention at this stage the mask msk is removed and silicon oxide sio 2 is deposited as an interlayer film is 1 then annealing.