PURPOSE: To obtain a TFT which can enhance TFT characteristics such as mobility, a threshold voltage, an S factor and the like, which is excellent in a dielectric breakdown strength frequency characteristic thanks to the removal effect of impurities and particles which are stuck to a surface layer on a polycrystal silicon semiconductor layer.

CONSTITUTION: In the manufacturing method of a semiconductor device, a polycrystal silicon film is formed on a glass substrate 1, the polycrystal silicon film is patterned in a prescribed shape, a semiconductor layer 2 is formed, and a gate insulating film 4 is then formed on the glass substrate 1 and the semiconductor layer 2. In the manufacturing method, before the gate insulating film 4 is formed, the semiconductor layer 2 is cleaned by a mixed solution of ammonia and hydrogen peroxide water, the film thickness of a surface layer 3 on the semiconductor layer 2 which is etched by its cleaning operation is set at 5nm or lower, and, in succession, the semiconductor layer is then cleaned by a mixed solution of hydrochloric acid and hydrogen peroxide water.