Conventional thin film transistors have a structure such as illustrated, for example, in JAPAN' DISPLAY 86, pages 196-199, published in 1986 and generally illustrated in FIG. 2A, which is a plan view of the structure and FIG. 2B is a cross sectional view taken along the line B-B' of FIG. 2A. The thin film transistor comprises an insulating substrate 201, e.g., glass, quartz or sapphire, and a sour