1. Field of the Invention
The invention of the present application relates to a semiconductor device having a circuit which is configured of thin film transistors (hereinbelow termed “TFTs”), and a method of fabricating the semiconductor device. By way of example, it relates to an electrooptical device which is typified by a liquid crystal display panel, and an electronic equipment in which such an electrooptical device is installed as a component.
Incidentally, here in this specification, the “semiconductor device” is intended to signify general devices which can function by utilizing semiconductor properties, and electrooptical devices, semiconductor circuits and electronic equipment are all the semiconductor devices.
2. Description of Related Art
In recent years, notice has been taken of technology wherein thin film transistors (TFTs) are constructed using a semiconductor thin film (having a thickness on the order of several˜a few hundred nm) which is formed on a substrate having an insulating surface. The TFTs are extensively applied to ICs and electron devices such as electrooptical devices, and it is especially hurried to develop them as the switching elements of an image display device.
Hitherto, liquid crystal display devices have been known as image display devices. The liquid crystal display device of active matrix type has come to be often employed because an image of higher definition than by the liquid crystal display device of passive type can be obtained. In the active matrix type liquid crystal display device, a display pattern is formed on a screen by driving pixel electrodes arranged in the shape of a matrix. More specifically, voltages are applied between selected ones of the pixel electrodes and ones of counter electrodes corresponding to the selected pixel electrodes, whereby a liquid crystal layer interposed between the pixel electrodes and the counter electrodes is optically modulated, and the optical modulation is recognized as the display pattern by an observer.
The applications of such an active matrix type liquid crystal display device have widened, and a higher definition, a higher aperture efficiency and a higher reliability have been more required together with the larger area of a screen size. Besides, enhancement in productivity and reduction in cost have been more required at the same time.
In the prior art, an amorphous silicon film is suitably employed as an amorphous semiconductor film for the reason that it can be formed on a substrate of large area at a low temperature of or below 300. Also, TFTs of inverse stagger type (or bottom gate type) each having a channel forming region formed of an amorphous semiconductor film are often employed.