1. Field of the Invention
The present invention relates to a liquid crystal display device and an electronic device using the liquid crystal display device. The present invention particularly related to a liquid crystal display device using a thin film transistor in a pixel portion, and an electronic device using the liquid crystal display device.
2. Description of the Related Art
In recent years, techniques to form thin film transistors using a semiconductor thin film (with a thickness of approximately several nanometers to several hundreds of nanometers) which is formed over a substrate having an insulating surface have been put to practical use in many electronic devices. Thin film transistors are put to practical use particularly as switching elements in pixel portions of liquid crystal display devices, and research and development is still actively pursued.
As a switching element of a liquid crystal display device, a thin film transistor using an amorphous semiconductor film is used for large panels, and a thin film transistor using a polycrystalline semiconductor film is used for small panels. As a method of forming a polycrystalline semiconductor film, there is known a technique in which a pulsed excimer laser beam is shaped into a linear laser beam by an optical system and an amorphous silicon film is crystallized by being irradiated while being scanned with the linear laser beam.
Further, as a switching element of an image display device, a thin film transistor using a microcrystalline semiconductor film is used (see Patent Document 1: Japanese Published Patent Application No. 4-242724, Patent Document 2: Japanese Published Patent Application No. 2005-49832, and Patent Document 3: U.S. Pat. No. 5,591,987). In addition, as a manufacturing method of a thin film transistor with a purpose of improving characteristics of an amorphous semiconductor film, a method of forming an amorphous silicon film over a gate insulating film, forming a metal film over a top surface thereof, and then irradiating the metal film with a diode laser to modify the amorphous silicon film into a microcrystalline silicon film, is known (see Non Patent Document 1: Toshiaki Arai and others, SID '07 DIGEST, 2007, p. 1370-1373). According to this method, the metal film formed over the amorphous silicon film is provided for converting the light energy of the diode laser into thermal energy, and should be removed afterwards to complete a thin film transistor. In other words, it is a method in which the amorphous silicon film is heated only by conduction heating from the metal film, so that a microcrystalline silicon film, which is a microcrystalline semiconductor film, is formed.