1. Field of the Invention
An embodiment of the present invention relates to a thin film transistor and a manufacturing method thereof, and further relates to a display device and an electronic device.
2. Description of the Related Art
In recent years, thin film transistors (hereinafter, referred to as TFTs) each including a thin semiconductor film (with a thickness of approximately several nanometers to several hundreds of nanometers) over a substrate having an insulating surface (e.g., a glass substrate) have been attracting attention. TFTs are widely used for ICs (integrated circuits) and electronic devices such as electro-optical devices. The development of TFTs especially as switching elements of image display devices typified by liquid crystal display devices and the like has been accelerated. In image display devices such as liquid crystal display devices, TFTs each using an amorphous semiconductor film or a polycrystalline semiconductor film are mainly used as a switching element.
The mobility of a TFT using an amorphous semiconductor film is low. In other words, the current driving capability is low. Thus, when a protection circuit is provided with a TFT using an amorphous semiconductor film, a large-sized TFT needs to be provided for adequate measure against electrostatic breakdown, which causes inhibition of a narrower frame. In addition, when the large-sized TFT is provided, parasitic capacitance between a scan line electrically connected to a gate electrode and a signal line electrically connected to a source electrode or a drain electrode is increased, which causes an increase in power consumption.
On the other hand, a TFT using a polycrystalline semiconductor film has higher mobility than a thin film transistor using an amorphous semiconductor film by two or more digits and a pixel portion of a liquid crystal display device and a driver circuit in the periphery thereof can be provided over the same substrate. However, a process of the TFT using a polycrystalline semiconductor film is more complicated than that of the TFT using an amorphous semiconductor film due to crystallization of a semiconductor film, introduction of an impurity element (doping), and the like. Therefore, there are problems of low yield and high cost. As a formation method of a polycrystalline semiconductor film, a technique is widely known in which the shape of a pulsed excimer laser beam is processed into a linear beam by an optical system and an amorphous semiconductor film is irradiated with the linear laser beam while being scanned by the linear laser beam to be crystallized.
As a switching element of an image display device, a TFT using a microcrystalline semiconductor film are known as well as the TFT using an amorphous semiconductor film and the TFT using a polycrystalline semiconductor film (see Patent Document 1).
[Reference]
    [Patent Document 1] Japanese Published Patent Application No. 2009-044134