A field-effect transistor (FET), which is a type of semiconductor devices, is a transistor configured to apply gate voltage to excite a carrier in a semiconductor in a state that an electric field is applied to a channel so that electric current passed through between a source and a drain can be controlled.
Since the FET is capable of switching by applying gate electrode, the FET is used as various switching elements, or amplifier elements. As the FET has a flat structure as well as using low gate electric current, the FET can be easily produced compared to a bipolar transistor. Moreover, high integration of the FET can be also easily carried out. For these reasons, the FET has been used in many of integrated circuits of current electric devices.
Among them, the FET is applied as a thin film transistor (TFT) in an active-matrix display.
As for an active-matrix flat panel display (FPD), a liquid crystal display (LCD), an organic electroluminescence (EL) display (OLED), and electronic paper have been recently realized.
These FPDs are typically driven by a driving circuit containing a TFT, in which amorphous silicon or polycrystalline silicon is used in an active layer. There are demands for the FPD to be increased in the size, definition, and driving speed thereof. Along with these demands, there is a need for TFTs, which have high carrier mobility, small aging variations in the properties, and small variations between the elements.
Currently, in addition to silicon, an oxide semiconductor has been attracted attentions as a semiconductor of the active layer. Among them, InGaZnO4 (a-IGZO) has characteristics that film formation thereof can be carried out at room temperature, it is amorphous, and it has high mobility of around 10 cm2/V·s.
Therefore, developments thereof for practical use have been actively conducted (see, for example, NPL 1).
The FET typically contains a protective layer for the purpose of protecting a semiconductor layer serving as the active layer. Various researches have been also conducted on the protective layer.
For example, a SiO2 layer (see, for example, PTL 1, and PTL 2), a SiNx layer (see, for example, PTL 1), a SiON layer (see, for example, PTL 3), and an Al2O3 layer (see, for example, PTL 4) are disclosed as a protective layer of the FET. Moreover, a composite oxide layer, in which SiO2 forms a composite with Al or B, is disclosed as a protective layer of the FET (see, for example, PTL 5).
When the SiO2 layer, and the composite oxide layer are formed on the silicon semiconductor layer, oxide semiconductor layer, metal lines, and oxide lines, there are however a problem that a crack or pealing is formed in a heating process of a post processing. Moreover, the SiON layer, SiNx layer, and Al2O3 layer have a problem that signal delay is caused due to parasitic capacitance.
Moreover, use of an organic material in the protective layer has been disclosed.
For example, a polyimide resin layer (see, for example, PTL 6), and a fluororesin layer (see, for example, PTL 3) are disclosed as a protective layer of the FET.
However, a typical organic material has a problem that deterioration of TFT properties is caused as the organic material is brought into contact with an oxide semiconductor. Moreover, the fluororesin layer causes relatively small deterioration of the TFT properties, but which is not sufficiently small for use.
Accordingly, there is currently a need for a field-effect transistor, which enables high speed operation, and exhibits high reliability.