1. Field of Invention
An aspect of the invention relates to a thin-film transistor, a method of producing a thin-film transistor, an electronic circuit including a thin-film transistor, a display including a thin-film transistor, and an electronic device including a thin-film transistor.
2. Description of Related Art
In recent years, thin-film transistors using an organic material behaving as a semiconductor in electrical conduction (organic semiconductor material) have been developed. Thin-film transistors of this type have an advantage that a semiconductor layer can be produced by a process using a solution without needing a high-temperature process or a high-vacuum process. The thin-film transistors of this type are also advantageous in that they can be in a thin and light form, they have good flexibility, and they need low material cost. Because of those advantages, they are expected to be used as switching devices in a flexible display or the like.
It has been proposed to produce a thin-film transistor using organic materials for its gate electrode, gate insulating layer, source electrode, drain electrode, organic semiconductor layer, and alignment layer. An example of such a thin-film transistor may be found, for example, in 2000 International Electron Device Meeting Technical Digest, p. 623-626. This thin-film transistor can produced by the following production process.
First, a partition wall, which will be converted in a next step into an alignment layer, is formed on a substrate such that an area in which to form a source and an area in which to form a drain are surrounded by the partition wall, and a source electrode and a drain electrode are formed in the respective areas surrounded by the partition wall. The partition wall is then rubbed in a direction parallel to a channel direction thereby converting the partition wall into an alignment layer.
Thereafter, an organic semiconductor material is coated on the alignment layer and the organic semiconductor material is heated to a temperate at which the organic semiconductor material changes into a liquid crystal phase. Thereafter, the organic semiconductor material is cooled rapidly. As a result, an organic semiconductor layer aligned in a direction along the channel length is obtained. Thereafter, a gate insulating film is formed on the organic semiconductor layer, and a gate electrode is formed on the gate insulating film.
One of physical characteristics that determine the performance of the thin-film transistor is a carrier mobility of the semiconductor layer. The operating speed of the thin-film transistor increases with increasing carrier mobility of the semiconductor layer. However, the carrier mobility of the organic semiconductor layer is generally two or more orders of magnitude smaller than that of semiconductor layers formed of an inorganic material such as silicon, and thus it is very difficult to realize a thin-film transistor using an organic semiconductor layer having high performance and operable with a small driving voltage.
To improve the carrier mobility, investigation on many types of organic materials for organic semiconductor layers has been made. The carrier mobility depends on the gate voltage applied to the semiconductor layer via the gate electrode and also on the relative dielectric constant and the thickness of the gate insulating layer. Thus, it is also important to select a proper material for the gate insulating layer and a proper process of producing the gate insulating layer. In this regard, it has been proposed to dispose an alignment layer such as that described above to align the organic semiconductor layer in a particular direction.
However, sufficient investigation has not been performed on the optimum layer structure, and there is room for improvement in the layer structure. For example, in a case in which after an alignment layer and an organic semiconductor layer are formed, a gate insulating layer and a gate electrode are formed on the organic semiconductor layer, there is a restriction that the gate insulating layer and the gate electrode must be formed in such a manner that does not cause degradation in characteristics of the organic semiconductor layer.
In other words, when the organic semiconductor layer is formed, if the organic semiconductor material is exposed to a temperature higher than a temperate at which the organic semiconductor layer changes into a liquid crystal phase, the organic semiconductor layer is brought into a randomly aligned state, and, as a result, a great reduction in carrier mobility occurs. Besides, if the organic semiconductor layer is exposed to a temperature higher than that temperature, it loses properties of semiconductor. Another problem with the organic semiconductor layer is that it is easily damaged by an etchant such as a sulfuric acid used in photolithography process.
For the above-described reasons, high-temperature film deposition techniques such as plasma CVD or sputtering and photolithography process cannot be used to form the gate insulating film and the gate electrode. Any material that needs a similar micro fabrication technique cannot be employed. Thus, when a thin-film transistor is formed using an organic semiconductor layer, a high enough carrier mobility of the organic semiconductor layer is not achieved, and thus a high driving voltage is required, and the operating speed is low.