Thin film transistors (TFT) are broadly used as switching elements for display devices such as liquid crystal display, etc. A cross sectional structure of a typical conventional TFT is shown in FIG. 8. As shown in FIG. 8, TFT comprises a gate electrode and an insulating layer in this order on the substrate, and further, comprises a source electrode and a drain electrode formed above the insulating layer having a predetermined distance between them. Over the insulating layer exposing between the electrodes, a semiconductor layer is formed having partial surfaces of each electrodes. In TFT with such a structure, the semiconductor layer forms a channel region and an electric current flowing between the source electrode and the drain electrode is controlled by a voltage applied to the gate electrode resultantly causing an On-Off operation. A transistor of the type having the above operation is generally called as a field effect transistor (FET). With regard to the FET, two kinds of types, i.e., an n-type FET and a p-type FET are known depending on whether the electric current flows caused by electrons or by holes. Above two kinds of FET are frequently used as a pair being known as complementary metal oxide semiconductor (CMOS), and because employing CMOS realizes a circuit for driving with low power consumption, the FET technology became essential which is incapable of lacking in current integrated circuits.
Conventionally, TFTs were prepared employing amorphous silicon or polycrystalline silicon, however, there were problems that making screens large in display devices or so with the use of TFTs is accompanied by significantly soaring in manufacturing cost because a chemical vapor deposition (CVD) equipment used for preparing TFTs employing the silicon is very expensive. Further, because a film-forming process of the amorphous silicon or the polycrystalline silicon is carried out under an extremely high temperature, causing a limitation in kinds of the material employable as a substrate for TFT, there was the problem that a lightweight polymer film substrate or so is unemployable.
For the purpose of overcoming such a problem, a TFT with the use of an organic substance replacing the amorphous silicon or the polycrystalline silicon is proposed. With regard to the film-forming process for preparing a TFT employing organic substances, a vacuum vapor deposition process or a coating process is well known. Those film-forming processes enable not only to realize making screens large in display devices while suppressing soaring in manufacturing cost but also to relatively reduce a process temperature required for film-forming. Accordingly, a practical use of the TFT employing an organic substance is highly expected because of an advantage in little limitation in a selection of material for a substrate and as a result, a large number of report about TFT employing an organic substance are published. Examples of the report include as follows: F. Ebisawa et al. Journal of Applied Physics, vol. 54, p 3255, 1983; A. Assadi et al. Applied Physics Letter, vol. 53, p 195, 1988; G. Guillaud et al. Chemical Physics Letter, vol. 167, p 503, 1990; X. Peng et al. Applied Physics Letter, vol. 57, p 2013, 1990; G. Horowitz et al. Synthetic Metals, vol. 41-43, p 1127, 1991; S. Miyauchi et al. Synthetic Metals, vol. 41-43, 1991; H. Fuchigami et al. Applied Physics Letter, vol. 63, p 1372, 1993; H. Koezuka et al. Applied Physics Letter, vol. 62, p 1794, 1993; F. Garnier et al. Science, vol. 265, p 1684, 1994; A R. Brown et al. Synthetic Metals, vol. 68, p 65, 1994; A. Dodabalapur et al. Science, vol. 2568, p 270, 1995; T. Sumimoto et al. Synthetic Metals, vol. 86, p 2259, 1997; K. Kudo et al. Thin Solid Films, vol. 331, p 51, 1998; K. Kudo et al. Synthetic Metals, vol. 102, p 900, 1999; K. Kudo et al. Synthetic Metals, vol. 111-112, p 11, 2000; etc.
Further, with regard to the organic substance employable in an organic compound layer of TFT, a multimer such as conjugate polymer or thiophene (disclosed in Japanese Unexamined Patent Application Laid-Open Nos. Hei 8-228034, Hei 8-228035, Hei 9-232589, Hei 10-125924, Hei 10-190001, etc.) metallophthalocyanine compound (disclosed in Japanese Unexamined Patent Application Laid-Open No. 2000-174277, etc.) or condensed aromatic hydrocarbon such as pentacene (disclosed in Japanese Unexamined Patent Application Laid-Open Nos. Hei 5-55568, 2001-94107, etc.) is used singly or as a mixture in combination with another compound each other. However, almost all those organic substances employed for the TFT are materials of p-type FET, and materials of n-type FET are extremely limited, as for the mobility, and to express the performance is small. With regard to the material of n-type FET, for example, Japanese Unexamined Patent Application Laid-Open No. Hei 10-135481 only discloses 1,4,5,8-naphthalenetetracarboxyldiunhydride (NTCDA), 11,11,12,12-tetracyanonaphth-2,6-quinodimethan (TCNNQD), 1,4,5,8-naphthalenetetracarboxyldiimide (NTCDI), etc.; and Japanese Unexamined Patent Application Laid-Open No. Hei 11-251601 only discloses phthalocyanine fluoride.
On the other hand, there is an organic electroluminescence (EL) device as a device similarly using an electronic conduction. However, the organic EL device generally forces to feed electric charges by applying a strong electric field of 106 V/cm or greater across a thickness direction of a ultra-thin film of 100 nm or thinner, whereas it is necessary for the organic TFT to feed charges for several μm or longer with high-speed under an electric field of 105 V/cm or smaller and accordingly, an enhanced electric conductivity becomes necessary for the organic substance itself.
Despite the above circumstances, the conventional compounds in the n-type organic TFT had problems in fast response as transistor because its capability for moving electrons was poor, because a field-effect mobility of electron was small, and because response speed was slow. Further, on/off ratio was also small. The above On-Off ratio is defined as a value obtained by dividing an amount of an electric current flowing between a source and a drain when some gate voltage is applied (On) by an amount of an electric current flowing there when any gate voltage is not applied (Off). A word “On electric current” usually means an amount of a (saturated) electric current at a time when the electric current between the source and the drain saturates while increasing the drain voltage.
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