JP2725587B2 - Field-effect transistor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a field-effect transistor using a .pi.-conjugated polymer for a semiconductor layer, and more particularly, to a poly-substituted thiophene having a chlorosilyl group as a semiconductor layer. The present invention relates to a field-effect transistor having a large off ratio and advantageously used as a switching element.

[0002]

2. Description of the Related Art π-conjugated polymers such as polythiophene and polythienylenevinylene exhibit semiconducting properties, and have flexibility not found in inorganic materials such as silicon and gallium arsenide. In addition, a π-conjugated polymer which is soluble in an organic solvent can be synthesized by introducing a substituent, and a thin film can be formed by a simple method such as a spin coating method or a dipping method (immersion method). For this reason, rectifiers and field-effect transistors using a π-conjugated polymer have been experimentally manufactured, and certain characteristics have been obtained.

As a conventional example using a π-conjugated polymer as a semiconductor layer of a field-effect transistor, there is known a method in which a π-conjugated polymer thin film is formed by electrolytic polymerization (see, for example, JP-A-62-85467). ), A solution of a π-conjugated polymer or a solution of a π-conjugated polymer precursor (for example, JP-A-5-110069). FIG. 2 shows an example of the element structure of these conventional π-conjugated polymer field effect transistors.

[0004] Studies have been made to apply a field effect transistor using an organic material to a pixel driving element of an active matrix type liquid crystal display. In this case, the on / off ratio is high, that is, the off current (current flowing between the source and drain when the gate voltage is 0 V) is small, and the on current (when the voltage is applied to the gate electrode, between the source and drain) A large flowing current) is required for improving the contrast and increasing the response speed.

[0005] In order to reduce the off current, it is necessary that the conductivity of the semiconductor layer at the time of off is low. As for the on-state current, the value of the field-effect mobility described below is important.

In general, in a field effect transistor, when a sufficient voltage is applied between the source and the drain, a current _ID flowing between both electrodes is known to be expressed by the following equation (where ON Only current is considered).

[0007] _{I D = (W / 2L)} μ FE C (V G -V th) 2 (II) (W: channel width, L: channel length, mu _FE: field-effect mobility, C _i: a gate insulating capacitance per unit area of the film, V _G: gate voltage, V _th: threshold voltage), where the field-effect mobility (mu _FE) is determined from the relationship between the oN current and the gate voltage of the field-effect transistor ,
It represents the effective carrier mobility of the current flowing through the semiconductor layer when turned on. As can be seen from this equation, in order to obtain a large on-current in a field effect transistor, it is necessary to use (II)
The field effect mobility (μ _FE ) in the equation needs to be large.

[0008] Early π-conjugated polymer field-effect transistors had low on-current and low on-off ratio.
Various attempts have been made to improve the on-current and the on / off ratio.

In Japanese Patent Application Laid-Open No. H5-110069, by applying a new material to a semiconductor layer, a considerably high 1 × 10 ⁻¹ is used as an example of using a π-conjugated polymer as a semiconductor layer of a field-effect transistor. A value of the field effect mobility of cm ² / V · s is obtained. However, in this example, the off current (the current flowing between the source and the drain when the gate voltage is 0 V) has also increased, and the on / off current has increased.
It has not led to an improvement in the off ratio.

In order to give priority to improving the on / off ratio, there is an example of a π-conjugated polymer field effect transistor which realizes an on / off ratio of 5 digits by reducing the off current (Applied. Physics Letter,
(Applied Physics Letter),
62, 1794, 1993). But in this case,
The field-effect mobility is limited to 2 × 10 ⁻⁴ cm ² / V · s, and a large on-current cannot be obtained. As described above, it has conventionally been difficult to simultaneously reduce the off-current and increase the on-current.

As a technique for increasing the ON current and increasing the ON / OFF ratio, two types of organic semiconductor layers having different carrier densities are stacked on a gate electrode, and two organic semiconductor layers are formed in response to voltage application from the gate electrode. There is one in which the electric conductivity between a source and a drain is changed by moving carriers between layers (Japanese Patent Application Laid-Open No. 5-48094). However, this method requires a two-step procedure for forming a semiconductor layer, and is complicated.

Many attempts have been made to control the arrangement of molecular chains by stretching in order to improve the conductivity of the π-conjugated polymer film, and many anisotropies in the conductivity of the stretched film have been observed. Have been. It is also known that the conductivity increases as the crystallization of the molecular chains progresses. Thus, the arrangement state of the molecular chains has a great influence on the electric conduction characteristics. From this, it is considered that the field-effect mobility of the π-conjugated polymer field-effect transistor largely depends on the arrangement state of the molecular chains in the semiconductor layer.

Applying a voltage between the source and the drain and applying a voltage to the gate electrode causes an on-current to flow between the source and the drain because a conduction channel is formed near the interface between the semiconductor layer and the insulating layer. It is considered to be. On the other hand, even when a conduction channel is not formed near the interface, if a voltage is applied between the source and the drain, a region other than the vicinity of the semiconductor layer / gate insulating film interface (bulk) unless the semiconductor layer is a perfect insulator. A small amount of current flows between the source and the drain via This is the cause of the off current. When the conductivity of the entire semiconductor layer is increased by doping or improving the carrier mobility of the entire semiconductor layer, the on-current is increased, but the off-current flowing through the bulk is also increased.

That is, the arrangement of molecular chains is controlled only near the interface with the insulating layer without increasing the carrier mobility of the current flowing through the bulk of the semiconductor layer, and the carrier mobility is reduced only for the current flowing through this portion. If it is increased, the on-current can be increased without increasing the off-current.

Π- in the form of being oriented laterally on the surface of a solid
As a technique for forming a conjugated polymer, there is a method using a chemisorption monomolecular film (for example, see Japanese Patent Application Laid-Open No.
No. 86531). In this method, a molecule having a Si-Cl group and a π-conjugated monomer moiety is chemically adsorbed on a solid surface to form a monomolecular film, and then a polymerization reaction is caused to cause a π-conjugated molecule fixed on the solid surface. This is to obtain a system polymer. However, in this method, only a monolayer of a π-conjugated polymer ultrathin film can be obtained by a series of operations.
When used as a semiconductor layer of a field effect transistor,
It is difficult to pass a sufficiently large current.

[0016]

As described above, conventionally, a field-effect transistor using a π-conjugated polymer for a semiconductor layer can simultaneously improve the on-current and reduce the off-current. It was difficult. In addition, the prior art for solving this problem has a complicated manufacturing method.

The present invention has been made to solve this problem, and has as its object to provide a field-effect transistor which has a large field-effect mobility and a large on / off ratio and can be manufactured by a simple method. .

[0018]

Means for Solving the Problems The present inventors have applied an electric field effect to a semiconductor layer obtained by applying a solution of a polymer represented by the following general formula (I) to a solid surface and drying the solution. The present inventors have found that a type transistor exhibits a higher field-effect mobility and an on / off ratio than those using a conventional π-conjugated polymer for a semiconductor layer, and have reached the present invention.

That is, the field-effect transformer according to the present invention
The semiconductor has a semiconductor layer represented by the general formula (I) (where R₁ Is carbon
Number 4 or more and 22 or lessAlkyleneGroup,Oxyalkylene
Group,AlkenyleneGroup,PhenyleneSelected from the group
R_Two, R_Three Is -Cl, -H, -CH_Three , -C_Two 
H_Five Π-conjugated polymer represented by:
It is characterized by consisting of.

[0020]

Embedded image

Here, in the general formula (I), the adsorptive substituent moiety (-R ₁ SiR ₂ R ₃ Cl) may be bonded to either the 3-position or 4-position of the thiophene skeleton. In other words, the adsorptive substituent may not be bound to the same side of all monomer units in the polymer. R ₁ is the number of carbon atoms
3 is the length the material follows poor solubility in nonaqueous organic solvents, inconvenient in a semiconductor forming process to be described later. Further, when a material having a carbon number of 23 or more for R ₁ is used for the semiconductor layer, problems such as insufficient field effect mobility and poor molecular aggregation occur. Accordingly, R ₁ is in the range of 4 to 22 carbon atoms, most preferably 6 to 10 carbon atoms.
A number of carbon atoms is preferred.

The gate electrode, source electrode and drain electrode are not particularly limited as long as they are conductive materials.
Gold, silver, nickel, chromium, copper, iron, tin, antimony lead, tantalum, indium, aluminum, zinc, magnesium, their alloys, conductive metal oxides such as indium and tin oxide, or doping Inorganic and organic semiconductors with improved conductivity, such as silicon single crystal, polysilicon, amorphous silicon, germanium, graphite, polyacetylene, polyparaphenylene, polythiophene, polypyrrole, polyaniline, polythienylenevinylene, polyparaphenylenevinylene, and the like. Can be As the source electrode and the drain electrode, those having low electric resistance at the contact surface with the semiconductor layer are preferable among the above.

As the insulating film, any material may be used as long as the polymer represented by the general formula (I) exhibits an adsorbing action, but a film having a hydroxyl group (-OH) on its surface is preferable. This is because a dehydrochlorination reaction occurs with the chloro group (-Cl) in the general formula (I), and a covalent bond is formed. Further, those having a high dielectric constant and a low conductivity are preferable. Examples include silicon oxide, silicon nitride, aluminum oxide, titanium oxide, tantalum oxide, polyethylene, polyimide, and the like.

As a method for manufacturing a semiconductor layer, the general formula (I)
Any method may be used as long as the polymer chain near the interface of the gate insulating film works so as to orient the polymer horizontally at the interface, but this polymer may be used in a non-aqueous organic solvent. A method of dissolving, applying and drying is preferred. Examples of the non-aqueous organic solvent include, but are not limited to, chloroform, dichloromethane, toluene, xylene and the like. Examples of the coating method include a spin casting method, a dipping method, a dropping method, a letterpress or intaglio printing method, and an ink jet method. Further, it is desirable that impurities are not mixed into the semiconductor layer as much as possible. This is because impurities serve as dopants to generate unnecessary carriers, which causes an increase in off-state current.

The structure of the field-effect transistor in the present invention is not limited to a thin film type, but may be a three-dimensional type such as a cylindrical type.

[0026]

When the material represented by the general formula (I) is dissolved in an organic solvent and applied on the surface of the gate insulating film, the surface of the gate insulating film and the π-conjugated polymer chlorosilyl An adsorption action acts between the base portion and the base portion, thereby affecting the arrangement state of the polymer chains near the gate insulating film interface. Specifically, it is considered that the polymer chains in this region are horizontally oriented at the interface. Thus, a semiconductor layer in which the electric conductivity in the region near the gate insulating film interface is different from that of the bulk is obtained. That is, the conductivity of the bulk of the semiconductor layer can be reduced, and the carrier mobility of the current flowing along the interface near the semiconductor layer / gate insulating film interface can be increased. Therefore, when operated as a field-effect transistor, the field-effect mobility can be high and the on / off ratio can be high.

According to this method, the control of the conductivity over the entire semiconductor layer (the control of the carrier density by doping,
It is easier to simultaneously improve the field-effect mobility and the on / off ratio than the method of controlling the carrier mobility by the molecular chain length).

Further, since the semiconductor layer can be formed by one solution application, the manufacturing process is simple.

[0029]

EXAMPLES Examples will be shown below, but the present invention is not limited to the examples.

(Embodiment 1) FIG.
1 shows a structure of a conjugated polymer field-effect transistor. First, chromium was deposited on a non-alkali glass substrate 1 to form a gate electrode 2. Next, 500
After depositing a 0 angstrom silicon nitride film 3, chromium and gold were deposited in this order, and a source electrode 4 and a drain electrode 5 were formed by ordinary photolithography.
Subsequently, the substrate was subjected to a surface hydrophilization treatment by immersing the substrate in hydrofluoric acid having a concentration of 10%, and then the substrate was treated with 2% by weight of poly [3- (dimethylchlorosilyl) hexylthiophene].
The semiconductor layer 6 was formed by immersing it in a xylene solution and then slowly pulling it up.

By the above procedure, a π-conjugated polymer field effect transistor having a channel length of 20 μm, a channel width of 2 mm, and a semiconductor layer thickness of about 1 μm was obtained. This transistor had a field-effect mobility of 1 × 10 ⁻¹ cm ² / V · s and an on / off ratio of about 5 digits.

(Example 2) The poly [3-
A π-conjugated polymer field effect transistor was completed according to Example 1, except that (dimethylchlorosilyl) hexylthiophene) was changed to poly [3- (dimethylchlorosilyl) octylthiophene]. This transistor had a field effect mobility of 9 × 10 ⁻² cm ² / V · s and an on / off ratio of about 5 digits.

(Example 3) The poly [3-
Π according to Example 1 except that (dimethylchlorosilyl) hexylthiophene] was changed to a 90: 1 copolymer of poly [3- (dimethylchlorosilyl) octylthiophene] and poly (3-methylthiophene). -Completed a conjugated polymer field effect transistor. This transistor had a field effect mobility of 4 × 10 ⁻² cm ² / V · s and an on / off ratio of about 5 digits.

[0034]

As described above, according to the present invention, a .pi.-conjugated polymer field effect transistor having a large field effect mobility and a large on / off ratio can be obtained by a simple manufacturing method.

[Brief description of the drawings]

FIG. 1 is a sectional view of a field-effect transistor showing one embodiment of the present invention.

FIG. 2 is a cross-sectional view showing an example of a conventional π-conjugated polymer field effect transistor.

[Explanation of symbols]

 1,7 substrate 2,12 gate electrode 3,11 gate insulating film 4,9 source electrode 5,10 drain electrode 6,8 semiconductor layer

Claims (2)

(57) [Claims] 1. A field effect transistor having a semiconductor layer and a gate insulating film, wherein the semiconductor layer is mainly composed of a π-conjugated polymer represented by the general formula (I). The field-effect transistor characterized by the above-mentioned. Embedded image (Wherein R ₁ is an alkylene group having 4 to 22 carbon atoms, O
Kishiarukiren group, alkenylene group, one selected from phenylene group, R _2, R ₃ are each -Cl, -H, -
CH ₃ , —C ₂ H ₅ ) 2. The field effect transistor according to claim 1, wherein a hydroxyl group (—OH) exists on the surface of the gate insulating film.