Active solid-state devices (e.g. – transistors – solid-state diode – Organic semiconductor material
Reexamination Certificate
1995-05-15
2001-08-21
Chaudhuri, Olik (Department: 2503)
Active solid-state devices (e.g., transistors, solid-state diode
Organic semiconductor material
C257S066000, C257S369000
Reexamination Certificate
active
06278127
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to thin film transistors (TFTs), more specifically, to TFTs that comprise organic active layer material.
BACKGROUND OF THE INVENTION
Thin film transistors (TFTs) are known, and are of considerable commercial significance. For instance, amorphous silicon-based TFTs are used in a large fraction of active matrix liquid crystal displays.
TFTs with an organic active layer are also known. See, for instance, F. Garnier et al.,
Science
, Vol. 265, pp. 1684-1686; H. Koezuka et al.,
Applied Physics Letters
, Vol. 62 (15), pp. 1794-1796; H. Fuchigami et al.,
Applied Physics Letters
, Vol. 63 (10), pp. 1372-1374; G. Horowitz et al.,
J. Applied Physics
, Vol. 70 (1), pp. 469-475, and G. Horowitz et al.,
Synthetic Metals
, Vol. 41-43, pp. 1127-1130. These devices typically are field effect transistors (FETs). Such devices potentially have significant advantages over conventional TFTs, including a potentially simpler (and consequently cheaper) fabrication process, the possibility for low temperature processing, and compatibility with non-glass (e.g, plastic) substrates. Bipolar transistors that utilize both p-type and n-type organic material are also known. See, for instance, U.S. Pat. No. 5,315,129. S. Miyauchi et al.,
Synthetic Metals
, 41-43 (1991), pp. 1155-1158, disclose a junction FET that comprises a layer of p-type polythiophene on n-type silicon.
However, despite considerable research and development effort, “organic” TFTs have not yet reached commercialization, at least in part due to relatively poor device characteristics of prior art organic TFTs.
An important device characteristic of a switching transistor is the on/off ratio of the source/drain current. Prior art organic TFTs typically have relatively low on/off ratios. For instance, H. Fuchigami et al. (op. cit.) recently reported a device that had carrier mobility comparable to amorphous silicon, but had an on/off ratio of only about 20 at −30V gate-source voltage. That paper also discloses purification of semiconducting materials to reduce the carrier scattering by impurities.
H. Koezuka et al. (op. cit.) report attainment of an on/off ratio (modulation ratio) of the channel current of about 10
5
in a device with doped polypyrole-coated (a highly conducting polymer) source and drain contacts. According to these authors, this is the highest on/off ratio achieved in organic FETs. Nevertheless, the reported on/off ratio is still substantially smaller than on/off ratios typically available in conventional FETs and demanded for many potential applications of organic TFTs. Furthermore, the organic TFT had very low carrier mobility (2×10
−4
cm
2
/V·s), and thus would not have been suitable for high-speed operation. European patent application No. 92307470.2 (publication No. 0 528 662 A1) discloses an organic FET that comprises a first organic layer that constitutes a channel between source and drain electrodes and is in contact with a second organic layer that is disposed between the gate electrode and the source and drain electrodes. The first and second organic layers are of the same conductivity type but differ in their carrier concentration.
In view of the potential significance of organic TFTs, it would be desirable to have available such devices that have improved characteristics, including improved on/off ratio of the source/drain current. This application discloses such devices, and a method of making the devices.
U.S. patent application Ser. No. 08/353,032, filed Dec. 9, 1994 by A. Dodabalapur et al., discloses articles that comprise improved organic thin film transistors and a method of making the transistors, and U.S. patent application Ser. No. 08/404,221, filed Mar. 15, 1995 by R. C. Haddon et al., discloses a C
60
-based organic transistor. See also A. Dodabalapur et al.,
Science
, Vol. 268, p. 270 (1995). All of these are incorporated herein by reference.
Definitions and Glossary
An “organic semiconductor” herein is a material that contains a substantial amount of carbon in combination with other elements, or that comprises an allotrope of elemental carbon (excluding diamond), and exhibits charge carrier mobility of at least 10
−3
cm
2
/V·s at room temperature (20° C.). Organic semiconductors of interest herein will typically have conductivity less than about 1S/cm at 20° C.
A “p-type” (“n-type”) organic semiconductor herein is an organic semiconductor in which the Fermi energy is closer to (farther from) the energy of the highest occupied molecular orbital (HOMO) of the molecules or aggregates present in the material than it is to (from) the energy of the lowest unoccupied molecular orbital (LUMO). The term is also intended to mean an organic semiconductor which transports positive charge carriers more (less) efficiently than negative carriers. Positive (negative) carriers are generally referred to as “holes” (“electrons”).
An organic “p-n junction” herein is the contact region between a p-type and a n-type organic semiconductor.
SUMMARY OF THE INVENTION
In a broad aspect the invention is embodied in an article that comprises a novel organic TFT that can have substantially improved characteristics (e.g., on/off ratio), as compared to prior art organic TFTs. Some embodiments of the invention can exhibit p-channel or n-channel transistor behavior, depending on biasing conditions, and need not necessarily have high on/off ratio.
Specifically, the organic TFT comprises organic material, spaced apart first and second contact means (e.g., gold electrodes) in contact with the organic material, and third contact means that are spaced from each of the first and second contact means and that are adapted for controlling, by means of a voltage applied to the third contact means, a current between the first and the second contact means. Significantly, the organic material comprises a layer of a first organic material of a first conductivity type and a layer of a second organic material of a second conductivity type that is in contact with the layer of the first organic material at least in a region between the first and second contact means and forms a p-n junction with the layer of first organic material, the layer of the first organic material being in contact with each of the first and second contact means and being not in contact with the third contact means. The third contact means generally can be identified with the gate contact in prior art devices, and the first and second contact means with the source and drain contacts of prior art devices.
Exemplarily, a TFT according to the invention has exhibited an on/off ratio of more than 10
6
, substantially higher than the ratios exhibited by prior art organic TFTs. The exemplary TFT according to the invention furthermore exhibited relatively high carrier mobility, in excess of 0.003 cm
2
/V·s. Desirably, TFTs according to the invention exhibit an on/off ratio greater than 10
5
at an operating gate voltage, and a carrier mobility of at least 3×10
−3
cm
2
/V·s, all at 20° C.
The layer of the first organic material in TFTs according to the invention can comprise any of the organic materials known to be suitable for use as the active layer in organic TFTs. Among these materials are polythiophene and substituted derivatives thereof such as poly(3-hexylthiophene and poly(3-octylthiophene) polythienylenevinylene, &agr;-hexathienylene (&agr;-6T) and substituted derivatives thereof such as &agr;, &ohgr;-dihexyl-&agr;-6T. Other suitable first organic materials are disclosed in U.S. Pat. No. 5,315,129 and in G. Horowitz et al.,
Synthetic Metals
., Vol. 41-43, pp. 1127-1130, both incorporated herein by reference. Exemplarily, the first organic material is selected from polymers of thiophene of degree of polymerization greater than three (and typically less than 9), polymers of substituted derivatives of thiophene, and poly(thienylenevinylene). Recently we have shown that 2, 2′-bis (benzo [1,2-b: 4,5-b′]dithiophene can have p-type mobility of >10
−4
cm
2
/V·s in
Dodabalapur Ananth
Haddon Robert Cort
Katz Howard Edan
Torsi Luisa
Agere Systems Guardian Corp.
Chaudhuri Olik
Peralta Ginette
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