Modern semiconductor electronics is faced with the problems of operating devices at further decreasing of the design rules. One of them is the impossibility of operating field transistors at a channel length less than 1000 Å. New technology, which called to replace a semiconductor one, should find a new physical principle of operating electronic devices. In our previous papers it was proposed to use the electrical instability of a low-dimensional molecular system as the physical principle of functioning molecular electronic devices in new technology (Krieger 1993, Krieger 1996),. In this paper, we would like to focus attention on the possibility of using this phenomenon for designing a new type of field effect transistors (FET). This principle can allow the creation of FET with a long channel less than 100 Å. In this approach there are several ways of designing FET.
In this paper we are going to present the first results on the creation of molecular transistors based on the effect of electron instability in one-dimensional systems. In these transistors we use the two most simple, but effective method of control conductivity of molecular compounds. These are:
changing the charge per lattice period;
variation of the transfer integrals between the neighboring one-dimensional molecular systems;
As a one-dimensional molecular system we use conjugate polymer (polyphenylacetylene (PPA)). Only for a given polymer was it possible to receive aggregates with high concentration of the introduced molecules. For the first method, tetracyanoquinodimethane (TCNQ) molecules were use. These molecules and PPA form the charge transfer system . The value of the transferred charge in this aggregate must depend on the value of an applied electrical field. In the last case, variation of the transfer integrals between the neighboring one-dimensional molecular systems will be achieved by inserting molecules with a high polarizability and a large dipole moment located in the PPA (Szablewsky et al.1997). For it we use tetracyanoquinodimethane derivatives, which have a large dipole moment. With the purpose of reaching higher solubility tetracyanoquinodimethane derivatives the following compounds were synthesized:
From the indicated compounds circumscribed in the literature only (II)
is found, that the essential increase of solubility in organic solvents
is reached only in the case of compounds (I, IV). The most qualitative
composite film were obtained because of PPA with the introduced molecules
TCNQ and, synthesized molecules (I). Only for a given polymer was it possible
to receive aggregates with a high concentration of the introduced molecules.
As working material of the transistor were used:
Aggregate PPA with (I) molecules;
PPA with TCNQ molecules
For research of the electrophysical properties of obtained aggregates, the test structures of the FET fabricated by a photolithography method are illustrated in Fig. 1. The channel length of the transistor is 2.5 micron, and width of a channel 100 micron.
The relations of a current through a channel of the transistor (source - drain) were measured depending on the voltage value applied between a source and gate (Fig. 2). For (a) and (b) examples the change of (source - drain) current more than a thousand time was revealed, at increase of (source - gate) voltage value up to 15-18 volt. In initial (c) polymer of noticeable changes of a current was not observed. The behavior in it cases is similar to the data, which had been presented late (Garnier 1996) and can be explained by the usual bulk semiconductor characteristics of a molecular compound.
In such a manner the polymeric aggregates consisted of conjugate polymers and acceptor (donor) molecules or molecules with a large dipole electrical moment and high polarizability have electrophysical properties and can be considered as the working material of molecular transistors. This type of FET is the first real molecular transistor which using intramolecular electrical property molecular compounds and illustrate the possibility of designing devices based on phenomenon electron instability of one-dimensional systems.
Garnier F. (1996), Thin film transistors based on molecular semiconductors,
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Krieger Ju.H. (1993), Molecular electronics: Current state and future
trends. J.Struct. Chemistry, 34, pp. 896-904.
Krieger Ju.H. (1996), Electronic instability of one-dimensional molecular
system as physical principle for design of electronic devices. Extended
Abstracts (The Third European Conference on Molecular Electronics, Leuven,),
Szablewsky M., Thomas P.R., Thronton A., Bloor D., et al (1997), Higly
Dipolar Nonlinear Adducts of Tetracyano-p-quinodimetane: Synthesis,
Physical Characterization, and Theoretical Aspects, J.Am.Chem.Soc., 119,
Dr. Juri H. Krieger
Institute of Inorganic Chemistry
Lavrentiev ave. 3, 630090, Novosibirsk, Russia