05-10-2012, 03:06 PM
Organic Semiconductor Thin Film Transistors
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Abstract
In recent years, signi¯cant advances have been made in the ¯eld of organic
semiconductors. These materials are basic building blocks of a number of
devices such as organic light-emitting diodes and particularly organic thin
¯lm transistors on which this seminar focuses. The fundamental proper-
ties of organic semiconductors (materials, bonding and charge transport) are
described in the ¯rst part. The second part focuses on organic thin ¯lm tran-
sistors. It describes the basics of transistor operation, the structure and fab-
rication of organic FETs and its main characteristics. The last part presents
the di®erent possible applications of organic transistors and also adresses the
main problems that need to be solved in order to utilize the potential that
these devices posess.
Introduction
Transistors that use organic semiconductors (OS) as the active layer are
steadily approaching amorphous silicon transistors in terms of performance.
These organic devices are presently of great interest beacuse they posess
many bene¯ts over their silicon counterparts, including lower cost, greater
durability and compatability with glass and plastic substrates [1]. Organic
thin ¯lm transistors (OTFTs) have great promise in electronic applications.
Perhaps the most attractive application for OTFTs is displays, where they
can be used as switching devices for active-matrix °at-panel displays (AMF-
PDs) based either on liquid crystals or organic light-emitting diodes (OLEDs).
Given their compatibility with polymeric substrates, OTFTs may enable the
production of °exible computer screens. Other applications of OTFTs in-
clude smart cards and electronic identi¯cation tags.
Organic Semiconductor Fundamental
Properties
In order to study organic transistors we ¯rst describe the properties of the
organic layer in these devices. The basic building blocks of organic semi-
conductors are organic molecular crystals (OMCs). They are formed by
molecules which interact with the relatively weak Van der Waals forces. The
most common OMCs are polyacenes, perylenes and pyrenes. Another type
of organic semiconductors, those based on conjugated polymers has rapidly
been developing in recent years. Polymers are very common materials in
our every-day lives. From an electrical point of view, polymers have long
been considered as insulators. Conducting polymers can be of metallic or
semiconductor nature yet keeping the mechanical °exibility and they can be
processed with plastic manufacturing techniques.
Charge Transport in OS
Detectable electrical conductivity can be found mainly in molecular crystals
containing molecules with polyconjugated bond system such as aromatic hy-
drocarbons (benzene) and other conjugated compounds and polymers. The
¼ electrons in these compounds are potential sources of free charge carriers.
In inorganic crystalline semiconductors the energy states of charge carriers
are described in terms of the valence and conduction band. If a charge is in
the conduction band it is virtualy free and delocalized. This so called band
model for electron energy states in inorganic crystal lattices is a consequence
of the large number of atoms that form the lattice. Each atom contributes
its energy states (orbitals) to the lattice but because of their large number,
the states overlap and they cannot be distinguished from eachother [6].
Organic Thin Film Transistors
In order to assembe an organic transistor, basic principles of its operation
must be known. Almost all organic transistor designs reported in literature
are ¯eld e®ect transistor (FET) designs. In contrast to the bipolar n-p-n or p-
n-p transistors, FETs are unipolar transistors. Unipolar transistors use only
one type of semiconductor channel which is called p-channel or n-channel,
depending on the type of majority carriers. The semiconductor layer is placed
between or onto two electrodes, the source and drain (Figure 6).
Conclusion
Although the organic transistors attract interest, the work is mostly limited
to the research ¯eld. So far, there is no commercial product based on or-
ganic TFTs. Several issues need to be adressed in order to realize useful
applications. One major problem is the low e±ciency of these organic de-
vices compared to the inorganic ones, since organic transistors render only
low currents despite the large operating voltages. Some of the reasons for this
are high trap densities at the grain boundaries and the semiconductor-gate
insulator interface. In order to produce high-ordered, high mobility semi-
conductors new production techniques should be developed. Until then, the
organic transistor will remain inferior to its inorganic counterpart.