 Progress in Organic Transistor Performance
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Gazing at the Far Side of Silicon
Research using organic materials (compounds based on carbon) as the material for
transistors began in 1984. Although the
research in Japan led the world at that time,
the carrier mobility of those materials was
nowhere near that of even amorphous
silicon and the technology was far from
practical.
The performance of organic transistors improved greatly at the start of the 1990s. Then
in 1997, instead of thiophene, the original
organic material used in this research,
pentacene was used, and a carrier mobility
of the order of 1cm2/Vs, which is comparable
to that of amorphous silicon, was reported.
Additionally, Bell Laboratories caused an uproar by announcing an organic transistor
based on a self-assembled monolayer film,
an event that became the catalyst that
created a worldwide boom. In recent years,
sessions related to organic transistors at
international conferences have become standing
room only events.
There are Still Mysteries, and
Dreams are Just Beginning
Even if it were the case that characteristics
similar to amorphous silicon appeared at the
research and development phase, that in
itself would not be enough to cause a shift to
a new generation. There were still many
issues to overcome at the basic research stage
to meet the performance requirements for
transistors. In the first place, the operating mechanisms of the organic transistor have not
yet been fully understood. Although operation
of the organic transistor is in principle
the same as that of silicon field-effect
transistor (FET), how carriers flow in organic
materials is not adequately understood.
However, Sony’s Fusion Domain Laboratory,
Materials Laboratories has now explicated
one part of those principles, namely the path
that carriers pass through. Before we discuss
into that explanation, we would first like to
summarize why researchers around the world
are taking pains to make organic transistors
practical.
Changing the Manufacturing Process
and the Semiconductor Concept
Should we use silicon (inorganic) or should
we use organic materials? It turns out that
the largest difference between them is in the
manufacturing processes.
Transistors using silicon require a complicated
and high precision manufacturing process
that creates electronic circuits by starting
with a single-crystal silicon substrate and
applying a variety of processes such as lithography,
the addition of impurities, deposition,
and etching. Large scale and expensive equipments,
such as clean rooms and vacuum systems,
is required.
In contrast, with organic transistors, one can
take advantage of the features of organic
materials and, by dissolving them in a solvent,
use printing technologies such as rotary
press or inkjet printing to create circuits simply.
Since these are low-temperature processes,
they have excellent compatibility with
plastic substrates. It is possible to “print” pixels
and transistors on a thin plastic sheet and
to manufacture large-screen displays that are
light and flexible.
Furthermore, since the flexibility in formation
is increased, for example, one can print
circuits on curved surfaces, applications such
as artificial skin for robots have been
announced.
Sony’s Viewpoint (1)
The Discovery of a Detour
The relationship between device size and
carrier mobility remained a major obstacle
to realizing the rich possibilities inherent in
the organic transistor.
In field-effect transistors, response becomes
faster and higher integration densities become
possible the smaller the size of the
device, that is, the shorter the path (gate
length) through which carriers flow in the
organic semiconductor layer from the source
electrode to the drain electrode. (See the figure
above.)
However in organic transistors, the carrier
mobility is reduced greatly as this gate length
becomes shorter. This is one of the main
factors preventing the practical use of
organic transistors.
Why does the carrier mobility fall? Sony has
defined that the cause is that since loss (contact
resistance) occurs in the current flow
from the source electrode to the semiconductor layer, that loss becomes
relatively larger as the gate length is reduced.
Sony analyzed the conduction mechanism
in that section and showed that the path that
the carriers pass through (the effective channel
layer) is a thin layer (about 3 nm) consisting
of just a few molecules. Previously,
carriers did not flow smoothly, since the
source electrode and the effective channel
layer were not actually in contact due to the thickness of the titanium or other material
used to bond the source electrode to the gate
insulating film.
This was like coming across a detour with
poor paving at some point on a superhighway.
Sony’s Viewpoint (2)
Using Self Assembly
Based on this knowledge, Sony thought that
carrier flow could be made more efficient by
reducing the thickness of the bonding layer
and increasing the area of contact between
the source electrode and the organic semiconductor
layer. What Sony used instead of
titanium was a self-assembled monolayer
(SAM) of organic material. This is a film that
makes use of the property that, when certain
processing is applied to the substrate, organic
molecules attach themselves to the substrate
in individual molecule units due to a
chemical reaction. This film has a thickness
of under 1 nm.
By sandwiching this film between the source
electrode and the gate insulating film, Sony
reduced the contact resistance and achieved
transistor performance in which carrier mobility
does not fall when the gate length is
reduced. (Carrier mobility was increased by
a factor of more than 50 over previous Sony
devices.)
Furthermore, Sony succeeded in using organic transistors that adopt this technology
for pixel switching and operating a 2.5-inch
monochrome transmissive TN LCD display
(160 × 120 pixels). Sony announced these
results at the ISSCC 2004 (IEEE International
Solid-State Circuits Conference) held in San
Francisco in February this year.
Although this work is still at the research
stage, Sony received a strong positive response
to this announcement that showed, in
a clearly visible manner, that practical use of
organic transistors is actually possible. Questions
continued long after the session had
ended. |
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Photomicrograph of an LCD Display Driven by Organic Transistors
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Vol.38
