[ODCAD] Slow Mobility :Organic Semiconductor as Device Material
Organic semiconductor,usually, has very low conductivity. Eeven in
crystal phase of the organic semiconductor, its conduction is still
poor. Conductivity is
contributed by two factors (mobility x concentration).
Let us consider simple case in which there is one charge carrier (say
hole) in the material. The "Concentration" is the
concetration of charge carrier whose intrinsic value is usually low
in organic semconductor. This factor is not discussed here. The 2nd
factor is the mobity of the charge carrier.
The charge carrier mobility in organic material is usually less than
1 cm2/V-s. The best so far is about 5 cm2/V-s for crystal organic
semiconductor. For polymer, the mobility is usually <0.0001 cm2/V-s.
There are many reasons to cause this for organic material and they
are not discussed here.
Is low conductivity good or bad for the application of device?
The answer is "depends". If the application is involved high current
going through the material. That is bad because the device requires
high voltage to operate resulting large heat generation. Both high
energy and large heat should be avoided in device design.
However, if the application just requires sensitivity of the device
to the change of operation condition (say change of voltage), for
example, field effect transistor. The operation then just involves
small current because of the low conductivity. This is particular
true when the device has very thin film (um or nm range) of the
material.
Well, one short coming of slow mobility is the speed. If the
device requires quick response, slow mobility will be issue. Some
device may need speed that can be reached by organic material. For
example, display device for TV, PDA, computer monitor etc, do not
mind the response time in micron sec or even longer time. This is
particularly true for large area electronic product. A few
applications such as logic transistor used for CPU require very fast
response of the device. Slow mobility will be big issue. That is why
the industry put a lot of effort to improve the mobility of organic
material. Mobility of charge carrier of material used in transistor
is a significant indicator of the material.
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