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For an ideal FET, an electrical current starts to flow from the source to the drain as the gate voltage
V
exceeds a threshold voltage V
GS
however, the current I
is small, but different from zero for V
DS
Important: Both gate and drain voltage share the same ground, more specifically V
potentials are measured with respect to the source terminal.
In this section, the mathematical model describing an (ideal) field effect transistor is briefly
introduced; please refer Physics of Semiconductor Devices
Organic Electronics: Materials, Manufacutring and Applications
effect working principles.
The standard field effect transistor model (FET model for short) is valid under the condition that the
density of the charge Q in the drain-source channel depends only on the distance from the source
(see figure 13). In other words, the charge can be approximately described by one-variable function,
i.e. Q≈ Q(y). This approximation is referred in the literature as gradual channel approximation.
Figure 13. Schematic of bottom gate/bottom contact OFET device.
Since, for a given device, the drain current depends on both drain and source voltage, I
variable function, i.e. I
=I
DS
purposes, a field effect transistor is more conveniently described by two correlated family of curves:
Output Characteristics (I-V) obtained by keeping the gate voltage constant and varying the
drain voltage only I
Transfer Characteristics (TC) obtained by keeping the drain voltage constant and varying the
gate voltage only I
6
Physics of Semiconductor Devices, 2
7
Organic Field-Effect Transistors, Zhenan Bao, Jason Locklin, CRC Press, 2007.
8
Organic Electronics: Materials, Manufacturing, and Applications, Hagen Klauk, John Wiley & Sons, 2006.
Ossila Ltd
and a second voltage V
Th
(V
,V
), requiring a 3D plot for its representation. For practical
DS
GS
DS
= I
(V
,V
) = I
DS,IV
DS
GS,cosnt
DS
= I
(V
,V
) = I
DS,TC
DS
GS
DS,cosnt
nd
Edition, Sze, S.M., Wiley Interscience, 1981.
Copyright © 2009-2015
is applied to the drain. In a real device,
DS
< V
.
T
GS
6
, Organic Filed Effect Transistors
8
for detailed descriptions of the field
(V
)
DS
DS
(V
)
DS
GS
and V
GS
DS
7
and
is a two-
DS
34
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