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34
For an ideal FET, an electrical current starts to flow from the source to the drain as the gate voltage
V
GS
exceeds a threshold voltage V
Th
and a second voltage V
DS
is applied to the drain. In a real device,
however, the current I
DS
is small, but different from zero for V
T
< V
GS
.
Important:
Both gate and drain voltage share the same ground, more specifically V
GS
and V
DS
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
6
,
Organic Filed Effect Transistors
7
and
Organic Electronics: Materials, Manufacutring and Applications
8
for detailed descriptions of the field
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
DS
is a two-
variable function, i.e. I
DS
=I
DS
(V
GS
,V
DS
), requiring a 3D plot for its representation. For practical
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
DS,IV
= I
DS
(V
GS,cosnt
,V
DS
) = I
DS
(V
DS
)
Transfer Characteristics (TC) obtained by keeping the drain voltage constant and varying the
gate voltage only
I
DS,TC
= I
DS
(V
GS
,V
DS,cosnt
) = I
DS
(V
GS
)
6
Physics of Semiconductor Devices
, 2
nd
Edition, Sze, S.M., Wiley Interscience, 1981.
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.
