Page 143 - Organic Electronics in Sensors and Biotechnology

120 Cha pte r F o u r

not enable identification of the frequency at which every single point
was measured. This can be avoided by plotting both the real and imag-
inary parts of the impedance over frequency. The two plots can be com-
bined into one if two different y axes are used. As an alternative to the
real and imaginary parts, the absolute value and the phase of the
impedance can also be plotted (Bode plot).
In this chapter the Bode plot is modified by multiplying the abso-
lute value of the impedance by the frequency f. This is done to elimi-
nate the frequency dependence of the capacitive impedance Z =
C
1/(2π f C). By this the capacitive behavior of the impedance can be
investigated more closely.

4.1.2 The IS of an Organic MIS Structure
The Field-effect transistor (FET) is probably the most important device in
electronics due to its amplifying and switching properties. For this rea-
son it is of great interest to understand the metal-insulator-semiconductor
(MIS) structure which constitutes a FET by adding a source, drain, and
gate contact. The transistor is switched on by accumulating charge car-
riers at the semiconductor-insulator interface, creating a conducting
channel between the source and the drain contact. It is switched off by
depleting this channel of charge carriers. This is done by applying an
appropriate gate voltage. Assuming a p-type semiconductor, a nega-
tive voltage switches on the transistor and a small positive voltage
switches it off. However, a sufficiently large positive voltage will accu-
mulate minority charge carriers in the channel (inversion).
An ideal MIS structure behaves as a capacitor whose dielectric
layer thickness varies with voltage. At accumulation and inversion
the thickness corresponds to the thickness of the insulating layer
whereas at depletion the thickness of the depletion zone must be
added. The characteristics of the corresponding C(V) curve give
information not only about the layer thicknesses but also about sev-
eral semiconductor parameters such as the charge carrier density. 3, 4
This method of device characterization, which is well established for
silicon devices, can, however, not be applied on organic MIS structures.
Because of the lower charge carrier density, mobility inversion does not
occur. Moreover, the semiconductor layer has complicated contact prop-
erties compared to silicon. Finally, also the dielectrics used in organic
FETs often do not have the ideal properties of silicon oxide regarding
their leakage behavior, mobile ion density, and so on. For these reasons
the EC of an organic MIS structure is much more complex, complicating
the measurement of C(V) characteristics. To determine quantitatively the
EC of the organic device, its IS must be measured.
An organic MIS under bias basically consists of three capacitive
layers. The first layer corresponds to the insulator, and the other two
correspond to the depletion zone and the (remaining) bulk of the
organic semiconductor. Since all three layers can show leakage to a