Detection of charged macromolecules by means of fi eld-effect devices (FEDs): possibilities and limitations - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 193

C - V curve but the polarity of the voltage is reversed; a positive voltage causes accu-
mulation and a negative voltage inversion). In the accumulation region (C i C sc ), the

capacitance of the whole EIS structure is determined by the geometrical capacitance of
the insulator, C C i , and corresponds to the maximum capacitance of the system. For a

sensor application, the more useful range represents, however, the depletion region of the
C - V curve.
The position of the C - V curve along the voltage axis is infl uenced by several solid-
state as well as electrochemical parameters which are gathered in the equation for the
fl at-band voltage V fb of the EIS structure [1, 54]:

W
q

QQ
C

S i ss

VE ϕ χ (2)

fb ref sol

i

Here, W S is the work function of electrons in the semiconductor, q is the elementary
charge (1.6

10 19 C), Q i and Q SS are charges located in the oxide and the surface
and interface states, respectively, E ref is the potential of the reference electrode, and
χ sol is the surface-dipole potential of the solution. Because in expression (2) for the
fl at-band voltage of the EIS system all terms can be considered as constant except for
ϕ

(which is analyte concentration dependent), the response of the EIS structure with
respect to the electrolyte composition depends on its fl at-band voltage shift, which can
be accurately determined from the C - V curves.
Generally, there are a number of ways in which the adsorption and binding of
charged macromolecules (in particular, DNA immobilization and hybridization) can
affect the electrochemical properties of the analyte-FED interface. In the case of fi eld-
effect devices, two basic effects are usually considered:
● a geometrical capacitance effect (due to the displacement of electrolyte by mac-
romolecules and change in the “effective” thickness of the gate insulator and thus
change in the “effective” gate capacitance), and
● charge effects resulting in a change of the fl at-band voltage of the EIS
hetero-structure.
Dependent on the type of doped semiconductor substrate as well as on the sign of
the molecular charge, these two effects can affect the sensor signal in the same direc-
tion, or in the opposite direction and thus, to some extent, they might even compensate
each other.
The presence of an additional molecular layer on the surface of the FED can lead
to a shift of the C-V curve of the original EIS structure along both the capacitance and
voltage axis, as it is exemplarily shown in Fig. 7.4 for the case of a layer of negatively
charged macromolecules. The shift of the C-V curve along the capacitance axis, C

(decrease of the maximal capacitance in the accumulation region), is usually due to an
additional series capacitance C ML of the molecular layer. More generally, it is a series
impedance, because usually monolayers of charged macromolecules do not represent
a perfectly homogeneous and tightly packed, electrically blocked layer, but a rather
much less dense layer with interstitial spaces permeable to ions, and therefore their
resistivity is much lower than that of the underlying gate insulator. For a particular

∆