Detection of charged macromolecules by means of fi eld-effect devices (FEDs): possibilities and limitations - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 185

RE

(Bio-)chemical
recognition
element

Analyte

Gate insulator

Si

FIGURE 7.1 Schematic of an EIS hetero-structure. For the (bio-)chemical sensing, suitable chemi-
cal or biological recognition elements need to be functionally coupled to the sensitive surface of the FED.
Chemical and/or biological recognition elements are, e.g., ionophores, enzymes, immunospecies, DNA, liv-
ing cells, microorganisms and receptors. RE: reference electrode.

create regions of excess charge in a semiconductor substrate is common to all of them.
FEDs are sensors with an external modulation possibility of the threshold voltage ( V th )
or fl at-band voltage ( V fb ) by means of the interface potential analyte (test sample)-
sensor chip. This fi nally varies the electric fi eld inside the insulator of the FED yield-
ing a modulation of the space-charge region in the semiconductor at the silicon/
insulator interface (e.g. change of the capacitance of an EIS structure or conductance
of the inversion channel of an ISFET). The basic common structure of all FEDs for
(bio-)chemical sensing is the electrolyte-insulator-semiconductor system that is sche-
matically shown in Fig. 7.1.
Originally, FEDs are derived from either MIS (metal-insulator-semiconductor)
capacitors or IGFETs (insulated-gate fi eld-effect transistors), where an analyte and a
reference electrode have replaced the gate electrode. FEDs are basically surface-charge
measuring devices (they detect the charge in a capacitive way) and therefore they are
very sensitive for any kind of electrical interaction at or nearby the gate insulator/elec-
trolyte interface. Therefore, nearly each (bio-)chemical reaction leading to chemical
or electrical changes at this interface can be measured by means of an ISFET, capaci-
tive EIS sensor or LAPS. For this purpose, suitable chemical or biological recogni-
tion elements need to be functionally coupled to the sensitive surface of the respective
FED. Changes in the chemical composition will induce changes in the surface charge
of the gate insulator and in the potential drop at the electrolyte/insulator interface, con-
sequently modulating the current in the ISFET's channel, the capacitance of the EIS
sensor or the photocurrent of the LAPS. These devices have been shown to be versatile
tools for detecting pH, ion concentrations, enzymatic reactions, cellular metabolisms
and action potentials of living cells, etc. More recently, the possibility of application
of FEDs for the detection of charged macromolecules, like DNA, proteins, and poly-
electrolytes, has been shown. For more detailed information concerning the operation
principle and different types of ISFET, EIS and LAPS, see, e.g., [1-8].