For example, there have recently been outstanding advances in the field of ISFET biosensors for use in biosensing research, including the progress of the enzyme-immobilized FET which detects H+ ion concentration, the DNA (deoxyribonucleic acid)-modified FET based on DNA hybridization detection, and the cell-based FET for cell metabolism sensing or the measurement of extracellular potential. Currently, the use of ISFET technology encompasses a wide range of applications in a variety of areas, and those in the biomedical and environmental monitoring areas are particularly noteworthy. In the following, this paper reviews recent advances and developments in the bio-analytical use of ISFET-based biosensors.2.
?Operating Principle of FET-Based BiosensorsIn general, a field-effect transistor (FET) consists of three terminals; the source, drain, and gate.
The voltage between the source and drain of the FET regulates the current flow in the gate voltage. Specifically, the current-control mechanism is based on an electric field generated by the voltage applied to the gate. The current is also conducted by only one type of carrier (electrons or holes) depending on the type of FET (n-channel or p-channel). A positive voltage applied to the gate causes positive charges (free holes) to be repelled from the region of the substrate under the gate. These positive Brefeldin_A charges are pushed downward into the substrate, leaving behind a carrier-depletion region.
The depletion region is populated by the bound negative charge associated with the acceptor atoms.
These charges are ��uncovered�� because the neutralizing holes have been pushed downward into the substrate . The positive gate voltage also pulls negative charges (electrons) from the substrate regions into the channel region. When sufficient electrons GSK-3 are induced under the gate, an induced thin n-channel is in effect created, electrically bridging the source and drain regions. The channel is formed by inverting the substrate surface from p-type to n-type (inversion layer). When a voltage is applied between the drain and source with the created channel, a current flows through this n-channel via the mobile electrons (n-type FET).
In the case of a p-type semiconductor, applying a positive gate voltage depletes carriers and reduces the conductance, whereas applying a negative gate voltage leads to an accumulation of carriers and an increase in conductance (the opposite effect occurs in n-type semiconductors). The applied gate voltage generates an electric field which develops in the vertical direction. This field controls the amount of charge in the channel, and thus it determines the conductivity of the channel.