In 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 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 [5]. The positive gate voltage also pulls negative charges (electrons) from the substrate regions into the channel region. When sufficient electrons 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. The gate voltage applied to accumulate a sufficient number of electrons in the channel for a conducting channel is called the threshold voltage (VTH). Note that VTH for an nchannel (p-channel) FET is positive (negative). With these properties, the FET can be configured as a biosensor by modifying the gate terminal with molecular receptors or ion-selective membranes for the analyte of interest. The binding of a charged biomolecule results in depletion or accumulation of carriers caused by change of electric charges on the gate terminal. The dependence of the channel conductance on gate voltage makes FETs good candidates for electrical biosensors because the electric field generating from the binding of a
charged biomolecule to the gate is analogous to applying a voltage to a gate. In general, the drain
current of the FET-type biosensor is defined as follows: