ION-SENSITIVE FIELD-EFFECT TRANSISTOR (ISFET)­BASED CHEMICAL SENSORS 191 3.7.3. CUTOFF REGION If the gate voltage is less than the threshold voltage, no inversion layer is formed. Consequently, the MOSFET behaves like two P­N junctions connected back to back, preventing flow of current in either direction. It acts as an open circuit in this region of operation. Figure 4.12b displays typical ISFET characteristics. ISFET output characteristics clearly resemble those of the parent MOSFET device except that the gate source voltages are replaced by pH values. 4. THEORY OF pH SENSITIVITY OF THE ISFET The pH sensitivity characteristic of the ISFET is linear, as shown in Figure 4.13. Because the surface hydroxyl group of the gate dielectric may donate or accept protons from the solution, making an originally neutral surface hydroxyl group a negatively or positively charged site, re- spectively, to maintain charge neutrality the surface charge is counterbalanced by an equal and opposite charge in the electrolyte and the resulting charge distribution produces a double-layer potential differ- ence at the gate (Yates et al. 1974; Siu and Cobbold 1979). Figure 4.14 shows the formation of the dou- ble layer on the surface of a solid. Helmholtz envisaged a double-layer model in which the excess charge Gate-source voltage V GS (milliVolts) 0 100 200 300 400 500 600 700 800 900 0 Typical slope = 58- 59 mV/pH for Ta 2 O 5 - gate ISFET; Nerstian limit = 59.2 mV/pH 2 4 6 8 10 pH 12 14 Figure 4.13. Representative pH response characteristics of Ta 2 O 5 -gate ISFET.