Chapter 11
Chemical Sensors Based on Semiconductor Electronic Devices
A potentiometric ion sensor cell develop a potential difference (voltage) that depends on the activity of the target ion. This voltage is applied to a semiconductor electronic device called field-effect transistor (FET) that is included in the measuring instrument. This FET imparts a very high input impedance that prevents current flow through the cell and allows the measurement to be performed under equilibrium conditions. The current flowing through the FET depends on the applied voltage and additional electronic circuitry allows the voltage to be measured.
A further step in the development of potentiometric sensors was achieved by integrating a ion-responsive membrane with a FET-type structure (P. Bergveld, 1970). In this way, the potential difference developed at the membrane-solution interface directly affects the electrical parameters of the device and the response signal is represented by an electrical parameter of the FET structure.
As in the case of standard potentiometric sensors, FET-based ion sensors can be utilized not only for ion determination but also as transducers in sensors based on recognition processes that give rise to ions, such as enzymatic reactions and gas dissolution that alter the ionic composition of the solution.
A standard FET includes a metal layer separated by a thin insulator film from the semiconductor component of the device. If the metal film is able to interact with a gas, ...
