Field-effect transistor silicon nitride

Figure 15-29 Operation of a chemicalsensing field effect transistor. The transistor is coated with an insulating Si02 layer and a second layer of Si3N4 (silicon nitride), which is impervious to ions and improves electrical stability. The circuit at the lower left adjusts the potential difference between the reference electrode and the source in response to changes in the analyte solution such that a constant drain-source current is maintained.

Semiconductor electrodes ion-selective field effect transistors (ISFETs) are semiconductor devices used to measure ionic species in solution. They are sometimes called chemical field-effect transistors or ChemFETs. The transistor is coated with silicon nitride, which is in contact with the test solution via an analyte-sensing membrane and also connected to a reference electrode. A variation in the concentration of the analyte ions changes proportionally with the voltage of the ISFET. ISFETs are rugged, have a faster response time than membrane electrodes and can be stored dry. 

Ion-Selective Field Effect Transistors. Ion-selective field effect transistors (ISFETs) are semiconductor devices related to the solid-state detectors used in spectroscopy (discussed in Chapter 5). In this case, the surface of the transistor is covered with silicon nitride, which adsorbs H ions from the sample solution. The degree of adsorption is a function of the pH of the sample solution and the adsorption of H" " ions results in a change in the conductivity of the ISEET channel. The cell requires an external reference electrode. ISEET pH sensors can be made extremely small (about 2 mm ) and are extremely rugged, unlike the fragile glass bulb pH electrode. They have rapid response times and can operate in corrosive samples, slurries, and even wet solids such as food products. The sensor can be scrubbed clean with a toothbrush, stored in a dry condition, and does not require hydrating... 

The metal oxide silicon field effect transistor (MOSFET) has the basic structure as shown schematically in Figure 10.1 (an n-channel device is shown and will be discussed, although p-channel devices work equally well). The device consists of two n-type diffusions (called the source and the drain) in a p-type substrate separated by a p-type area. This area is covered by a thin (50 nm) insulator layer (generally silicon dioxide or silicon dioxide overlaid with silicon nitride) which is called the gate insulator. On top of the gate insulator there is a metallic contact called the gate electrode, which in conventional devices is made from aluminium. The source and drain diffusions are... 

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