The Selectivity and Temperature Characteristics of Ion Sensitive Field Effect Transistors

Abstract

The possibility of constructing inexpensive solid state transducers for measuring the concentration of ions in solutions or of certain gases has prompted a good deal of research on ion sensitive field effect transistors lately. In this paper there are presented some results of an experimental investigation on the chemical and electrical characteristics of one configuration of this sort of field effect transistor. The devices constructed for the study were basically metal oxide silicon field effect transistors with the metal gate omitted. It has been found that these devices are selectively sensitive to hydrogen ions, contrary to what would be expected from experimental data on standard glass electrodes. In fact, from our data one concludes that there is little or no response to the common alkali ions. Although the drain current (or effective gate voltage) as a function of pH is approximately linear, the sensitivity is less than that predicted by the Nernst relation and usually is equivalent to about 30 mV/decade pH. In addition, these devices exhibit a memory or hysteresis effect. The behaviour of ion sensitive field effect transistors when subjected to changes in temperature is dominated by the properties of the semiconductor. Depending on the bias the temperature coefficient can be negative or positive or even be zero.