Operation of an ISFET with non-insulated substrate directly exposed to the solution

The use of a probe-type n-channel depletion-mode Si3N4-ISFET with the substrate in direct contact with solution is described. A proper mode of ISFET operation is achieved when it is electrically isolated from the solution by a reversely biased diode formed by an n-type source-channel- drain region and a p-type substrate. The ohmic contact used to bias the substrate is at the same time utilized to short-circuit the photovoltage generated at the p-n junction in order to reduce the ISFET light sensitivity. The pH sensitivity of the ISFETs is linear with a slope 55 ± 1.5 mV pH-1 (n = 6) in the pH range 2 to 10. In the case of cell and enzyme biosensors based on a differential pair of ISFETs , the non-encapsulated substrate of the sensor chip itself may be used as a quasi-reference electrode.