Abstract
Ion sensitive field effect transistors (ISFETs) have been used as analogue continuous time chemical sensors. Each ISFET had its own ion sensitive membrane as part of it. Allowing two ISFETs to share the same ion sensing membrane, a pH inverter was built. Chemical field effect transistors (CHEMFETs) are ISFETs' derivatives. It is demonstrated here that using the ISFET based chemical switch it is possible to choose a chemical concentration threshold that when crossed the circuit changes state. A threshold of pH 6.75 was chosen and electrical output was shown to switch for pH changes from 6.5 to 7.0 and vice versa. This is effectively a digital CHEMFET basic building block as it serves as an input for further processing. This circuit can be used for chemical thresholding operations, as a digital CHEMFET transducer, in adverse applications including biomedical, chemical, and environmental industries.