A mathematical model for the calculation of the output characteristics of amorphous silicon hydrogenated (a-Si:H) ion-sensitive field-effect transistors (ISFET) is developed, which depends on the integration of the conductivity channel versus gate voltage curve at fixed drain voltage. Single curve integration was changed to integration with many simple lines to obtain the ID − VD characteristics using computer programming. The results of this model were tested with those of experiments. © 2011 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.