The consequences of a constant substrate bias on the operation of ISFETs in a source and drain follower circuit are briefly discussed. The biasing of the substrate becomes necessary as only one substrate contact is available on a chip that contains several ISFETs. Although the gain of the follower circuits therefore becomes smaller than unity, the transfer is practically linear and the application of the ISFETs is not restricted.
van der Schoot, B., Voorthuyzen, H., & Bergveld, P. (1987). The use of a multi-ISFET sensor fabricated in a single substrate. Sensors and actuators, 12(4), 463-468. https://doi.org/10.1016/0250-6874(87)80065-3