A single-chip pH sensor fabricated by a conventional CMOS process

A pH sensor fabricated on a single chip by an unmodified commercial CMOS process is described The sensor comprises a circuit for making differential measurements between ISFET and REFET both biased by the same reference electrode. The ISFET has a floating electrode structure and uses the silicon nitride passivation layer as the pH sensitive material. It is found to have a large threshold voltage caused by charge trapped on the floating electrode. A large reference electrode voltage is therefore required to bias the ISFET, this is shown to cause the operating point to drift. However the threshold voltage has been successfully reduced by exposure to UV radiation. The REFET is formed by deposition of a PVC membrane onto an ISFET and the reference electrode is simply a metal contact. The complete circuit provides an output voltage proportional to pH and operates from a single 5V supply.