Abstract: A bio-sensor employs an ion senstive field effect transistor (ISFET) comping a source 1 and a drain 3 formed in a substrate, an ion sensitive gate 2 placed between the source 1 and drain 3, an ion sensitive film 4 formed on the ion sensing gate 2, an immobilized enzyme membrane 5 defined on the ion sensitive film 4 and, a Pt electrode 6 formed on the ion sensitive film 4. The sensor has a Pt eleletrode being capable of sensing all biological substances which generate H.sub.2 O.sub.2 in enzyme reaction, whereby having the high sensitivity and the rapid reaction time.

Abstract: A measuring circuit with a biosensor utilizing ion sensitive field effect transistors having a simplified structure and is advantageous to integration. The measuring circuit comprises two ion sensitive FET input devices composed of an enzyme FET having an enzyme sensitive membrane on the gate and a reference FET, and a differential amplifier for amplifying the outputs of the enzyme FET and the reference FET. The drift phenomena of the ISFETs due to the use of a non-stable quasi-reference electrode as well as the temperature dependence thereof can be eliminated by the differential amplifier consisting of MOSFETs having the same channel as the ISFETs. The ISFET biosensor and the measuring circuit can be integrated into one chip.