cut into individual sensors with a known dicing method the thus produced isfet as an h ion sensor exhibited selectivity of about 50 mv ph like a conventional sensor and no insulation breakdown was found over a long period of use with the method described above an isfet in which the element portion is completely passivated can be manufactured in a normal semiconductor manufacturing process in which a large number of elements are formed on a single wafer and are cut into devices by dicing furthermore an si substrate can be used in place of an sos substrate for this reason the mass production of these isfets is enabled thus reducing their manufacturing cost in addition to the above advantages an si layer on the element forming side is sufficiently thick for example when a 3 inch standard wafer is used the si layer can have a thickness of 450 u03bcm with this structure deep impurity diffusion is allowed and resistance of wiring extending to the contact layers can be reduced in contrast to this in the sos structure since the si layer is an epitaxially grown film its thickness is limited e g 20 u03bcm or less when the sos substrate is used it is necessary to dope al from the.