Methods for fabricating ion sensitive field effect transistors (ISFETs) with SnO.sub.2 extended gates. A SnO.sub.2 detection film is formed on a substrate by sol-gel technology to serve as an extended gate. The SnO.sub.2 detection film is electrically connected to a conductive wire, and an insulating layer is formed on the surface of the ISFET but part of the SnO.sub.2 detection film and the conductive wire are left exposed. The exposed conductive wire is electrically connected to a gate terminal of a MOS transistor.