Electrochemical and bio sensors using metal oxide semiconductors and method of making the same are described herein. The sensor includes a gate electrode, a dielectric layer over the gate electrode, a channel layer over the dielectric layer, and source and drain electrodes formed on the channel layer to provide a field effect transistor structure. The channel layer is a metal oxide semiconductor film that has a substantially uniform thickness of at least 3 nm thick and less than 10 nm thick. The metal oxide semiconductor film is functionalized with molecules attached thereto that are open to make contact with a fluid for detection of at least one component or at least one physical or chemical property of the fluid.