polymers such as polyvinyl chloride pvc plasticizers such as bis 2 ethylhexyl sebacate dos ion selective materials such as c 29 h 37 n 3 o 6 and electronegative ion complexes such as potassium tetrakis 4 chlorophenyl borate the conductive wire 16 may be a copper wire the insulating layer 24 may comprise epoxy resin the epoxy resin possesses excellent insulation low water and electrolyte permeation high adhesion to sensing membrane silicon wafer and substrate chemical stability anti corrosion processibility proper mobility before solidification high mechanical strength after solidification and small volume contraction after immersing suitable for packaging the sensing unit 14 in the extended gate ion sensitive field effect transistor egfet the calcium ion sensing membrane is separated from a gate thereof to isolate the metal oxide semiconductor field effect transistor mosfet and the test solution reducing instability of semiconductor devices and avoiding signal interference generated from the test solution a method for fabricating a calcium ion sensor in an embodiment of the invention is disclosed in fig1 a metal oxide semiconductor field effect transistor 12 is provided a sensing unit 14 comprising a substrate 18 a ruthenium dioxide membrane 20 and a calcium ion sensing membrane 22 is then provided the ruthenium dioxide membrane 20 is formed on the substrate 18 by for example radio frequency sputtering the calcium ion sensing membrane 22 is formed on the ruthenium dioxide membrane 20 a conductive wire 16 is then provided to connect the metal oxide semiconductor field effect transistor 12 and the sensing unit 14 the sensing unit 14 is further covered by an insulating layer 24 exposing the calcium ion sensing membrane 22 to be in contact with the test solution the substrate 18 may be a p type silicon substrate the calcium.