thicker the field oxide is the less the capacitance of the equivalent gate capacitor is and change of the threshold voltage influenced by hole charge is greater in other words the conducting condition between two conductive contacts i e the drain 36 and the source 38 adjacent to a thicker field oxide i e the field oxide 32 is more susceptible to charge accumulation and is thus more sensitive to exposure to radiation the field oxide 32 in the detecting transistor 30 should thus be thickened to a thickness that is thicker than any other isolating field oxide in semiconductor wafer 25 so that current conduction between the two conductive contacts drain 36 and source 38 in the detecting transistor 30 is changed by radiation before other field oxides isolating ordinary transistors are influenced in this arrangement current conduction change between the two conductive contacts drain 36 and source 38 in the detecting transistor 30 means that the semiconductor wafer 25 has received a certain dosage of radiation if the semiconductor wafer 25 continues to be subjected to radiation exposure the normal functioning of the semiconductor wafer 25 will be adversely influenced so the influence of radiation on the operations of the semiconductor wafer 25 can be forewarned by the detecting transistor 30 there are many possible embodiments to measure current conduction between the two conductive contacts in the detecting transistor 30 please refer to fig3 fig3 is a circuit schematic diagram of an embodiment of a sensor 50 according to the present invention the sensor 50 uses a detecting circuit.