Abstract: The invention relates to a radiation dosimeter having a pair of insulated gate field effect transistors integrated into the same silicon substrate, in which each of the transistors are operable in a bias mode and a test mode. A circuit element for biasing each of the transistors, during said test mode is provide, so that one of the transistors is more sensitive to ionizing radiation than the other of the transistors. A circuit element is provided for determining, during the test mode, the difference in the threshold voltages of the transistors, whereby the difference voltage is indicative of the radiation dose, and a circuit element is provided for continuously switching the transistors between the bias mode and the test mode, whereby the period of operation of the transistors in the test mode time period is small in comparison to the period of operation of the transistors in the bias mode.