A new type of direct reading semiconductordosimeter has been investigated as a radiation detector for photon and electron therapy beams of various energies. The operation of this device is based on the measurement of the threshold voltage shift in a custom‐built metal oxide‐silicon semiconductorfield effect transistor(MOSFET). This voltage is a linear function of absorbed dose. The extent of the linearity region is dependent on the voltage controlled operation during irradiation. Operating two MOSFETS at two different biases simultaneously during irradiation will result in sensitivity (V/Gy) reproducibility better than ±3% over a range in dose of 100 Gy and at a dose per fraction greater than 20×10−2 Gy. The modes of operation give this device many advantages, such as continuous monitoring during irradiation, immediate reading, and permanent storage of total dose after irradiation. The availability and ease of use of these MOSFETdetectors make them very promising in clinical dosimetry.