Experimental study of MOS capacitors as wireless radiation dose sensors

The RADiation sensitive Field Effect Transistor (RADFET) has been conventionally used to measure radiation dose levels. These dose sensors are calibrated in such a way that a shift in threshold voltage, due to a build-up of oxide-trapped charge, can be used to estimate the radiation dose. In order to estimate the radiation dose level using RADFET, a wired readout circuit is necessary. Using the same principle of oxide-trapped charge build-up, but by monitoring the change in capacitance instead of threshold voltage, a wireless dose sensor can be developed. This RADiation sensitive CAPacitor (RADCAP) mounted on a resonant patch antenna can then become a wireless dose sensor. From the resonant frequency, the capacitance can be extracted which can be mapped back to estimate the radiation dose level. The capacitor acts as both radiation dose sensor and resonator element in the passive antenna loop. Since the MOS capacitor is used in passive state, characterizing various parameters that affect the radiation sensitivity is essential. Oxide processing technique, choice of insulator material, and thickness of the insulator, critically affect the dose response of the sensor. A thicker oxide improves the radiation sensitivity but reduces the dynamic range of dose levels for which the sensor can be used. The oxide processing scheme primarily determines the interface trap charge and oxide-trapped charge development; controlling this parameter is critical to building a better dose sensor.