Readout techniques for linearity and resolution improvements in MOSFET dosimeters

Abstract

Novel measurement methods are presented for improving the linearity and the Signal-to-Noise-Ratio of the radiation response of pMOS transistors used as dosimeters. These methods focus on the modification of the readout process of the dosimetric parameter based, until now, on source voltage measurement at constant drain current. We propose to include an additional drain current to extend the linear range and to pulse the drain currents to reduce the electronic noise level. The response of a low-cost pMOS transistor, the 3N163 of Vishay-Siliconix (USA), has been studied in unbiased mode, without connections between the readout system and the sensor, aiming for patient comfort and greater ease for clinical use. Four different measurement methods have been discussed, depending on the polarization transistor currents during readout: only one DC current, one pulsed current, two DC currents and, finally, two pulsed currents. The standard deviation of the source voltage was measured with one DC and one pulsed current, obtaining a 20% reduction for the pulsed mode compared to the DC mode. A linear range, defined as the accumulated dose that produces 5% of the sensitivity reduction, was measured at 6.8 Gy for only one DC current, and at 10.3 Gy for two DC currents. The linear range was also extended for pulsed currents, from 8.8 Gy with one current to 16.9 Gy with two pulsed currents. The two pulsed current method provides the highest linear range and doubles the value measured with one DC current.