Method of monitoring radiation using a floating gate field effect transistor dosimeter, and dosimeter for use therein

Abstract

An insulated gate field effect transistor dosimeter has a source and drain defining a channel region, a floating gate having a first portion extending over the channel region, and a second, larger portion extending away from said region, a control gate having at least a portion thereof overlapping a first part of the floating gate, and a charging gate overlapping a second part of the floating gate. The area of the second part of the floating gate is much smaller than the area of the first part, and the charging gate is separated from the channel region by the control gate. The dosimeter is charged, before irradiation, by connecting the source, drain and control gate to a common ground and applying a potential difference between the charging gate and the common ground. The charge is supplied to the floating gate by a path which does not require a significant electric stress to be created in the region of the gate oxide and the channel. The dosimeter may comprise two such transistors fabricated on a common substrate, conveniently with a common source. The pair of transistors may be charged by maintaining the sources, drains and control gates within the normal maximum operating voltage relative to each other, and applying different potential differences between the two charging gates, respectively, and the substrate. Following irradiation, the absorbed radiation does is determined by measuring the difference between the threshold voltages of the two transistors. Preferably, the transistors have charges of opposite polarities. This differential arrangement reduces the effects of temperature variations and enhances sensitivity.