An apparatus comprises a conducting substrate layer, a dielectric layer formed over the conducting substrate layer, a channel formed over at least a portion of the dielectric layer and first and second source/drain regions formed on respective first and second portions of the channel. The channel comprises a thin-film carbon material. The conducting substrate layer, the dielectric layer, the channel and the first and second source/drain regions are configured such that exposure to radiation causes a change in a threshold voltage of the apparatus.