A method of forming a transistor comprises forming a conducting substrate layer, forming a dielectric layer over the conducting substrate layer, forming a channel over at least a portion of the dielectric layer and forming first and second source/drain regions contacting 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 formed such that exposure to radiation causes a change in a threshold voltage of the transistor.