A single device for emitting and detecting photons. The device comprises a semiconductive layer (3), active material (5), further dielectric layer (17) and overlying electrode (25). In a first mode of operation an electrical field is applied between the semiconductive layer (3) and the overlying electrode (25). This enables photons to be emitted from the active material (5). In a second mode of operation, the semiconductive layer (3) constitutes a channel of a field effect transistor (23). The field effect transistor further comprises source electrode (11), drain electrode (15), gate electrode (13) and dielectric layer (19). Photons absorbed by the active material (5) causes charge to be transferred to the semiconductive layer (3), thereby changing the channel resistance. A plurality of such devices can be arranged in a configurable array.