An apparatus comprising: a plurality of sensors (501) arranged in an array (500), each sensor having a source electrode (504), a drain electrode (503), a gate electrode (505) and a channel, wherein the source electrode and drain electrode are elongate and the channel has a channel width defined by the longitudinal extent of the source and/or drain electrode and a channel length defined by the separation between the source and drain electrodes; a common conductive or semiconductive layer (506), which may be made of graphene, comprising the channels of the sensors (501) and arranged to extend over the plurality of sensors of the array and configured to be in electrical contact with at least the source electrode and the drain electrode of each sensor; and wherein the source electrode or drain electrode of each sensor forms a substantially continuous sensor perimeter at least along the channel width, which substantially encloses the other electrode of each sensor to inhibit the flow of charge carriers beyond the sensor perimeter to inhibit crosstalk between sensors in the array.