An apparatus comprising: a substrate, a plurality of electrode pairs (403, 404, 503, 504) arranged in an array on the substrate, each electrode pair (403, 404, 503, 504) comprising a source electrode (404, 504) and a drain electrode (403, 503); and a common conductive or semiconductive layer (406, 506) arranged to extend over the plurality of electrode pairs (403, 404, 503, 504) of the array and configured to be in electrical contact with the source electrode (404, 504) and the drain electrode (403, 504) of each electrode pair (403, 404, 503, 504) to create an array of sensors, wherein each sensor has a source electrode (404, 504), a drain electrode (403, 503) and a channel formed by part of the common conductive or semiconductive layer (406, 506), one of the source (404, 504) and the drain (403, 503) electrodes configured to receive a flow of charge carriers for injecting into the channel and the other configured to provide a current sink for said flow of charge carriers from the channel, wherein the source electrode (404, 504) and drain electrode (403, 503) are elongate and the channel has a channel width defined by the longitudinal extent of the source (404, 504) and/or drain (403, 503) electrode and a channel length defined by the separation between the source (404, 504) and drain (403, 503) electrodes; and wherein the source electrode (404, 504) or drain electrode (403, 503) of each sensor forms a substantially continuous sensor perimeter at least along the channel width which substantially encloses the other electrode (403, 404, 503, 504) of the respective sensor to inhibit the flow of charge carriers beyond the sensor perimeter to inhibit crosstalk between sensors in the array, wherein each sensor includes a gate electrode (405, 505) arranged for modulating the conductivity of the channel between the drain electrode (403, 503) and the source electrode (404, 504) and a dielectric material is interposed between the gate electrode (405, 505) and the common conductive or semiconductive layer (406, 506), and wherein the common conductive or semiconductive layer (406, 506) is graphene. a plurality of electrode pairs (403, 404, 503, 504) arranged in an array on the substrate, each electrode pair (403, 404, 503, 504) comprising a source electrode (404, 504) and a drain electrode (403, 503); and a common conductive or semiconductive layer (406, 506) arranged to extend over the plurality of electrode pairs (403, 404, 503, 504) of the array and configured to be in electrical contact with the source electrode (404, 504) and the drain electrode (403, 504) of each electrode pair (403, 404, 503, 504) to create an array of sensors, wherein each sensor has a source electrode (404, 504), a drain electrode (403, 503) and a channel formed by part of the common conductive or semiconductive layer (406, 506), one of the source (404, 504) and the drain (403, 503) electrodes configured to receive a flow of charge carriers for injecting into the channel and the other configured to provide a current sink for said flow of charge carriers from the channel, wherein the source electrode (404, 504) and drain electrode (403, 503) are elongate and the channel has a channel width defined by the longitudinal extent of the source (404, 504) and/or drain (403, 503) electrode and a channel length defined by the separation between the source (404, 504) and drain (403, 503) electrodes; and wherein the source electrode (404, 504) or drain electrode (403, 503) of each sensor forms a substantially continuous sensor perimeter at least along the channel width which substantially encloses the other electrode (403, 404, 503, 504) of the respective sensor to inhibit the flow of charge carriers beyond the sensor perimeter to inhibit crosstalk between sensors in the array, wherein each sensor includes a gate electrode (405, 505) arranged for modulating the conductivity of the channel between the drain electrode (403, 503) and the source electrode (404, 504) and a dielectric material is interposed between the gate electrode (405, 505) and the common conductive or semiconductive layer (406, 506), and wherein the common conductive or semiconductive layer (406, 506) is graphene.