A detector device comprises a substrate (50), a source region (S) and a drain region (D), and a channel region (65) between the source and drain regions. A nanopore (54) passes through the channel region, and connects fluid chambers (56,58) on opposite sides of the substrate. A voltage bias is provided between the fluid chambers, the source and drain regions and a charge flow between the source and drain regions is sensed. The device uses a nanopore for the confinement of a sample under test (for example nucleotides) close to a sensor. The size of the sensor can be made similar to the spacing of adjacent nucleotides in a DNA strand. In this way, the disadvantages of PCR based techniques for DNA sequencing are avoided, and single nucleotide resolution can be attained.