An apparatus is provided which includes an array of impurity ions disposed in an insulating region, a semiconductor region adjacent to the insulating region, an array of electrometers arranged to detect charge carriers in the semiconductor region and an array of sets of at least one control gate configured to apply an electric field to the insulating region and semiconductor region. Each control gate is operable to cause at least one charge carrier in the semiconducting material region to bind to the impurity ion without the at least one charge carrier leaving the semiconductor material region. The electrometers are operable to detect whether the at least one charge carrier is bound to the impurity ion.