A semiconductor device (300a) comprising: a substrate (302) having a first surface (303); an n-type well (304) extending from the first surface (303) into the substrate (302) and configured to form a depletion region (306) in the substrate (302) around the n-type well (304); an insulating layer (340) extending over the first surface (303) of the substrate (302) from the n-type well (304), the insulating layer (340) configured to form an inversion layer (342) in the substrate (302) extending from the n-type well (304) adjacent to the first surface (303); wherein a p-type floating channel stopper (370a) is provided, configured to extend through the inversion layer (342) to reduce electrical coupling between the n-type well (304) and at least part of the inversion layer (342), and is electrically disconnected from a remainder of the substrate (320) outside of the depletion region (306).