A lateral semiconductor device has a semiconductor layer on an insulating layer on a semiconductor substrate. The semiconductor layer has a region of a first conduction type and a region of a second conduction type with a drift region therebetween. The drift region is provided by a region of the first conduction type and a region of the second conduction type. The first and second conduction type drift regions are so arranged that when a reverse voltage bias is applied across the first and second conduction type regions of the semiconductor layer, the second conduction type drift region has an excess of charge relative to the first conduction type drift region which varies substantially linearly from the end of the drift region towards the first conduction type region of the semiconductor layer to the end of the drift region towards the second conduction type region of the semiconductor layer.