Trace amounts of current which continue to flow between source and drain parts of transistors when the voltage is removed. This unwanted leakage current wastes power and becomes even more of a problem as transistors get smaller and more numerous. As inversion layers (the conductor in a transistor) get smaller, the amount of current it can conduct also is reduced to the level of the leakage current, making it hard to tell the difference between "on" and "off" transistors, meaning the transistor's primary purpose – transmitting I's and O's of binary – becomes lost.