Transistors are either on or off in digital circuits, ideally. A simple inverter is two transistors stacked on top, one connects the output to ground when on, t

Transistors are either on or off in digital circuits, ideally. A simple inverter is two transistors stacked on top, one connects the output to ground when on, the other connects the output to vdd (the supply voltage) when on. Of course only one can be on at a time, as otherwise there would be a direct connection between power and ground via the output wire. The residual current is a measure of how 'off' a transistor is. As planar transistors scaled down, it was harder and harder to fully turn them off, so in effect every transistor was consuming an inordinate amount of power even when off by enabling this leakage path from vdd to ground. FinFET improved this by having the gate that controls if the transistor is on or off on 3 sides (the fin) , which meant the transistor really could be turned off more completely.