Leak Off Current in Transistors refers to the curent which passes through the transistor, even when it is switched off and the PN regions are not in forward bias.
Leak-off current in modern transistors is very small. The leak off current varies inversely with the thickness of the semiconductor layer.
Leak-off Current is caused by design imperfections in the transistors. the Leak-off Current is of the order of micro amperes or nano amperes. The leak-off current is considered to be significant only when the transistor is designed to be of very low power rating.