In electronics, leakage is the gradual transfer of electrical energy across a boundary normally viewed as insulating, such as the spontaneous discharge of a charged capacitor, magnetic coupling of a transformer with other components, or flow of current across a transistor in the "off" state or a reverse-polarized diode.
It is a result of the dielectric material not being a perfect insulator and having some non-zero conductivity, allowing a leakage current to flow, slowly discharging the capacitor.
This sort of leakage is undesirable because the current flowing through the alternate path can cause damage, fires, RF noise, or electrocution.
These leakage currents are becoming a significant factor to portable device manufacturers because of their undesirable effect on battery run time for the consumer.
In semiconductor devices, leakage is a quantum phenomenon where mobile charge carriers (electrons or holes) tunnel through an insulating region.
Other than tunneling via the gate insulator or junctions, carriers can also leak between source and drain terminals of a Metal Oxide Semiconductor (MOS) transistor.
Efforts to minimize leakage include the use of strained silicon, high-κ dielectrics, and/or stronger dopant levels in the semiconductor.
In heterostructure field-effect transistors (HFETs) the gate leakage is usually attributed to the high density of traps residing within the barrier.