What is the name of the effect where electrons tunnel through a barrier?

The effect where electrons tunnel through a barrier is called quantum tunneling. It occurs when a particle passes through a potential-energy barrier that classical physics says it lacks enough energy to cross.

The effect follows from quantum mechanics’ wave description of matter. An electron’s wave function does not abruptly become zero at a narrow barrier; it extends into and sometimes beyond it. The probability of transmission falls exponentially as the barrier becomes higher or wider, which is why tunneling is especially important for lightweight particles such as electrons.

Quantum tunneling was demonstrated in semiconductor devices by Leo Esaki in 1957. It also explains alpha decay, helps power the scanning tunneling microscope, and allows superconducting Cooper pairs to cross insulating barriers. The related Josephson effect is used in sensitive detectors and quantum technologies.

A common mix-up is calling this the photoelectric effect. The photoelectric effect ejects electrons when light supplies enough energy; tunneling is barrier penetration without classically surmounting the barrier.