- The passage of a particle, such as an electron, through a potential barrier that could not be crossed according to classical mechanics. quantum mechanics, however, takes into account the wavelike properties of particles, and provides for the existence of a probability of an electron passing through this region. The wave function of such a particle allows for a given probability that the particle will appear on the other side of said barrier. The tunnel effect, for instance, explains alpha decay, and is employed, for example, in tunnel diodes. Also called tunneling (1).