"DARTH VADER" wrote in
oups.com:

What exactly is electron tunelling?

In classical physics, if a particle has energy A and it needs energy B to
get past a barrier, and A is less than B, then the particle simply won't
get past the barrier. This is true even if A is "close" to B or if the
energy needed after the particle gets past the barrier is equal to or less
than A.

In quantum mechanics, there is a probability that the particle will pass
the barrier even if A is less than B. The closer A is to B, and the
shorter the time that the particle needs the higher energy to pass, the
higher the probability is that the particle will get past. The effect is
called tunneling.

--
Steve Gray