pic taken from :www.nanoscience.comQuantum mechanics tells us that electrons have both wave and particle like properties. Tunneling is an effect of the wavelike nature.
The top image shows us that when an electron (the wave) hits a barrier, the wave doesn't abruptly end, but tapers off very quickly - exponentially. For a thick barrier, the wave doesn't get past.
The bottom image shows the senario if the barrier is quite thin (about a nanometer). Part of the wave does get through, and therefore some electrons may appear on the other side of the barrier..Read more here: nanoscience
