What is meant by tunnelling phenomenon?

A tunnel diode is a high conductivity two-terminal P-N junction doped heavily about 1,000 times higher than a conventional junction diode. Due to heavy doping depletion layer width is reduced to an extremely small value of order of 10-5nm, reverse breakdown voltage is also reduced to a very small value (approaching zero) resulting in appearance of diode to be broken for any reverse voltage and a negative resistance section is produced. It's found that reduced depletion layer can result in carriers 'punching through' the junction with the velocity of light even when they don't possess enough energy to overcome the potential barrier. Result is that large forward current is produced at relatively low forward voltage (less than 100 mV). Such a mechanism  of  conduction  in  which  electrons  (possessing  very  little  energy)  punch through a barrier directly rather than climbing over it is known as tunnelling phenomenon.