sunilkumar wrote:

to find a way to make a tunnel in the barrier,That means doing some work to overcome the potential barrier.If you have any more questions on this then i will explain you promptly.

Actually, if you increase the energy to overcome the potential barrier, then you are not "tunneling".

Take a baseball trapped in a box. As long as you do not significantly jiggle the box, the baseball will never fly out. In the classical picture the baseball is trapped forever. Baseballs cannot "tunnel" through the box.

But on smaller scales (like the size of an electron), it turns out that particles do not act like baseballs that have a particular position. Rather they behave more like spread-out waves. The description of how they behave is called quantum mechanics. The energy term for the particle's wave (similar to our baseball's kinetic energy) can be negative in a particular region. So the wave can leak through a region where the potential energy is actually bigger than the total energy. So if we stick an electron in the same box, then most of it's wave is in the box, but some of it lies outside of the box.

When the electron wave interacts with other things, for example when try to view it in a microscope, it quits acting so spread out. When you are looking at the elctron, it "looks" like a baseball and not a wave. So when you look for it, you will usually find it in regions where the wave is large, like the center of the box. Yet sometimes, you could see it outside of the box. When this happens, we say the electron tunneled out of the box.

Also see this:

http://en.wikipedia.org/wiki/Quantum_tunneling