Impact ionization is said to be generation of electron-hole pairs with help of free electrons and holes. I have the intuition on how a free electron would be able to generate electron hole pair. I can't understand how a hole can do so.Please help.
A hole is just a low energy location in the valence band, where electrons tend to jump.
A hole could be a vacancy left behind by an electron jumping to the conduction band or to another hole or it could be a low energy orbital of an acceptor atom imbedded, via a p-type doping, into semiconductor to promote conductivity in the valence band.
When, under the influence of an electric field, a valence electron jumps to a hole and leaves another hole behind, another electron could jump into that new hole, setting off a chain of jumps and thus creating a current. This could make an impression that a hole itself is moving, giving a rise to the concept of a hole current, which flows in the direction opposite to the direction of the electron current.
In fact, of course, we still have moving electrons, and, similar to free electrons in the conduction band, they could hit neutral atoms and, given a strong enough field, generate new electron-hole pairs.