The recombination procedure is represented with returning the electron from the CB where it is free into a hole in the VB where it is in a bond. The excess energy of the electron is lost as a photon of energy. This type of recombination results in the emitted light from light emitting diodes. The recombination process between an electron and a hole like every other process in nature, must obey the conservation law of energy. The wave function of an electron in the CB will have the momentum and similarly electron wave function in the VB will have a momentum. Conservation of linear momentum during recombination process requires that when the electron drops from the CB to the VB, its wave vector remain the same, K_vb= K_cb. The states with this are right at the top of the valance band, they are essentially empty (certain holes) this kind of recombination is called Direct Recombination. This type of recombination is highly probable in gallium arsenide etc. In elemental semiconductor crystal for example Si and Gallium the minimum or lowest energy of CB is not directly above the maximum of the VB. Hence, electron at the bottom of the CB there fore cannot recombine directly with a hole at the top of the VB. The law of conservation of momentum is not allowed in this case and therefore momentum is not allowed in this case and therefore momentum must change. The recombination process in these elemental semiconductors occurs via a recombination centres at an energy level E which is represented below E_c in the band gap. When an electron approaches the centre at E_r, it is captured. The electron is then localized and bound to this centre and “waits” therefore for a hole with which it can recombine. And then it can fall down into the top of the VB. The energy of electron is lost to lattice vibration (as sound) via the recoiling of the body (recombination centre). Emitted lattice vibrations are known as phonons. A phonon is a quantum of energy associated with atomic vibrations in the crystals analogues to the photon.