DIRECT BAND GAP AND INDIRECT BAND GAP SEMICONDUCTORS
In semiconductor physics, the band gap of a semiconductor is always one of two types,
(A) Direct band gap
(B) Indirect band gap
DIRECT BAND GAP SEMIONDUCTOR
The band gap represents the minimum energy difference between the top of the valence band and the bottom of the conduction band
However, the top of the valence band (valence band, VB maxima) and the bottom of the conduction band (conduction band minima, CB minima) are not generally at the same value of the electron momentum.
Minimum-energy state in the conduction band (CB -minima) and the maximum-energy state in the valence band (VB-maxima) are each characterized by a certain crystal momentum and k- vector (propagation constant or wave vector) in the Brillouin zone.
If the k-vectors and are the same, for conduction band minima (CB-minima) and valence Band maxima (VB- maxima) it is called a “Direct band gap semiconductor”
The band gap is called “direct” if the momentum of electrons and holes is the same in both the conduction band and the valence band; an electron can directly emit a photon.
In case of “direct band semiconductor ” electron in conduction band (CB) minima ,recombine directly with the holes in valence band (VB) maxima without change in momentum as well as kinetic energy ,so energy will be emitted in the form of light , this phenomenon is called as “SPONTANEOUS EMISSION “
In direct band gap semiconductor Energy is conserved by means of emitting a photon, such transitions are called as radiative transitions.
Relative carrier life time is small in case of direct band gap semiconductor
Best example of direct band gap semiconductors are GaAs InAs, InSb GaN InN ZnO CdSe ZnS
GaAs emits light in Infra-red region
INDIRECT BAND GAP SEMICONDUCTOR
If the k-vectors (Propagation constant or wave vector) are the different for conduction band minima (CB-minima) and valence Band maxima (VB- maxima) it is called a “Indirect band gap semiconductor”
In this case momentum is not same for conduction band minima ( CB-minima) and valence band maxima ( VB –maxima)
In an “indirect” gap, a photon cannot be emitted because the electron must pass through an intermediate state and transfer momentum to the crystal lattice.
In case of indirect band gap semiconductors during excitation there is change in momentum , K.E. as well as direction and path of electron
In indirect band gap semiconductors energy emits in the form of heat
Carrier life time in case of indirect band gap semiconductor is greater than the carrier life time of direct band gap semiconductor
Best examples of indirect band gap semiconductors are Si , Ge ,C (diamond) ,GaP