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if a element or compound have a direct bandgap, when an electron gives energy and goes to conduction band from valence band, when it want to recombinate,
it release a photon. But for indirect band gap when it want to recombinate, it dissipate its energy as heat in the crystal.
Direct band gap material is one in which the peaks of conduction and valency bands
coinside ..so hole r electron can make transitions fro each band directly..
Incase indirect band gap material,heir peaks are not in same line..so they need some other particle for transition..which is a photon wich releases heat..
Thats wht they told! This heat they say is the acustic mode! You have these two: optic and acustic, the acustic only happens when you have an indirect transition!
plz see the book solid state devices by streetman
see the chapter number 3 u will be able to understand abt direct and indirect semiconductors.
this concept is very useful in lasers
In direct bandgap the minimum of the conduction band and the maximum of the valence band coincide when considered along the momentum axis and hence the transition is a momentum and energy conserving process and hence this transition takes place with the emission of a photon.... only materials with direct bandgap can emit photon eg:- GaAs..... This is the main reason that researches are being done to make Si a direct bandgap material which is by nature indirect bandgap material.....
In semiconductors and insulators, electrons are confined to a number of bands of energy, and forbidden from other regions. The term "band gap" refers to the energy difference between the top of the valence band and the bottom of the conduction band, where electrons are able to jump from one band to another. The other gaps are between bands are between a pair of filled or a pair of empty bands, which are unimportant to the properties of the semiconductor.
The conductivity of intrinsic semiconductors is strongly dependent on the band gap. The only available carriers for conduction are the electrons which have enough thermal energy to be excited across the band gap.
Band gap engineering is the process of controlling or altering the band gap of a material by controlling the composition of certain semiconductor alloys, such as GaAlAs, InGaAs, and InAlAs. It is also possible to construct layered materials with alternating compositions by techniques like molecular beam epitaxy. These methods are exploited in the design of heterojunction bipolar transistors (HBTs), laser diodes and solar cells.
The distinction between semiconductors and insulators is a matter of convention. One approach is to consider semiconductors a type of insulator with a low band gap. Insulators with a higher band gap, usually greater than 3 eV, are not considered semiconductors and generally do not exhibit semiconductive behaviour under practical conditions. Electron mobility also plays a role in determining a material's informal classification.
Band gap decreases with increasing temperature, in a process related to thermal expansion. Special purpose integrated circuits such as the DS1621 exploit this property to perform accurate temperature measurements. Band gap also depends on pressure. Bandgaps can be either direct or indirect bandgaps, depending on the band structure.
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