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Band Gap

Need to Know

  • What is band gap?
  • What is the relation between band gap and absorption of light?
  • How does band gap influence the efficiency of cell?
Bandgap in semiconductor

Every solid has its own characteristic energy-band structure. This variation in band structure is responsible for the wide range of electrical characteristics observed in various materials. In semiconductors and insulators, electrons (Links to an external site.) are confined to a number of bands (Links to an external site.) 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. Electrons are able to jump from one band to another. However, in order for an electron to jump from a valence band to a conduction band, it requires a specific minimum amount of energy for the transition. The required energy differs with different materials. Electrons can gain enough energy to jump to the conduction band by absorbing either a phonon (Links to an external site.) (heat) or a photon (Links to an external site.) (light).

Indirect vs direct band gap transitions.gif

An electron can be excited with a photon for a direct band gap. For an indirect band gap, where the conduction band is not directly above the electron, a phonon assisted transition is needed where the electron is first excited with a photon and then pushed side ways with a phonon. For certain conductors, the indirect band gap requires less energy to excite the electron, such as silicon, so only phonon assisted transitions occur until the photon has enough energy to excited an electron with a direct transition.

A semiconductor (Links to an external site.) is a material with a small but non-zero band gap that behaves as an insulator at absolute zero but allows thermal excitation of electrons into its conduction band at temperatures that are below its melting point. In contrast, an insulator (Links to an external site.) has a much larger band gap requiring higher energies to excite an electron or to transfer heat and electrical current. In conductors (Links to an external site.), the valence and conduction bands may overlap, so they may not have a band gap. This, essentially, creates a sea of electrons easily transferring heat or electrical current through the material.

The conductivity (Links to an external site.) of intrinsic semiconductors (Links to an external site.) is strongly dependent on the band gap. The only available charge carriers for conduction are the electrons that have enough thermal energy to be excited across the band gap and the electron holes (Links to an external site.) that are left off when such an excitation occurs.

Band-gap engineering is the process of controlling or altering the band gap of a material by controlling the composition of certain semiconductor alloys (Links to an external site.), such as GaAlAs, InGaAs, and InAlAs. It is also possible to construct layered materials with alternating compositions by techniques like molecular-beam epitaxy (Links to an external site.). These methods are exploited in the design of heterojunction bipolar transistors (Links to an external site.) (HBTs), laser diodes (Links to an external site.) and solar cells (Links to an external site.).

From Wikipedia

Band Gap of selected materials

Group Material Symbol Band gap @ 302K
IV <a title="Diamond" >Diamond</a> <a title="Carbon" >C</a> 5.5
<a title="Copper oxide" >Copper oxide</a> Cu2O 2.1
III–V <a title="Gallium arsenide" >Gallium arsenide</a> GaAs 1.43
III–V <a title="Gallium nitride" >Gallium nitride</a> GaN 3.4
III–V <a title="Gallium phosphide" >Gallium phosphide</a> GaP 2.26
IV <a title="Germanium" >Germanium</a> Ge 0.67
IV–VI <a title="Lead sulfide" >Lead sulfide</a> PbS 0.37
IV <a title="Silicon" >Silicon</a> Si 1.11
IV–V <a title="Silicon nitride">Silicon nitride</a> Si3N4 5
IV–VI <a title="Silicon dioxide" >Silicon dioxide</a> SiO2 0

Shockley Queisser Limits (Links to an external site.)