Band gap

In solid state physics, a band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference (in electron volts) between the top of the valence band and the bottom of the conduction band in insulators and semiconductors. This is equivalent to the energy required to free an outer shell electron from its orbit about the nucleus to become a mobile charge carrier, able to move freely within the solid material. In conductors, the two bands often overlap, so they may not have a band gap.

Contents

In semiconductor physics

Semiconductor band structure.

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; electrons are able to jump from one band to another. 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 (heat) or a photon (light).

A material with a small but nonzero band gap which behaves as an insulator at absolute zero but allows thermal excitation of electrons into its conduction band at temperatures which are below its melting point is referred to as a semiconductor. A material with a large band gap is called an insulator. In conductors, the valence and conduction bands may overlap, so they may not have a band gap.

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 think of semiconductors as a type of insulator with a narrow band gap. Insulators with a larger 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.

The band gap energy of semiconductors tends to decrease with increasing temperature. When temperature increases, the amplitude of atomic vibrations increase, leading to larger interatomic spacing. The interaction between the lattice phonons and the free electrons and holes will also affect the band gap to a smaller extent[1]. The relationship between band gap energy and temperature can be described by Varshni's empirical expression,

E_g(T)=E_g(0)-\frac{\alpha T^2}{T+\beta}, where Eg(0), α and β are material constants.[2]

In a regular semiconductor crystal, the band gap is fixed owing to continuous energy states. In a quantum dot crystal, the band gap is size dependent and can be altered to produce a range of energies between the valence band and conduction band.[3] It is also known as quantum confinement effect.

Band gaps also depend on pressure. Band gaps can be either direct or indirect, depending on the electronic band structure.

Mathematical interpretation

Classically, the ratio of probabilities that two states with an energy difference ΔE will be occupied by an electron is given by the Boltzmann factor:

e^{\left(\frac{-\Delta E}{kT}\right)}

where:

e is the exponential function
ΔE is the energy difference
k is Boltzmann's constant
T is temperature

At the Fermi level (or chemical potential), the probability of a state being occupied is ½. If the Fermi level is in the middle of a band gap of 1 eV, this ratio is e–20 or about 2.0•10–9 at the room-temperature thermal energy of 25.9 meV.

Photovoltaic cells

The band gap determines what portion of the solar spectrum a photovoltaic cell absorbs.[4] A luminescent solar converter uses a luminescent medium to downconvert photons with energies above the band gap to photon energies closer to the band gap of the semiconductor comprising the solar cell.[5]

List of band gaps

Material Symbol Band gap (eV) @ 300K Reference
Silicon Si 1.11 [6]
Selenium Se 1.74
Germanium Ge 0.67 [6]
Silicon carbide SiC 2.86 [6]
Aluminium phosphide AlP 2.45 [6]
Aluminium arsenide AlAs 2.16 [6]
Aluminium antimonide AlSb 1.6 [6]
Aluminium nitride AlN 6.3
Diamond C 5.5
Gallium(III) phosphide GaP 2.26 [6]
Gallium(III) arsenide GaAs 1.43 [6]
Gallium(III) nitride GaN 3.4 [6]
Gallium(II) sulfide GaS 2.5
Gallium antimonide GaSb 0.7 [6]
Indium(III) nitride InN 0.7 [7]
Indium(III) phosphide InP 1.35 [6]
Indium(III) arsenide InAs 0.36 [6]
Zinc oxide ZnO 3.37
Zinc sulfide ZnS 3.6 [6]
Zinc selenide ZnSe 2.7 [6]
Zinc telluride ZnTe 2.25 [6]
Cadmium sulfide CdS 2.42 [6]
Cadmium selenide CdSe 1.73 [6]
Cadmium telluride CdTe 1.49 [8]
Lead(II) sulfide PbS 0.37 [6]
Lead(II) selenide PbSe 0.27 [6]
Lead(II) telluride PbTe 0.29 [6]
Copper(II) oxide CuO 1.2 [9]
Copper(I) oxide Cu2O 2.1 [10]

In photonics and phononics

In photonics band gaps or stop bands are ranges of photon frequencies where, if tunneling effects are neglected, no photons can be transmitted through a material. A material exhibiting this behaviour is known as a photonic crystal.

Similar physics applies to phonons in a phononic crystal.

Materials

  • Aluminium gallium arsenide
  • Boron nitride
  • Indium gallium arsenide
  • Indium arsenide
  • Gallium arsenide
  • Germanium
  • Metallic hydrogen

List of electronics topics

See also

References

  1. ↑ H. Unlu (1992). "A Thermodynamic Model for Determining Pressure and Temperature Effects on the Bandgap Energies and other Properties of some Semiconductors". Solid State Electronics 35: 1343–1352. doi:10.1016/0038-1101(92)90170-H. 
  2. ↑ Temperature dependence of the energy bandgap
  3. ↑ “Evident Technologies”
  4. ↑ Nanoscale Material Design
  5. ↑ Nanocrystalline luminescent solar converters, 2004
  6. ↑ 6.00 6.01 6.02 6.03 6.04 6.05 6.06 6.07 6.08 6.09 6.10 6.11 6.12 6.13 6.14 6.15 6.16 6.17 6.18 6.19 Streetman, Ben G.; Sanjay Banerjee (2000). Solid State electronic Devices (5th ed.). New Jersey: Prentice Hall. p. 524. ISBN 0-13-025538-6. 
  7. ↑ Wu, J. (2002). "Unusual properties of the fundamental band gap of InN". Applied Physics Letters 80: 3967. doi:10.1063/1.1482786. 
  8. ↑ Madelung, Otfried (1996). Semiconductors - Basic Data (2nd rev. ed.). Springer-Verlag. ISBN 3-540-60883-4. 
  9. ↑ Elliott, R. J. (1961). "Symmetry of Excitons in Cu2O". Physical Review 124: 340. doi:10.1103/PhysRev.124.340. 
  10. ↑ Baumeister, P.W. (1961). "Optical Absorption of Cuprous Oxide". Physical Review 121. 

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