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In semiconductors, the band gap of a semiconductor can be of two basic types, a direct band gap or an indirect band gap. The minimal-energy state in the conduction band and the maximal-energy state in the valence band are each characterized by a certain crystal momentum (k-vector) in the Brillouin zone. If the k-vectors are different, the ...
The optical band gap (see below) determines what portion of the solar spectrum a photovoltaic cell absorbs. [18] Strictly, a semiconductor will not absorb photons of energy less than the band gap; whereas most of the photons with energies exceeding the band gap will generate heat. Neither of them contribute to the efficiency of a solar cell.
The Brus equation or confinement energy equation can be used to describe the emission energy of quantum dot semiconductor nanocrystals in terms of the band gap energy E gap, the Planck constant h, the radius of the quantum dot r, as well as the effective mass of the excited electron m e * and of the excited hole m h *.
Energy band gaps can be classified using the wavevectors of the states surrounding the band gap: Direct band gap: the lowest-energy state above the band gap has the same k as the highest-energy state beneath the band gap. Indirect band gap: the closest states above and beneath the band gap do not have the same k value.
A, B, and C depend on the band structure of the material. They are positive constants such that 4C − B 2 > 0. Finally, n(∞), a constant greater than unity, represents the value of n at E = ∞. The parameters B 0 and C 0 in the equation for n(E) are not independent parameters, but depend on A, B, C, and E g. They are given by:
Since the energy of a photon is inversely proportional to its wavelength, QE is often measured over a range of different wavelengths to characterize a device's efficiency at each photon energy level. For typical semiconductor photodetectors, QE drops to zero for photons whose energy is below the band gap .
In solid-state physics, an energy gap or band gap is an energy range in a solid where no electron states exist, i.e. an energy range where the density of states vanishes. Especially in condensed matter physics , an energy gap is often known more abstractly as a spectral gap , a term which need not be specific to electrons or solids.
Shockley and Queisser calculated that the best band gap for sunlight happens to be 1.1 eV, the value for silicon, and gives a u of 44%. They used blackbody radiation of 6000K for sunlight, and found that the optimum band gap would then have an energy of 2.2 kT s. (At that value, 22% of the blackbody radiation energy would be below the band gap.)