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Electron and hole mobility are special cases of electrical mobility of charged particles in a fluid under an applied electric field. When an electric field E is applied across a piece of material, the electrons respond by moving with an average velocity called the drift velocity, . Then the electron mobility μ is defined as =.
Electron and hole trapping in the Shockley-Read-Hall model. In the SRH model, four things can happen involving trap levels: [11] An electron in the conduction band can be trapped in an intragap state. An electron can be emitted into the conduction band from a trap level. A hole in the valence band can be captured by a trap.
The energy given to the electron by the photon "excites" it into the conduction band where it is free to move around within the semiconductor. The network of covalent bonds that the electron was previously a part of now has one fewer electron. This is known as a hole, and it has positive charge.
Hybrid solar cells exhibit material properties inferior to those of bulk silicon semiconductors. The carrier mobilities are much smaller than that of silicon. Electron mobility in silicon is 1000 cm 2 ·V −1 ·s −1, compared to 600 cm 2 ·V −1 ·s −1 in CdSe, and less than 10 cm 2 ·V −1 ·s −1 in other quantum dot
where D is the diffusion coefficient for the electron in the considered medium, n is the number of electrons per unit volume (i.e. number density), q is the magnitude of charge of an electron, μ is electron mobility in the medium, and E = −dΦ/dx (Φ potential difference) is the electric field as the potential gradient of the electric potential.
The two different mechanisms result in different charge mobilities. In disordered solids, disordered potentials result in weak localization effects (traps), which reduce the mean free path, and hence the mobility, of mobile charges. Carrier recombination also decreases mobility.
where the js are the current densities of electrons (e) and holes (p), the μs the charge carrier mobilities, E is the electric field, n and p the number densities of charge carriers, the Ds are diffusion coefficients, and x is position.
One or two of the valence electrons from each atom are able to move about freely within the crystal structure of the metal. [4] The free electrons are referred to as conduction electrons, and the cloud of free electrons is called a Fermi gas. [5] [6] Many metals have electron and hole bands. In some, the majority carriers are holes. [citation ...