Editing Electric current (section)

===Semiconductor===
{{Main|Semiconductor}}

In a [[semiconductor]] it is sometimes useful to think of the current as due to the flow of positive "[[electron hole|holes]]" (the mobile positive charge carriers that are places where the semiconductor crystal is missing a valence electron). This is the case in a p-type semiconductor. A semiconductor has [[electrical conductivity]] intermediate in magnitude between that of a [[electrical Conductor|conductor]] and an [[insulator (electrical)|insulator]]. This means a conductivity roughly in the range of 10<sup>−2</sup>  to 10<sup>4</sup> [[siemens (unit)|siemens]] per centimeter (S⋅cm<sup>−1</sup>).

In the classic crystalline semiconductors, electrons can have energies only within certain bands (i.e. ranges of levels of energy). Energetically,  these bands are located between the energy of the ground state, the state in which electrons are tightly bound to the atomic nuclei of the material, and the free electron energy, the latter describing the energy required for an electron to escape entirely from the material. The energy bands each correspond to many discrete [[quantum state]]s of the electrons, and most of the states with low energy (closer to the nucleus) are occupied, up to a particular band called the ''[[valence band]]''. Semiconductors and insulators are distinguished from [[metals]] because the valence band in any given metal is nearly filled with electrons under usual operating conditions, while very few (semiconductor) or virtually none (insulator) of them are available in the ''conduction band'', the band immediately above the valence band.

The ease of exciting electrons in the semiconductor from the valence band to the conduction band depends on the [[band gap]] between the bands. The size of this energy band gap serves as an arbitrary dividing line (roughly 4 [[electronvolt|eV]]) between semiconductors and [[Electrical insulation|insulators]].

With covalent bonds, an electron moves by hopping to a neighboring bond. The [[Pauli exclusion principle]] requires that the electron be lifted into the higher anti-bonding state of that bond. For delocalized states, for example in one dimension{{snd}}that is in a [[nanowire]], for every energy there is a state with electrons flowing in one direction and another state with the electrons flowing in the other. For a net current to flow, more states for one direction than for the other direction must be occupied. For this to occur, energy is required, as in the semiconductor the next higher states lie above the band gap. Often this is stated as: full bands do not contribute to the [[electrical conductivity]]. However, as a semiconductor's temperature rises above [[absolute zero]], there is more energy in the semiconductor to spend on lattice vibration and on exciting electrons into the conduction band. The current-carrying electrons in the conduction band are known as ''free electrons'', though they are often simply called ''electrons'' if that is clear in context.