There are also some organic semi-conducting compounds, but their analysis is beyond the scope of this article. A semiconductor is called intrinsic if its conductivity is the result of equal contributions from its own electrons and holes. The equation must then be expanded:. The mobilities are usually different. These terms add because the opposite charges move in opposite directions, resulting in a pair of like signs in each product.
For application in devices, semiconductors are rarely used in their pure or intrinsic composition. Under carefully controlled conditions, impurities are introduced which contribute either an excess or a deficit of electrons. Excess electrons neutralize holes so that only electrons are available for conduction. The resulting material is called n-type, n for negative carrier. An example of n-type material is Si with Sb, a column IV element with a column V impurity known as a donor. In n-type material, donor atoms remain fixed and positively ionized.
When a column III impurity is infused into a column IV element, electrons are bound and holes made available. That material is called p-type, p for positive carrier. Column III impurities are known as acceptors; in the material acceptor atoms remain fixed and negatively ionized. An example of p-type material is Si with Ga. Both n-type and p-type semiconductors are referred to as extrinsic. Thermal kinetic energy is not the only mechanism for the release of charge carriers in semiconductors.
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Photons with energy equal to the activation energy can be absorbed by a bound electron, which, in an intrinsic semiconductor, adds both itself and a hole as mobile carriers. These photons may be in the visible range or in the near infrared, depending on E G. In extrinsic semiconductors, photons of much lower energies can contribute to the pool of the prevailing carrier type, provided the material is cooled to cryogenic temperatures in order to reduce the population of thermally activated carriers.
This behavior is known as photoconductivity. Each separate variety of semiconductor is ohmic, with the conductivity constant at constant temperature. However, as the temperature is increased, the conductivity increases very rapidly. The concentration of available carriers varies in accordance with an exponential function:. The increase in available charge carriers overrides any decrease in mobility, and this leads to a negative value for a.
Indeed, a decrease in resistance with increasing temperature is a reliable indication that a substance is a semiconductor, not a metal. The Fermi level, E F , can be shown differently for intrinsic, n-type, and p-type semiconductors. However, for materials physically connected, E F must be the same for thermal equilibrium.
This is a consequence of the laws of thermodynamics and energy conservation. Thus, the behavior of various junctions, in which the interior energy levels shift to accommodate the alignment of the Fermi level, is extremely important for the semiconductor devices. Non-ohmic conduction is marked by nonlinear graphs of current vs. It occurs in semiconductor junctions, electrolytic solutions, some ionic solids not in solution, ionized gases, and vacuum tubes. Respective examples include semiconductor p-n diodes, battery acid or alkaline solutions, alkali halide crystals, the ionized mercury vapor in a fluorescent lamp, and cathode ray tubes.
Ionic conductivities are much lower than electronic, because the masses and diameters of ions make them much less mobile. While ions can drift slowly in a gas or liquid, their motions through the interstices of a solid lattice are much more restricted. Yet, with their thermal kinetic energy, ions will diffuse through a lattice, and in the presence of an electric field, will wander toward the appropriate electrode. In most instances, both ionic and electronic conduction will occur, depending on impurities.
Thus, for studies of ionic conductivity, the material must be a very pure solid. In gases, the gas atoms must be ionized by an electric field sufficient to supply the ionization energy of the gas in the tube. Electrons falling back into bound states produce the characteristic spectrum of the gas, qualitatively associated with color, e. The basic definition of a plasma in physics includes all material conductors, ohmic and nonohmic.
A plasma is a medium in which approximately equal numbers of opposite charges are present, so that the medium is neutral or nearly so. In a metal the negative electrons are separated from an equal number of positive ion cores. In a semiconductor there may be holes and electrons intrinsic , holes and ionized acceptors p-type , or electrons and ionized donors n-type. In an electrolytic solution and in an ionic solid there are positive and negative ions. An ionized gas contains electrons and positive ions. A small distinction among these may be made as to whether the medium has one or two mobile carriers.
In contemporary usage, the term plasma usually refers to extremely hot gases such as those used in the Tokamak for nuclear fusion experiments. High-energy plasmas are discussed in the article on fusion as a means of generating electric power. Lattice — The structure of atoms in a solid.
In a conducting material, ion cores make up the lattice. Potential difference — In a conductor carrying an electric current , it is the difference of potential energy per unit charge.
The remaining non-ohmic conduction category is the vacuum tube , in which a beam of electrons is emitted from either a heated cathode thermionic or a suitably illuminated cathode photoelectric , and moves through evacuated space to an anode. The beam in its passage is subjected to electrostatic or magnetic fields for control. The evacuated space cannot be classed either as a material with a definable conductivity or as a plasma, since only electrons are present. However, there are relations of current and voltage to be analyzed.
These graphs are generally nonlinear or linear over a limited range. But vacuum tubes are not called ohmic even in their linear ranges because there is no material undergoing the lattice behavior previously described as the basis for ohmic resistance.
Electrical conduction in the human body and other animal organisms is primarily ionic, since body fluids contain vital electrolytes subject to electrochemical action in organs. Further information is available in other articles, particularly those on the heart, the brain, and neurons.
See also Chemical bond ; Electrolyte ; Nonmetal. Ellse, Mark and Chris Honeywill. Electricity and Thermal Physics.
Why do metals conduct heat and electricity so well?
Cheltenham, UK: Nelson Thornes, Halliday, David, et al. Fundamentals of Physics. New York : Wiley, Cite this article Pick a style below, and copy the text for your bibliography. September 24, Retrieved September 24, from Encyclopedia. Then, copy and paste the text into your bibliography or works cited list. Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.
Conductivity is the term used to describe the ability of a material medium to permit the passage of particles or energy. Electrical conductivity refers to the movement of charged particles through matter. Thermal conductivity refers to the transmission of heat energy through matter. In metals, electrical conductivity and thermal conductivity are related since both involve aspects of electron motion.
Benjamin Franklin in his experiments with lightning leading to his invention of the lightning rod , reasoned that the charge would travel along the metallic rod. Alessandro Volta derived the concept of electrical potential from his studies of static electricity , and then discovered the principle of the battery in his experiments with dissimilar metals in common contact with moisture. Georg Simon Ohm found the direct proportion relating current and potential difference, which became a measure of the ability of various metals to conduct electricity.
The relation among these functions is known as Ohm's law.
Related An Electron Theory of Electric Conduction in Metals
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