- Electron avalanche
An electron avalanche is a process in which a number of
free electrons in a medium (usually a gas) are subjected to strong acceleration by an electric field, ionizing the medium's atoms by collision (called impact ionization), thereby forming "new" electrons to undergo the same process in successive cycles. Electron avalanches are essential to the dielectric breakdownprocess within gases. The process can culminate in corona discharges, streamers, leaders, or in a sparkor continuous arc that completely bridges the gap. The process extends to huge sparks — streamers in lightningdischarges propagate by formation of electron avalanches created in the high potential gradientahead of the streamers' advancing tips. Once begun, avalanches are often intensified by the creation of photoelectrons as a result of ultravioletradiation emitted by the excited medium's atoms in the aft-tip region.
A plasma begins with a rare natural 'background' ionization event of a neutral air molecule, perhaps as the result of
photoexcitationor background radiation. If this event occurs within an area that has a high potential gradient, the positively charged ionwill be strongly attracted toward, or repelled away from, an electrodedepending on its polarity, whereas the electron will be accelerated in the opposite direction. Because of the huge mass difference, electrons are accelerated to a much higher velocitythan ions.
High-velocity electrons often collide with neutral atoms inelastically, sometimes ionizing them. In a chain-reaction — or an 'electron avalanche' — additional electrons recently separated from their positive ions by the strong potential gradient, cause a large cloud of electrons and positive ions to be momentarily generated by just a single initial electron. However, free electrons are easily "captured" by neutral oxygen or water vapor molecules (so-called electronegative gases), forming negative ions. In air at STP, free electrons exist for only about 11
nanoseconds before being captured. Captured electrons are effectively removed from play — they can no longer contribute to the avalanche process. If electrons are being created at a rate greater than they are being lost to capture, their number rapidly multiplies, a process characterized by exponential growth. The degree of multiplication that this process can provide is huge, up to several million-fold depending on the situation. The multiplication factor "M" is given by
Where "X"1 and "X"2 are the positions that the multiplication is being measured between, and α is the ionization constant. In other words, one free electron at position "X"1 will result in "M" free electrons at position "X"2. Substituting the voltage gradients into this equation results in
Where "V" is the applied voltage, "V"BR is the breakdown voltage and "n" is an empirically derived value between 2 and 6. As you can see from this formula, the multiplication factor is very highly dependent on the applied voltage, and as the voltage nears the breakdown voltage of the material, the multiplication factor approaches infinity and the limiting factor becomes the availability of charge carriers.
Avalanche sustenance requires a reservoir of charge to sustain the applied voltage, as well as a continual source of triggering events. A number of mechanisms can sustain this process, creating avalanche after avalanche, to create a corona current. A secondary source of plasma electrons is required as the electrons are always accelerated by the field in one direction, meaning that avalanches always proceed linearly toward or away from an
electrode. The dominant mechanism for the creation of secondary electronsdepends on the polarity of a plasma. In each case, the energyemitted as photons by the initial avalanche is used to ionisea nearby gas molecule creating another accelerable electron. What differs is the source of this electron. When one or more electron avalanches occur between two electrodes of sufficient size, complete avalanche breakdowncan occur, culminating in an electrical sparkthat bridges the gap.
* [http://ece-www.colorado.edu/~bart/book/book/chapter4/ch4_5.htm Breakdown effects in semiconductors ]
Wikimedia Foundation. 2010.
Look at other dictionaries:
electron avalanche — elektroninė griūtis statusas T sritis fizika atitikmenys: angl. electron avalanche vok. Elektronenlawine, f rus. электронная лавина, f pranc. avalanche électronique, f … Fizikos terminų žodynas
electron avalanche — noun see avalanche I … Useful english dictionary
Avalanche breakdown — is a phenomenon that can occur in both insulating and semiconducting materials. It is a form of electric current multiplication that can allow very large currents to flow within materials which are otherwise good insulators. Explanation Avalanche … Wikipedia
Avalanche (disambiguation) — An avalanche is a cascade of snow down a slope.Groups and things that have been named after the avalanche include:Phenomena: *Rock avalanche *Avalanche breakdown, an electrical process *Avalanche effect, aka bit avalanche , a desirable property… … Wikipedia
avalanche électronique — elektroninė griūtis statusas T sritis fizika atitikmenys: angl. electron avalanche vok. Elektronenlawine, f rus. электронная лавина, f pranc. avalanche électronique, f … Fizikos terminų žodynas
avalanche — [ avalɑ̃ʃ ] n. f. • 1611; lavanche XVIe; mot savoyard lavantse; bas lat. labina « glissement de terrain », de labi (cf. labile); altér. d apr. 1. aval 1 ♦ Masse de neige qui se détache d une montagne, qui dévale en entraînant des pierres, des… … Encyclopédie Universelle
Avalanche photodiode — Avalanche photodiodes (APDs) are photodetectors that can be regarded as the semiconductor analog to photomultipliers. By applying a high reverse bias voltage (typically 100 200 V in silicon), APDs show an internal current gain effect (around 100) … Wikipedia
Avalanche-Photodiode — Avalanche Photodioden bzw. Lawinenphotodioden (englisch avalanche photodiode, APD), sind hochempfindliche und schnelle Photodioden. Sie nutzen den inneren photoelektrischen Effekt zur Ladungsträgererzeugung und den Lawinendurchbruch… … Deutsch Wikipedia
Avalanche transistor — An Avalanche Transistor is a bipolar junction transistor designed for operation in the region of its collector current/collector to emitter voltage characteristics beyond the collector to emitter breakdown voltage, called avalanche breakdown… … Wikipedia
avalanche effect — ▪ physics in physics, a sudden increase in the flow of an electrical current (electric current) through a nonconducting or semiconducting solid when a sufficiently strong electrical force is applied. The ability of most nonmetallic solids… … Universalium