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. // Definition An Insulator is a material or object which resists the flow of electric charge. ... A semiconductor is a solid whose electrical conductivity can be controlled over a wide range, either permanently or dynamically. ... Electric current is the flow of electric charge. ...

Explanation

Avalanche breakdown can occur within solids, liquids, or gases when the voltage applied across the insulating material is great enough to accelerate free electrons to the point that, when they strike atoms in the material, they can knock other electrons free. This phenomenon can be quite useful in semiconductor diodes such as the avalanche diode and avalanche photodiode, but in other situations, such as in MOSFET transistors, it can cause excessive current flow and destroy the device. When avalanche breakdown occurs within a solid insulating material it is almost always destructive. When an avalanche-like effect occurs without connecting two electrodes, it is referred to as an electron avalanche. Although there are some superficial similarities to Zener breakdown, this is a very different effect. now. ... The electron is a fundamental subatomic particle that carries an electric charge. ... Properties For other articles with similar names, see Atom (disambiguation). ... Types of diodes In electronics, a diode is a component that restricts the direction of movement of charge carriers. ... An avalanche diode is a diode (usually made from silicon, but can be made from another semiconductor) that is designed to break down and conduct at a specified reverse bias voltage. ... Avalanche photodiodes (APDs) are photodetectors that can be regarded as the semiconductor analog to photomultipliers. ... The metal-oxide-semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET), is by far the most common field-effect transistor in both digital and analog circuits. ... Assorted transistors The transistor is a solid state semiconductor device that can be used for amplification, switching, voltage stabilization, signal modulation and many other functions. ... An electron avalanche is a process in which a number of free electrons in a medium (usually a gas) are subjected to a strong electric field accelerate, ionizing the mediums atoms by collision (creating positive ions), forming new electrons to undergo the same process in successive cycles. ... Zener diode schematic symbol A Zener diode is a type of diode that permits current to flow in the forward direction like a normal diode, but also in the reverse direction if the voltage is larger than the rated breakdown voltage known as Zener knee voltage or Zener voltage. A...

The avalanche process

Avalanche breakdown is a current multiplication process that occurs only in strong electric fields, which can be caused either by the presence of very high voltages, such as in electrical transmission systems, or by more moderate voltages which occur over very short distances, such as within semiconductor devices. The electric field strength necessary to achieve avalanche breakdown varies greatly between different materials: in air, 3 MV/m is typical, while in a good insulator such as some ceramics, fields in excess of 40 MV/m can be needed. Field strengths used in semiconductor devices that exploit the avalanche effect are often in the 20–40 MV/m range, but vary greatly according the details of the device. In physics, an electric field or E-field is an effect produced by an electric charge (or a time-varying magnetic field) that exerts a force on charged objects in the field. ... Power line redirects here. ... Josephson junction array chip developed by NIST as a standard volt. ... The metre, or meter (symbol: m) is the SI base unit of length. ... Fixed Partial Denture, or Bridge The word ceramic is derived from the Greek word κεραμικος (keramikos, potters earth, or pottery). The term covers inorganic non-metallic materials whose formation is due to the action of heat. ...

Once the necessary field strength has been achieved, all that is necessary to start the avalanche effect is a free electron, and since even in the best insulators a tiny number of free electrons are always present, an avalanche will always occur. In devices that exploit the avalanche effect, the electric field is normally kept just below the threshold at which avalanche breakdown is possible, resulting in a current that is highly dependent on the generation of free electrons. In avalanche photodiodes, for example, incoming light is used to generate these free electrons. Avalanche photodiodes (APDs) are photodetectors that can be regarded as the semiconductor analog to photomultipliers. ...

As avalanche breakdown begins, free electrons are accelerated by the electric field to very high speeds. As these high-speed electrons move through the material they inevitably strike atoms, but if their velocity is not sufficient for avalanche breakdown (because the electric field is not strong enough) they are absorbed by the atoms and the process halts. If their velocity is high enough, when they strike an atom, they knock an electron free from it, ionizing it (and this is referred to as impact ionization for obvious reasons). Both the original electron and the one that has just been knocked free are then accelerated by the electric field and strike other atoms, in turn knocking additional electrons free. As this process continues, the number of free electrons moving through the material increases exponentially, often reaching a maximum in just picoseconds. The avalanche can result in the flow of very large currents, limited only by the external circuitry. Impact ionization is the process in a material by which one energetic charge carrier can lose energy by the creation of other charge carriers. ... A picosecond is an SI unit of time equal to 10-12 of a second. ...

Applications

If the current is not externally limited, the process normally destroys the material, and in situations such as power line insulators, this can take the form of an explosive breakdown of the insulator. In situations where the current is externally limited, such as avalanche diodes or avalanche photodiodes, this effect can be used to multiply normally tiny currents. In avalanche photodiodes, current gains of over a million can be achieved.

See also

• Avalanche diode
• Avalanche noise
• Avalanche photodiode
• Zener diode
• Electron avalanche

An avalanche diode is a diode (usually made from silicon, but can be made from another semiconductor) that is designed to break down and conduct at a specified reverse bias voltage. ... Avalanche photodiodes (APDs) are photodetectors that can be regarded as the semiconductor analog to photomultipliers. ... Zener diode schematic symbol A Zener diode is a type of diode that permits current to flow in the forward direction like a normal diode, but also in the reverse direction if the voltage is larger than the rated breakdown voltage or Zener voltage. A conventional solid-state diode will... An electron avalanche is a process in which a number of free electrons in a medium (usually a gas) are subjected to a strong electric field accelerate, ionizing the mediums atoms by collision (creating positive ions), forming new electrons to undergo the same process in successive cycles. ...

References

• Microelectronic Circuit Design — Richard C Jaeger — ISBN 0-07-114386-6
• The Art of Electronics — Horowitz & Hill — ISBN 0-521-37095-7
• University of Colorado guide to Advance MOSFET design