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LW1DSE > TECH 09.03.21 13:36l 146 Lines 7795 Bytes #999 (0) @ WW
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Subj: Avalanche Breakdowns
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From: LW1DSE@LU7DQP.#LAN.BA.ARG.SOAM
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* Avalanche breakdown *
***********************
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.
Contents
1 Explanation
2 The avalanche process
3 Applications
4 See also
5 References
1) Explanation
Avalanche breakdown can occur within insulating or semiconducting
solids, liquids, or gases when the electric field in the 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: the number of free
electrons is thus increased rapidly as newly generated particles become part
of the process. This phenomenon is usefully employed in special purpose
semiconductor devices such as the avalanche diode, the avalanche photodiode
and the avalanche transistor, as well as in some gas filled tubes. In general
purpose semiconductor devices such as common diodes, MOSFETs, transistors, it
poses an upper limit on the operating voltages since the associated electric
fields can start the process and cause excessive (if not unlimited) current
flow and destruction of 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, the physical origins of the two phenomena are very different.
2) 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.
In a good insulator such as some ceramics, fields in excess of 40 MV/m are
required.
Field strengths used in semiconductor devices that exploit the avalanche
effect are often in the 20 to 40 MV/m range, but vary greatly according
the details of the device.
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.
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. If their velocity isn't
sufficient for avalanche breakdown (because the electric field isn't strong
enough) they are absorbed by the atoms and the process halts. However, 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. When all electrons reach the anode, the process stops,
unless of course holes are created also.
For a bipolar junction transistor the strength of the base drive has an
important impact on the avalanche voltage. If a low impedance is connected to
the base then charge is quickly removed from the base which helps hold back
the avalanche process, but if the base is driven by a high impedance, such as
a current source, then the excess charges stay in the base and avalanche
occurs at a lower electric field.
3) Applications
If the current is not externally limited, the process normally
destroys the device where it has started, and in situations such as power
line insulators, this can take the form of an explosive breakdown of the
insulator. When avalanche current is externally limited, avalanche breakdown
can successfully serve to several purposes. In avalanche transistors and
avalanche photodiodes, this effect is used to multiply normally tiny currents,
thus increasing the gain of the devices: in avalanche photodiodes, current
gains of over a million can be achieved. Also, the phenomena is very fast,
meaning that avalanche current quickly follows avalanche voltage variations
or starting charge (number of free electrons available to start the process)
variations, and this gives to avalanche transistors and avalanche photodiodes
the capability of working in the microwave frequency range and in pulse
circuits. In avalanche diodes, this effect is mainly used to construct over
voltage protection circuits and voltage reference circuits: as a matter of
fact, avalanche breakdown and Zener breakdown are jointly present in each
avalanche diode, depending on breakdown voltage, which is the leading
contributing process to the avalanche current.
- | | +
- | . --> * | +
- | / | +
- | . --> * . --> * | +
- | \. --> * | +
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- | _ | +
E
Charged regions
Legends:
. = free electron(s)
* = atom(s)
= elecric field
n
(Number of ³ . Current tends to infinite
free electons) ³ .
³ .
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³ ω
³ .
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Ί Compilled from Wikipedia.com . Translatted to ASCII by LW1DSE Osvaldo Ί
Ί F. Zappacosta. Barrio Garay, Almirante Brown, Buenos Aires, Argentina. Ί
Ί Made with MSDOS 7.10's Text Editor (edit.com) in my AMD's 80486. Ί
Ί November 02, 2007 Ί
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Ί Osvaldo F. Zappacosta. Barrio Garay (GF05tg) Alte. Brown, Bs As, Argentina.Ί
Ί Mother UMC ζPC:AMD486@120MHz 32MbRAM HD SCSI 8.4Gb MSDOS 7.10 TSTHOST1.43C Ί
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