Avalanche breakout

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IU characteristics of Zener diodes. Zener and avalanche breakthrough in the lower left quadrant.

The avalanche breakthrough , also called avalanche breakthrough (from English avalanche , avalanche) is one of the three types of breakthroughs in semiconductor components . A breakdown of a pn junction is understood to mean the steep rise in the current at a certain reverse voltage when the diode is polarized in the reverse direction. The avalanche effect is triggered by the avalanche effect (also called avalanche effect , avalanche multiplication or carrier multiplication ). The avalanche effect is a reversible or reversible effect, provided that the permissible total power loss of the component is not exceeded.

description

Charge carriers that are moved through the space charge zone by an external electric field can knock the valence electrons of the Bravais lattice out of their bonds through impact ionization and thus lift them into the conduction band . If the external field strength is sufficiently high, the electrons have such a high energy that after a collision with the valence electrons they not only make them available as charge carriers, but do not recombine themselves, remain in the conduction band and can generate free charge carriers again. As a result, the number of free charge carriers in the conduction band grows exponentially like an avalanche.

In semiconductors, the degree of doping allows the width of the space charge zone and thus the avalanche breakdown voltage to be changed. In the case of an avalanche breakdown, the current increases very abruptly with the voltage compared to the Zener breakdown . When the temperature rises, the avalanche breakdown, in contrast to the Zener breakdown, only sets in at a higher voltage. In general, the Zener and avalanche effects work simultaneously. The breakdown voltages are in the range between slightly below 6 and 8-10 V.

The avalanche breakdown occurs in weakly doped pn junctions in Z-diodes ; the specific breakdown voltage is set by the strength of the doping. The positive temperature coefficient present at the avalanche breakdown for a specific Zener diode depends on the strength of the doping and, in contrast to the Zener effect, also on the breakdown voltage of the Zener diode. For example, for a Zener diode with a breakdown voltage of 8.2 V, the temperature coefficient is in the range from 3 mV / K to 6 mV / K, for a Zener diode with a breakdown voltage of 18 V it is 12-18 mV / K.

Due to the superposition and mutual compensation of the Zener and avalanche effects, by combining several diodes with different breakdown voltages, comparatively temperature-stable Zener diode circuits can be produced. In the case of Zener diodes with breakdown voltages of around 5.5 V, the two effects overlap to approximately the same extent. This fact is used with reference diodes and simple reference voltage sources in order to be able to obtain a reference voltage that is as temperature-independent as possible. The band gap references available in integrated circuits have significantly better temperature stability.

application

The avalanche effect is used in the following semiconductor components:

  • Avalanche diodes work with a very high reverse voltage and use the avalanche effect and the like. a. for voltage stabilization and resonant circuit damping ( IMPATT diode ) as well as for the construction of noise generators.
  • Avalanche photo diodes use the avalanche effect to amplify the photocurrent
  • Diodes and bipolar transistors can be protected from destruction by overvoltages through a controlled avalanche breakdown behavior. This is used, among other things, in avalanche transistors, which have comparatively high currents and short transit times.
  • Zener diodes with a breakdown voltage U Z > 5 V.

bibliography

  • Joachim Specovius: Basic course in power electronics - components, circuits and systems. 9th edition, Springer Verlag, Wiesbaden 2018, ISBN 978-3-658-21168-4 .
  • Stefan Goßner: Basics of electronics - semiconductors, components and circuits. 9th edition, Shaker Verlag GmbH, Aachen 2016, ISBN 978-3-8265-8825-9 .

Individual evidence

  1. Joachim Specovius: Basic course in power electronics - components, circuits and systems . 9th edition. Springer Fachmedien Wiesbaden GmbH, Beuth University of Applied Sciences Berlin Berlin, Germany 2018, ISBN 978-3-658-21168-4 , p. 15 .
  2. Stefan Goßner: Fundamentals of electronics - semiconductors, components and circuits . 9th edition. Shaker Verlag GmbH, Aachen 2016, ISBN 978-3-8265-8825-9 , p. 29 ff .
  3. ^ Zener and Avalanche Breakdown / Diodes, Engineering Sciences 154. Retrieved December 29, 2014 .