Avalanche diode

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Avalanche diodes or avalanche diodes are Zener diodes that use the avalanche effect. Corresponding doping profiles can create areas of high electrical field strength in avalanche diodes ( multiplication zone ), in which the number of free charge carriers in the conduction or valence band is multiplied by impact ionization and rises like an avalanche above the breakdown voltage. This effect has a wide range of applications and avalanche diodes include a. the (avalanche) transit time diodes ( IMPATT and TRAPATT diodes ), the suppressor diodes and the avalanche photodiodes . Zener diodes with high breakdown voltages, in which the avalanche breakdown predominates, can also be regarded as avalanche diodes.

Negative differential resistance

The voltage-current characteristic of an avalanche diode is similar to that of a Zener diode . Above the breakdown voltage, it has a negative differential resistance , in which the operating voltage with increasing current strength decreases. Many normal diodes also have such an area, which, however, is outside the intended working area.


Generation of vibrations

Avalanche diodes are suitable for setting up oscillator circuits for generating electrical oscillations , since an oscillating circuit can be undamped with the aid of the negative differential resistance . For microwave generation that comes IMPATT diode and related runtime diodes used.

Noise generator

In addition to thermal noise (Johnson-Nyquist noise) to the statistical process of the charge carrier movement produces so-called shot noise (engl. Shot noise ). The shot noise is proportional to the current flow and is additionally amplified by the avalanche effect ( excess noise ), which is why avalanche diodes can be used well as noise generators.

Avalanche photodiodes

Using the internal photoelectric effect , avalanche photodiodes can be used to detect photons and, in contrast to pin photodiodes, have an internal gain. For this purpose, they are operated in the reverse direction near the (avalanche) breakdown voltage, where the internal amplification is achieved due to the multiplication of charge carriers occurring in the multiplication zone.

If the avalanche photodiode is operated below the breakdown voltage, an output current proportional to the radiation power is obtained. The internal gain depends on the applied reverse voltage.

Above the breakdown voltage, special single-photon avalanche diodes (SPAD) are used, with which the counting of individual photons with counting rates of up to 10 MHz is possible. A single photon can easily trigger a measurable charge avalanche due to the prevailing high electric field strength , whereby special detector electronics ensure that the diode is not destroyed and then reset again (passive or active quenching ).

Reference voltage

With silicon diodes, the Zener breakdown and avalanche breakdown always occur at the same time, with the Zener breakdown dominating up to approx. 5 V and the avalanche breakdown dominating from approx. 6 V (in the range between 5 and 6 V, both effects are roughly in balance) . Zener diodes with breakdown voltages above 6 V can therefore be viewed as avalanche diodes and are used to generate reference voltages. Its kinking of the characteristic curve in the area of ​​the breakdown voltage is more pronounced and the avalanche breakdown, in contrast to the Zener breakdown, has a positive temperature coefficient.

Overvoltage protection

For overvoltage protection , suppressor diodes are operated in reverse direction parallel to the load or circuit to be protected. When a voltage peak occurs that is above the breakdown voltage of the suppressor diode, an avalanche breakdown occurs in the diode. The diode becomes low-resistance and a high transient current in the ampere range flows. This limits the voltage on the diode, and thus also on the load to be protected, to a safe value.

Rectifier diodes

Rectifier diodes of the Avalanche type show a specified behavior in the event of reverse breakdown , similar to suppressor diodes, which is specified in the data sheet as “ surge reverse power dissipation ”. Conventional rectifier diodes, on the other hand, can develop “ hot spots ” in the barrier layer and be damaged.

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