Avalanche-Diode

Avalanche diodes are special semiconductor diodes that exploit the effect of avalanche breakdown ( avalanche breakdown ). By appropriate doping profiles of high electric field strength can be generated (multiplication zone ) in which the number of free charge carriers in the conduction or valence band multiplies by impact ionization, and above the breakdown voltage avalanche increases in avalanche diode regions. This effect has a variety of applications and the avalanche diodes include the ( avalanche ) Runtime diodes ( IMPATT and TRAPATT diode), the suppressor diode and avalanche photodiodes. Zener diodes having high breakdown voltage, in which prevails the avalanche breakdown can be seen as avalanche diodes.

Negative differential resistance

The voltage-current characteristic of an avalanche diode is similar to a Zener diode. Above the breakdown voltage, it has a negative differential resistance, the voltage decreases with increasing current level at which during operation. Many normal diodes have such area that lies outside the intended operating range.

Applications

Generation of vibrations

Avalanche diodes are suitable for the construction of oscillator circuits for generating electric oscillations, since with the aid of the negative differential resistance of a resonant circuit may be de-attenuated. For microwave generation comes the IMPATT diode and related runtime diodes used.

Noise generator

In addition to thermal noise (Johnson - Nyquist noise ) the statistical process of charge carrier movement so-called shot noise generated ( engl. shot noise ). The shot noise is proportional to the current flow and is further enhanced by the avalanche effect ( engl. excess noise), so can be good avalanche diodes used as noise generators.

Avalanche photodiodes

Utilizing the photoconductive effect avalanche photodiodes may be used to detect photons and have, in contrast to the pin photodiode, an internal reinforcement. To be operated in the reverse direction near the ( avalanche ) breakdown voltage, where due to the carrier multiplication occurring in the multiplication region, the internal amplification is achieved.

The avalanche photodiode operates below the breakdown voltage, is obtained in a radiation power - proportional output current. The internal gain is dependent on the applied reverse voltage.

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

Reference voltage

For silicon diodes Zener breakdown and avalanche breakdown always occur simultaneously, with up to 5 V Zener breakdown and from about 6 V avalanche breakdown dominated (in the range betw 5-6 V to keep both effects roughly in balance ). Zener diodes with breakdown voltages of over 6 V can be regarded as avalanche diodes and are in the generation of the reference voltages applied. Their bending of the curve in the region of the breakdown voltage is more pronounced and the avalanche breakdown has in contrast to the Zener breakdown a positive temperature coefficient.

Surge Protection

For surge suppressor diodes are operated in the reverse direction parallel to the protected load or circuit. Upon occurrence of a spike voltage which is above the breakdown voltage of the suppressor, it takes the diode to the avalanche breakdown. The diode is low and it flows a high transient current in ampere range. This limits the voltage across the diode, and thus also to the protected load to a safe value.

Rectifier diodes

Rectifier diodes of the Avalanche have the reverse breakdown, similar suppressor, a specified behavior that is specified in the data sheet "surge reverse power dissipation ". Ordinary rectifier diodes, however, can "hot- spots" develop in the barrier layer and damaged.