What is a zener diode

Z diodes (Zener diodes / avalanche diodes)

The Zener diode is a silicon semiconductor diode that is operated in the reverse direction. In the reverse direction, the Zener or avalanche effect occurs with a silicon diode, in which the current increases suddenly from a certain reverse voltage (breakdown voltage). In the forward direction, the Zener diode works like a normal diode.
Typically, Zener diodes are used to stabilize pulsating DC voltages or to set reference voltages.

Differentiation: Zener diode and avalanche diode

Often one speaks of a "Zener diode", although it is not based on the Zener effect. That is why there is the name "Zener diode". As a rule, the term "Z-diode" is used for all diodes with a Zener or avalanche effect. A Zener diode may only be called a Zener diode with a breakdown voltage of 1.5 V to 5 V. In addition, it is an avalanche diode.

The Zener effect, on which the Zener diode is based, only occurs below a reverse voltage of 5 V, which Clarence Melvin Zener discovered. The avalanche effect or avalanche breakthrough occurs above 6.5 V. Between 5 and 6.5 V both effects are superimposed. Therefore, up to 5 V breakdown voltage, one speaks of the Zener voltage. The diodes are then called Zener diodes. Above 5 V breakdown voltage one speaks of the avalanche effect. The diodes are known as avalanche diodes or avalanche diodes.

  • Zener diodes: Zener diodes up to approx. 5 V.
  • Avalanche diodes: Zener diodes from approx. 5 V.

It is important to understand that, depending on the level of the breakdown voltage, both effects can occur alone or together. Another difference between the two effects is the opposing temperature behavior, which is compensated between 6 and 6.5 V. Zener diodes with a breakdown voltage of 6.2 V are very temperature-stable.

Since the characteristics of Zener and Avalanche diodes have similar Z-shaped characteristics, both types of diodes are referred to as Z diodes. The "Z" is not due to the Zener effect, but to the Z-shaped characteristic. It is scientifically correct to speak of Zener diodes in general and not Zener diodes.

Zener effect / avalanche effect / avalanche breakthrough

The following illustration and description must be seen taking into account the different effects. This means that the Zener effect, the avalanche effect and the avalanche breakthrough are "not" different names for the same effect, but should be considered separately.

The Zener effect is triggered by the electric field that, above a certain size, leads to the electrons being released from their crystal bonds. The electrons lead to the formation of the current I.z. From a certain voltage value UZ0, the Zener voltage, the Zener diode has a low resistance. As of the Zener voltage, the current I increasesz abruptly to.
The charge carriers released by the Zener effect are very strongly accelerated by the electric field. This causes more electrons to be pushed out of their crystal bonds. The barrier layer of the diode is flooded with free charge carriers. This is called the avalanche effect (shock ionization).
The Zener effect and the avalanche effect overlap with the Zener diode. This condition is known as a zener breakthrough. The sudden conductivity leads to a very high current in the reverse direction. If the current is too high, the Zener diode will be destroyed. Therefore, in the data sheet of a Zener diode, there is always a maximum permissible reverse current I.Zmax which must not be exceeded. The maximum permissible power loss P is just as importantdead. Both limit values ​​must not be exceeded and should be known and taken into account when dimensioning a circuit with a Zener diode.
If the reverse voltage falls below UZ0, then the barrier is immediately restored. The area between IZmin and I.Zmax is called the work area or breakthrough area.
During the production of the Zener diode, the Zener voltage can be set in the range from 2 to 600V by doping the silicon crystal.

Temperature dependence of the Zener diode

The temperature dependence of the Zener diode is a disadvantage, especially in measurement and control technology. This has a negative impact on applications where exact tension is required. That is why Zener diodes are often connected in series with positive and negative temperature coefficients TK. In the best case scenario, they cancel each other out or there is only a small residue left over. The temperature coefficient TK indicates the temperature dependency.
Sometimes normal silicon diodes are used to stabilize the temperature. The manufacturer has already made this connection in special temperature-compensated Zener diodes.

Circuit symbols

Designation of Zener diodes

descriptionAlternativesPdeadcasing
ZPD ...BZX83 ... / BZY88 ... / BZX55 ... / ZX71 ... / BZX79 ...500 mW DO35
ZPY ...BZX 29 ... / BZX 85 ... / BZY 92 .. / BZY 95 ... / BZY 96 ... / BZX 97 ... / ZD ...1.3 W DO41

Applications

Zener diodes are best suited for voltage stabilization in circuits with low power consumption. The voltage limitation of voltage peaks is also a possibility.
With a suitable Zener voltage, they are suitable as setpoint generators in measurement and control technology. Or where reference voltages are required.

Overview: semiconductor diodes

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