It is a special type of diode designed to operate reverse biased: the zener diode.
Zener Diodes
Today, we are going to take a look at the so-called zener diodes. They are a special kind of diodes that are build with an exactly known breakdown voltage. In circuit diagrams they are denoted as shown below. The first symbol is the standard IEC symbol, however, you can often also find the two symbols from the bottom row to be used in schematics. Beware, that especially the last one can be easily confused with the Schottky diode symbol. Luckily, in most cases the breakdown voltage is denoted right next to the symbol so that the component can be clearly identified as a zener diode.
Zener diodes act just like normal diodes when forward biased, however, that is not the way zener diodes are meant to be used. The typical zener diode circuits use it in reverse biased configuration, in which it starts to conduct, as soon as its known breakdown voltage is exceeded. This property can be used for a variety of different circuits like, e.g, overvoltage protection, voltage stabilization or voltage references.
Properties of Zener Diodes
What are the properties of zener diodes? Zener diodes are using highly doped silicon. Due to the high doping level the depletion region on the junction becomes very narrow. The narrow junction is what defines the main properties of zener diodes:
- Zener diodes have a low breakdown voltage, which makes them ill-suited for the use as rectifier diodes
- Due to the narrow depletion region, zener diodes have a high junction capacitance, which makes them ill-suited for the use as signal diodes
So you can see, that zener diodes have primarily disadvantages when used like normal diodes. It is their exactly known breakdown voltage that makes them special. The breakdown voltage is dependent on the exact doping levels. This makes it possible to design zener diodes for different breakdown voltages. The most important characteristics of a zener diode are:
- the zener voltage \(V_{Z}\) which is equivalent to the breakdown voltage \(V_{BR}\)
- the maximum power dissipation \(P_{tot}\)
The power dissipation can be calculated just like for normal diodes, but because zener diodes are used reversed biased, you need to multiply the current \(I\) with the zener voltage \(V_{Z}\) instead of the forward voltage \(V_F\):
\(P_{tot} = V_Z \cdot I\)
The graph below shows the I-V curve for zener diodes with different breakdown voltages. When forward-biased they are good conductors above 0.7 V, just like normal diodes. Reverse biased, each diode starts to conduct at its designated zener voltage.
You may notice, that the I-V curve for diodes with a low breakdown voltage differs from the one with higher breakdown voltages. For the higher voltage ones the curve is much more steep than for the lower voltage ones. This is because there are two different physical effects causing the breakdown: the zener effect and the avalanche breakdown. At voltages below 5 V the zener effect is dominant. It gives this diode its name and is named after Clarence Zener. At higher voltages the avalanche breakdown is the dominant effect.
You want to know the details? In the next few months we will dive more deeply into how these effects work and how zener diodes are used in different circuits.
