Let's learn more about the second important breakdown effect in zener diodes: the avalanche breakdown.
There are two noteworthy breakdown effects in zener diodes: the zener effect and the avalanche breakdown. This time, we are going to take a deeper look at the latter.
The avalanche breakdown is especially important to zener diodes with breakdown voltages above 5 V. It is identifiable by the strong increase of conductivity, once the breakdown voltage is exceeded. This can be easily seen in the I-V curve. Something drastic seems to happen here. Let's take a look at the underlying physical process, that does not only apply to zener diodes, but is of a more general significance. Ready to learn more?
Disclaimer: I'm not a physicist and semiconductors are an extremely complicated topic. I cannot guarantee for correctness of my explanations. For the ease of understanding some details might be left out or presented in an over simplified manner. If you think, that parts of this article need correction, contact me at firstname.lastname@example.org.
Luckily, the avalanche breakdown and the underlying physical process are much easier to understand than the zener effect. Similar to the zener effect, the avalanche breakdown is caused by high electric field strengths. The underlying physical effect is, however, a different one: impact ionization. Let's see what it is.
Even without externally applied fields electrons can move in the semiconductor material, which is e.g., caused by thermal energy. They randomly collide with other electrons and in this process give up part of their kinetic energy to them. It's an ever ongoing process of electrons colliding which each other, a constant back and forth of excitation and relaxation.
High electric field strengths always imply high acceleration forces to electrically charged particles like electrons. Things get interesting once the kinetic energy of electrons exceeds a certain level. At this point, once they hit another electron, they do not simply loose part of their energy to it, but set free new charge carriers.
This process is called impact ionization, where ionization in general is just another term for the creation of new charge carriers from an atom. The image below visualizes the ionization process. Due to the colliding electron the atom loses one of its electrons and itself becomes a positively charged ion. Since we talk about semiconductors, we are better off saying: the colliding electron causes there to be an additional free electron and a new electron hole in the semiconductor material.
What happens with this newly generated charge carries? Well, they get themselves accelerated by the electric field and start colliding with electrons in other atoms, where they can again cause impact ionization. This causes a chain-reaction, the so-called electron avalanche. The number of free charge carriers rises exponentially, and the material becomes more and more conductive.
If we look at how diodes work, we can easily see how such an electron avalanche causes their internal potential barrier to breakdown. Reverse biased diodes cannot conduct due to the lack of majority charge carriers in the depletion zone. Only a small leakage current can flow, due to the few minority charge carriers. These few minority charge carriers can, however, once the voltage exceeds a certain threshold, ionize other atoms within the depletion zone. All of a sudden, there are plenty of free charge carriers and the diode starts to conduct. The so-called avalanche breakdown has occurred.
The avalanche breakdown does not only affect zener diodes, but many other semiconductor components like standard diodes, transistors or MOSFETs. The only thing that differentiates an avalanche type zener diode from a normal diode is that the breakdown voltage is usually lower and well known.
Besides that, the same effect can also occur in insulators, though hopefully only at much higher voltages. Additionally, there are many more physical effects that have there root cause in impact ionization and an electron avalanche. To name a few examples, we are talking about things like gas discharge, electric sparks, and even lightning strikes. So, if you want to, there is a lot more to discover out there.