A diode is like a one-way valve in an electronic device. It basically keeps an electric current flowing in a single direction. If the current tries to flow in the opposite direction, the diode blocks it. If an electronic device requires a low voltage, a diode keeps high voltage currents away from it. You’ll find diodes in relay circuits and stepper motors which reduce the power of coils quickly so that voltage spikes don’t damage the electronic components or circuitry.
Several types of integrated circuits have connection pins with diodes in order to prevent their transistors from getting damaged by external voltages. But a regular solid diode does not allow current to flow if it is reverse-biased. The breakdown voltage is the limit of the current flow. If this limit is exceeded, the breakdown destroys the diode because of the overheating and strength of the current.
In a Zener diode, the breakdown voltage is reduced even more than in a regular diode. It features a p-n junction that is extremely doped, which lets electrons travel from the p-type material to the n-type material. The p-type has a valence band, and the n-type has the conduction band. If the Zener diode is reverse-biased, then the breakdown will be controlled, and the current flow will be allowed to sustain the voltage throughout the Zener diode to match the Zener voltage.
To give you an example, a reverse-biased Zener diode with 3.2 volts will drop 3.2 volts, but it won’t be an unlimited current. The Zener diode usually stabilizes the voltage of low-current applications. During the doping process, accurate control over the breakdown voltage can be achieved.
An American physicist named Clarence Melvin Zener discovered the Zener diode breakdown mechanism. Naturally, the mechanism was named after him because he was the one who discovered it. He studied the p-n junction, which is comprised of the p-type region and n-type region of semiconductor material. With these two regions together, they form the depletion region. The doping of this semiconductor material determines the depletion region’s width. The more the material is doped, the thinner the width gets.
The Zener breakdown happens in this thin region of depletion, where there are more free electrons. The reverse bias creates an electric field which spreads throughout the depletion region. The electric field becomes stronger and more intense as time goes on. The free charge carriers contain more kinetic energy as the electric field intensity grows stronger. This causes the charge carriers to go to different regions until they collide with P-type atoms and N-type atoms.
Do not confuse the Zener breakdown mechanism with the avalanche breakdown mechanism. The latter produces an avalanche effect, which is also present in the Zener breakdown. In fact, both mechanisms contain the same materials and produce the same effects. They even exist in the same diode, but there can only be one dominating mechanism. There is where the difference comes into play.
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For example, if you have a silicon diode with a maximum voltage of 5.6 volts, then the dominating mechanism will be the Zener breakdown and have a negative temperature coefficient. But if you have a diode that is over 5.6 volts, then it’ll experience an avalanche breakdown and a positive temperature coefficient.
Remember, the Zener and avalanche effects both exist within the same diode. Since one has a positive temperature coefficient, and the other has a negative temperature coefficient, they basically cancel one another out completely. That is one of the reasons why manufacturers of temperature-vital applications love to use the diode with 5.6 volts. If you have a device with more than 5.6 volts, then you’ll have a huge increase in the temperature coefficient.