At a Glance of Zener Diode
This Blog discusses Zener Diode and Zener effect. It also mentions the Characteristics of Zener Diode and different types of Breakdowns. It will clear your understanding about the Zener diode as a voltage regulator.
Follow the Block diagram as the Road map to understand this Blog.
History
The Zener diode is named after Clarence Melvin Zener. He discovered the Zener effect in 1934 while primarily theoretical studies of the breakdown of electrical insulators’ properties.
Later on, the Zener effect is implemented by Bells Lab to make the Zener effect.
What is the Zener effect?
Zener effect is the type of breakdown that occurs when the P-N junction diode is reverse biased.
During this, the free minority charge carrier of P-N junction tunnels from valance band to conduction band of semiconductor causes sudden reverse current flow.
When the high reverse bias voltage is applied to the P-N junction diode, the depletion region gets widen causing the minority charge carrier to cross the junction. This is discussed in our previous blog. As we increase the reverse voltage, the minority charge carrier also flows increases, causing the reverse current to increase.
The effect that occurs after applying an increased voltage to the Zener diode is called the Zener effect.
Zener diode
Zener diode is a heavily doped P-N junction diode with an extra thin depletion region.
It is a modified form of P-N junction silicon diode.
The P-N junction diode only works in forwarding bias conditions. If it is reverse biased, then due to heavy current diode probably gets burnt.
So the Zener diode is constructed to conduct reverse direction when a specified voltage is reached.
The flow of current is both in the forward as well as reverse direction.
Working of Zener diode
When the Zener diodes are forward biased, with anode voltage higher than cathode voltage, they behave the same as the PN junction diode.
When the anode voltage is lower than cathode voltage, i.e., the Zener diode is reverse biased.
At the start, a small amount of current will flow through the diode. As we increase the reverse voltage to predetermined breakdown voltage (Vz). The junction gets break down, and the maximum current starts flowing through it.
Breakdowns in Zener Diode
There are two types of breakdowns in the Zener diode
- Avalanche breakdown
- Zener breakdown
Avalanche breakdown
In the reverse bias condition, the current we are getting is due to the minority charge carrier. So in the reverse bias, as we increase the applied reverse voltage, the depletion region’s width will increase. Due to that, the immobile ions in this depletion region will also increase.
So due to the increase in immobile ions, the electric field in this region will become more robust due to this stronger electric field. Now the minority charge carriers that are in the vicinity of this depletion region will get accelerated. Now they can move more quickly through this depletion region. While moving, these free-charge carriers can collide with these silicon atoms.
But once the applied voltage reaches the breakdown voltage, the kinetic energy gained by this charge carrier will be such that they can knock off the valance electron of this silicon atom.
So if we see the same thing at the crystal level, then the accelerated electron may collide with the silicon atom. When it has enough kinetic energy, it can knock out a valance charge of this atom. After the collision, instead of one electron, we have two free electrons. Under the influence of the electric field, now this electron can collide with more atoms, and they can knock out the two more electrons. Similarly, these two atoms collide with two more atoms and knock out two more electrons. Similarly, these four electrons will collide with four more atoms. They can knock out four more electrons.
So due to this collision number of free charge carriers in the depletion region will increase drastically. Due to the increase in this charge carrier, we will see a sudden jump in reverse saturation current known as the avalanche breakdown effect. The voltage after which it occurs is known as the breakdown voltage.
Zener breakdown
P and N regions of the Zener diode are heavily doped. The number of free atoms in this P and N region will increase. The number of free-charge carriers in both regions will be more.
So due to this heavy doping width of this depletion region will be much narrower than the PN junction because now, when electron diffuses from the P side to the N side. It will be recombined to the junction itself. Due to this depletion region, we will now have more immobile ions than the P -N junction diode.
So due to this now built in the electric field inside this depletion region will be much stronger. When its diode is reverse bias, the external electric field will also add to the built-in electric field. Now, the electric field in this depletion region will become significantly stronger. At one particular voltage, it can knock out an electron from a silicon atom.
So due to this strong electric field, many charge carriers are generated. Due to the very narrow depletion region, they can tunnel through it. They can reach the other side of the depletion region. So in this way, very high electric field many charge carriers are generated in this depletion region, and suddenly a lot of current starts flowing in the reverse direction, so this effect is known as Zener breakdown effect, and the voltage at which this Zener effect starts is known as the Zener voltage
Characteristics of Zener diode
Following are the Characteristics of Zener Diode:
Image Source: Google| Image By Wikipedia
Forward characteristics
The first quadrant in the graph presents the relationship between forward current and voltage when the Zener diode is forward-biased.
During this condition, the Zener diode behaves like an ordinary PN junction diode. It shows identical characteristics during the forward bias condition (in the first quadrant).
But the current flowing through is high due to high doping concentration.
Reverse characteristics
The third quadrant in the graph represents the relationship between voltage and reverse current when the diode is reverse biased.
At the start, a small amount of leakage current flows through the diode. When the Zener voltage or knee voltage is achieved, the current starts to flow. The Zener breakdown voltage remains the same; only the reverse current increases as we increase the input voltage.
Zener voltage or knee voltage: the reverse bias voltage after which Zener diode starts conducting a significant amount of current through it is known as Zener Voltage.
Zener diode as a voltage regulator
A voltage regulator is a device that regulates the voltage. It is used to maintain the output voltage constant as per the required value. And to regulate the output voltage.
We cannot use the PN junction diode as a voltage regulator because of the low power rating and chances of damage
When we use the Zener diode as a voltage regulator, the resistor series is connected to the positive terminal of the DC supply to limit the current through the Zener diode.
The objective of the Zener diode as a voltage regulator is to maintain a constant voltage. Let us say if Zener voltage is 5v, then the voltage becomes constant at 5v.
The Zener diode is the best option for a voltage regulating device as its cost is low.
From the above characteristics, we have seen that after zener breakdown voltage, as we increase the supply voltage current gets increase, zener breakdown voltage remains constant, which is the voltage we get at the output of the Zener diode.
This means after the Zener breakdown, as we increase the supply voltage, only current increases. Still, the output voltage always remains constant (set value of say 5v).
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