What is the Zener Diode?
A semiconductor device that causes the current to flow in a backward or forward direction is the Zener diode which is usually doped to a large extent and possesses a p-n junction. It is specifically designed so that the flow of current happens in a reverse direction when a particular voltage is reached.
The features of the Zener diode are that it has a reverse breakdown voltage. What does this mean? Current that is conducted by the diode is constant in the reverse mode and the voltage drop is also constant, notwithstanding the amount of voltage or force applied. Hence it is thought in electronics that the Zener diode is extremely useful for voltage regulation in circuits.
How is the Zener Diode Circuit Depicted?
As we already know, the Zener diode functions in a reverse-bias mode. Hence, the negative terminal of the power supply is connected to the p-type material and the positive terminal is connected to the n-type material in the Zener diode. Because it has semiconductor material that is heavily doped, the diode has a thin depletion region.
How does the Zener Diode Work?
To enhance its conductivity, the Zener diode is doped with lots of impurities in the semiconductor material. Hence the depletion region thins out. So the application of electric fields across this depleted region is intensified even when a small voltage is applied to the system.
Now, what happens when there is no biassing in the Zener diode? The p-type semiconductor material has a valence band where electrons come together. This results in zero current flow across the diode. This band is then called the valence band electron. Once external energy is applied to this valence band, electrons start making their journey from one band to the next.
What is the situation when a reverse bias is applied across the Zener diode? It so happens that the diode conducts itself in a reverse-bias mode when the Zener voltage equals the supply voltage. We must remember that the voltage which is known as the Zener voltage is when the depletion region not only thins out but completely vanishes.
When a reverse bias is applied across the diode, the electric field intensifies as the depletion region has thinned out. The electrons thus move from the valence band of the p-type semiconductor material to the conduction band of the n-type semiconductor material. This movement destroys the barrier between the two materials. At this voltage and level of the current field, the diode conducts the current in a reverse bias direction.
VI Characteristics of the Zener Diode
When the diode is in forwarding bias mode, the Zener diode is nothing but an ordinary diode. However, when the reverse bias is applied, the reverse voltage rises. This causes a complete breakdown in the Zener diode.
This breakdown causes the current to flow in a reverse direction, at what is called a breakdown voltage. Graphs show us that the Zener diode possesses quite a bit of resistance and that the breakdown is not linear or vertical.
Hence, the Zener diode’s voltage is given by the following equation - V = VZ + IZRZ
Applications of the Zener Diode
Zener diodes are mainly used in industrial as well as commercial settings. A voltage stabiliser is used to regulate voltage. The fluctuating voltage here is converted to a constant voltage to be supplied to the power load. So the Zener diode is usually connected parallel to the load. Because it is able to maintain a constant voltage, it acts as a voltage regulator or stabiliser.
Commercial and industrial buildings use huge power-metre systems which are easily prone to metre overloads. However, these multimeters are connected parallel to the Zener diode which prevents accidental overloads upon the system. When an overload does occur, the majority of the current passes through the diode, hence the metre is protected.
Two Zener diodes are connected back to back and in opposite directions in a series with the resistance of the circuit, because of which a sine wave is easily converted into a square wave.
The Zener diode also has certain specifications for values such as Zener voltage, the minimum current required for the breakdown and maximum current, the power that can be dissipated by the Zener diode, temperature stability of the diode and the Zener resistance.
FAQs on Zener Diode
1. What is a Zener diode and how does it differ from a standard p-n junction diode?
A Zener diode is a special type of semiconductor diode designed to operate in the reverse breakdown region. Unlike a standard p-n junction diode which can be damaged by reverse breakdown, a Zener diode is specifically manufactured to handle it. The key difference lies in the doping concentration; a Zener diode is much more heavily doped, which results in a very thin depletion region and a sharp, well-defined reverse breakdown voltage (Zener voltage).
2. What is the working principle behind a Zener diode, especially in its reverse bias mode?
When a Zener diode is reverse-biased, a very small leakage current flows. As the reverse voltage is increased, it reaches the Zener voltage (Vz). At this point, the intense electric field across the thin depletion region is strong enough to pull electrons from the valence band of the p-type material to the conduction band of the n-type material. This process, called Zener breakdown, causes a large current to flow in the reverse direction while the voltage across the diode remains nearly constant at Vz.
3. How does a Zener diode maintain a constant output voltage to act as a voltage regulator?
To act as a voltage regulator, a Zener diode is connected in parallel with the load and in reverse bias with a series resistor. When the input voltage fluctuates:
- If the input voltage increases, the excess current flows through the Zener diode to the ground, maintaining a constant voltage across the load.
- If the input voltage decreases, the current drawn by the Zener diode reduces, but the voltage across it (and the load) remains stable at the Zener voltage (Vz), as long as the input voltage is above Vz.
4. What is the crucial difference between Zener breakdown and Avalanche breakdown?
The two breakdown mechanisms differ primarily in their cause and the voltage at which they occur.
- Zener Breakdown: Occurs in heavily doped diodes at low reverse voltages (typically < 6V). It is caused by the strong electric field directly forcing electrons to tunnel across the narrow depletion region.
- Avalanche Breakdown: Occurs in lightly doped diodes at higher reverse voltages. It is caused by accelerated minority charge carriers colliding with and ionising other atoms, creating an 'avalanche' of free charge carriers.
5. Why is a Zener diode always operated in the reverse breakdown region for voltage regulation?
A Zener diode is operated in its reverse breakdown region because this is the only region where it exhibits its unique voltage-stabilising property. In forward bias, it behaves just like a regular diode with a constant voltage drop of about 0.7V. However, in the reverse breakdown region, it can sustain a large change in current with almost no change in the voltage across it. This stable voltage characteristic is precisely what is needed for regulation.
6. Apart from voltage regulation, what are some other important applications of a Zener diode?
Besides being a voltage regulator, a Zener diode has several other applications in electronic circuits. The most common examples include:
- Wave Shaping Circuits: They can be used as clippers to limit parts of a signal to a certain voltage level, effectively converting sine waves into square waves.
- Voltage Reference: Due to their stable breakdown voltage, they serve as a precise voltage reference in power supplies and measurement instruments.
- Overvoltage Protection: They are used in surge protector circuits to divert high voltage spikes away from sensitive electronic components.
7. What do the V-I characteristics of a Zener diode show?
The V-I (Voltage-Current) characteristic graph of a Zener diode shows its behaviour in both forward and reverse bias. The graph illustrates:
- Forward Bias Region: It behaves like a normal diode, with current increasing exponentially after a forward voltage of about 0.7V.
- Reverse Bias Region: A very small leakage current flows until the voltage reaches the Zener voltage (Vz).
- Breakdown Region: At Vz, the graph shows a very sharp, almost vertical increase in reverse current, indicating that the current can change significantly while the voltage across the diode remains nearly constant.
8. How does the heavy doping of a Zener diode contribute to its unique function?
Heavy doping is the fundamental reason for a Zener diode's behaviour. This high concentration of impurity atoms leads to a very narrow depletion region. According to the relationship E = V/d, a narrow depletion region (d) means that a very high electric field (E) can be generated across it even with a small reverse voltage (V). This intense electric field is strong enough to initiate Zener breakdown at a predictable and low voltage, which is essential for its function as a voltage regulator.
