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Zener Diode
A Zener diode is very similar to a PN junction diode. However, it is usually operated in reverse bias mode. This is a heavily doped PN junction diode that has been particularly created. As a result, the Zener diode is a highly doped semiconductor diode that is engineered to work in the opposite direction.
The Zener diode is also a diode that is specifically built for maximising the breakdown region. It signifies that the diode’s n-type material is linked to the power supply’s positive terminal. The P-type material is also linked to the power supply’s negative end. Due to the strongly doped semiconductor material, the depletion region of the diode is extremely thin.
Zener Diode Specifications
Zener diodes come in a variety of configurations, including nominal working voltage, power dissipation, maximum reverse current, and packaging. The following are some of the most often used Zener Diode specifications:
1.Voltage (Vz): The reverse breakdown voltage, which can range from 2.4 V to around 200 V, is referred to as the Zener voltage. Even so, it may reach 1 kV, whereas the surface-mounted device’s maximum voltage is around 47 V.
2.Current (Iz -max.): At the rated Zener voltage Vz, the maximum current will range from 200 microamps to 200 amps.
3.Current (Iz-min.): The minimal current necessary to break down the diode will be between 5 and 10 mA.
4.Power rating: The product of the voltage across the diode and the current flowing through it determines the maximum power that can be dissipated by the Zener diode. 400 mW, 500 mW, 1 W, and 5 W are some common quantities.
5.Typically, a voltage tolerance of 5% is used.
6.The diode’s temperature stability is around 5 V.
7.Surface mount and leaded devices can be used as separate devices or as part of integrated circuits.
8.Zener resistance (i.e., Rz): The diode also has a resistance value, which may be seen in the I-V characteristics.
Working of Zener Diode
As the reverse voltage applied to the Zener diode increases, the breakdown voltage is reached, at which the Zener current reaches a high value. Further increases in reverse voltage will not raise the voltage across the Zener diode in the breakdown area; instead, they will increase the current. When the supply voltage changes, a constant value termed Zener voltage (Vz) is maintained across the Zener diode. As a result, it serves as a voltage regulator.
Taking reverse voltage along the negative X-axis and reverse current along the negative Y-axis yields the reverse characteristic. The reverse current increases to a huge value as the reverse voltage reaches a specific value, yet the voltage across the diode remains constant. Vz is the voltage at which the circuit breaks down.
V-I Characteristics curve of Zener Diode
The Zener diode goes through several different stages or zones, which are described here.
(a).The Zener diode receives forward voltage, which is positive voltage across its anode and cathode terminals, on the right half of the characteristics curve. In this region, the diode is forward biased. The current is tiny for a while during this time until the voltage hits a particular point, known as the threshold voltage, at which point it spikes exponentially up.
(b).When it comes to Zener diodes, the left half of the characteristics curve is more essential. The Zener diode receives positive voltage across its cathode and anode terminals at this point. In this area, the diode is reverse biased. When receiving reverse voltage, the current is initially quite low. The diode only has a little current running through it, known as the leakage current. When it reaches the breakdown voltage, the current skyrockets. Because of its extreme peaks, this current is known as the avalanche current.
(c).The breakdown voltage point is very significant, not only because of the avalanche current, but also because once the Zener diode’s voltage reaches this point, it remains constant at that voltage, even if the current across it increases dramatically. This makes the Zener diode valuable in voltage regulation applications.
(d).When the voltage across a Zener reaches this breakdown voltage, also known as a Zener diode’s Zener voltage, VZ, the voltage that a Zener lowers across itself will not grow. If a Zener diode has a Zener voltage of 5.1V and the voltage feeding the diode is about 5.1V, the Zener will drop 5.1V across its terminals. Even if the voltage (and current) powering it continues to increase, say to 12V, the Zener diode will keep its Zener voltage of 5.1V.
(e).This is the single most significant feature of a Zener diode, which permits it to operate as a voltage regulator in a circuit, as previously stated. Even if the voltage or current in the circuit increases, the voltage lowered across a Zener will not surpass its breakdown or Zener voltage, as shown by the I-V characteristics curve above.
Zener Breakdown
The failure is caused by the Zener breakdown phenomenon, which occurs when the voltage falls below 5.5 volts. It could also have an effect on ionisation that happens at 5.5 Volt. Both processes provide the same results and hence do not necessitate separate circuitry. However, the temperature coefficient for each process is different. The temperature coefficient of the Zener effect is negative, whereas the temperature coefficient of the impact effect is positive. Since the two temperature effects are nearly equivalent, they cancel each other out. Because of this, Zener diodes are the most stable throughout a large temperature range.
Avalanche Breakdown
The reverse saturation current is responsible for the avalanche breakdown mechanism. The PN-junction is made up of P-type and N-type materials. At the point where the P and N-type materials meet, a depletion area forms.
The P and N-type materials in the PN junction are not ideal, and they contain impurities, such as electrons in the p-type material and holes in the N-type material. The depletion region’s width varies. Their width is determined by the bias given to the P and N region’s terminals.
The electrical field across the depletion zone is increased by the reverse bias. When a strong electric field prevails across the depletion, the minority charge carrier’s velocity increases as it crosses the depletion region. These carriers collide with the crystal’s atoms. The charge carrier pulls the electrons off the atom due to the forceful collision.
The electron-hole pair is increased as a result of the collision. The electron-hole pairs are swiftly split and smash with the other atoms in the crystals as they induce in the high electric field. The process is ongoing, and as the electric field increases, reverse current begins to flow in the PN junction. The Avalanche Breakdown is the name for this process. The junction cannot revert to its previous position after the failure because the diode has entirely burned out.
Conclusion
A Zener diode is a semiconductor device made of silicon that allows current to flow in both directions. When a particular voltage is reached, the diode’s special, highly doped p-n junction is designed to conduct in the other direction.The Zener diode has a well-defined reverse-breakdown voltage at which it begins to conduct current and may operate in reverse-bias mode indefinitely without damage. Furthermore, the voltage drop across the diode remains constant over a wide range of voltages, making Zener diodes ideal for voltage regulation.
