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Zener Diodes
Zener Diodes are special types of diodes that can conduct current in both forward and reverse directions.
They are designed to have a low breakdown voltage, called the Zener voltage, which is the voltage at which the diode starts to allow reverse current.
Zener Diodes are useful for generating stable reference voltages, regulating power supplies, and protecting circuits from overvoltage.
In this blog post, we will explore the working principle, circuit symbol, V-I characteristics, and applications of Zener Diodes.
We will also compare the two types of breakdown mechanisms that occur in Zener Diodes: Zener breakdown and avalanche breakdown.
Working Principle of Zener Diode
A Zener Diode works like a normal diode when it is forward-biased, meaning that it allows current to flow from the anode to the cathode.
However, when it is reverse biased, meaning that the voltage across the diode is negative, it behaves differently. A small leakage current flows through the diode until the reverse voltage reaches the Zener voltage.
At this point, the diode undergoes a breakdown and allows a large current to flow in the reverse direction. This current is limited by the external resistor in series with the diode. The voltage across the diode remains constant at the Zener voltage, regardless of the changes in the applied voltage or the load resistance.
Circuit Symbol of Zener Diode
The circuit symbol of a Zener Diode is shown below. It is similar to a regular diode symbol, except that it has bent edges on the cathode side to indicate the Zener effect.
V-I Characteristics of Zener Diode
The V-I characteristics of a Zener Diode are shown below. They show how the voltage across and the current through the diode vary with different biasing conditions.
Forward Characteristics of Zener Diode
The forward characteristics of a Zener Diode are similar to those of a regular diode. The diode has a forward voltage drop of about 0.7 V for silicon and 0.3 V for germanium. The current increases exponentially with the increase in forward voltage.
Reverse Characteristics of Zener Diode
The reverse characteristics of a Zener Diode are different from those of a regular diode. The diode has a small leakage current until the reverse voltage reaches the Zener voltage.
Then, the diode undergoes a breakdown and allows a large reverse current to flow. The voltage across the diode remains constant at the Zener voltage, even if the reverse current changes.
Breakdown Mechanisms in Zener Diode
There are two types of breakdown mechanisms that occur in Zener Diodes: Zener breakdown and avalanche breakdown. Both mechanisms involve the generation of electron-hole pairs due to high electric fields in the depletion region of the diode.
Zener Breakdown
Zener breakdown occurs in heavily doped diodes with low Zener voltages (below 5 V). In this case, the depletion region is very narrow and the electric field is very high.
The high electric field causes quantum tunneling of electrons from the valence band of the p-type material to the conduction band of the n-type material. This results in a large reverse current.
Avalanche Breakdown
Avalanche breakdown occurs in lightly doped diodes with high Zener voltages (above 5 V). In this case, the depletion region is wide and the electric field is moderate.
The electric field accelerates some free electrons in the n-type material, which collide with atoms and knock out more electrons. This creates an avalanche effect, where more and more electrons are generated and contribute to the reverse current.
Applications of Zener Diode
Zener Diodes have many applications in electronic circuits, such as:
Voltage regulation: A Zener Diode can be used to provide a stable reference voltage for a power supply or a load circuit. The reference voltage is equal to the Zener voltage of the diode and does not change with variations in input voltage or load current.
Voltage clamping: A Zener Diode can be used to protect a circuit from overvoltage or electrostatic discharge. The diode clamps the voltage across the circuit to a safe level by conducting excess current to ground.
Voltage multiplier: A series of Zener Diodes can be used to create a higher output voltage than the input voltage by adding up their individual Zener voltages.
Waveform shaping: A Zener Diode can be used to clip or limit the peaks of an alternating voltage waveform to a desired level.