AIM
To study the voltage current characteristics of the Zener diode under forward bias and reverse
bias conditions, so as to obtain the Zener breakdown voltage in reverse biased condition.
INTRODUCTION
A Zener diode is a semiconductor device that permits current to flow in either forward bias or
reverse bias condition. The Zener diode consists of a special, heavily doped P-N junction,
designed to conduct in the reverse direction when a certain specified voltage is reached.
Figure 1:Diagram showing a symbol of the Zener diode
The breakdown voltage of a Zener diode is carefully set by controlling the doping level during
manufacture. If the diode is heavily doped, the Zener diode will breakdown at low reverse
voltages. On the other hand, if the diode is lightly doped, the Zener diode will breakdown at high
reverse voltages. Zener diode is heavily doped than the normal P-N junction diode. Hence, it has
very small depletion region, this in turn allows more electric current to flow through the Zener
diode as compared to the P-N junction diode.
WORKING OF THE ZENER DIODE
In a Zener diode high levels of impurities are added to the semiconductor material to make it
more conductive. Due to the presence of these impurities, the depletion region of the
semiconductor becomes very thin. The intensity of the electric field is increased across the
depletion region, due to heavy doping even if a small voltage is applied.
When no biasing is applied across the Zener diode, the electrons accumulate in the valence band
of the P-type semiconductor material and no current flow occurs through the diode.
The Zener diode works just like a normal P-N junction diode when placed in the forward bias
mode. However, when placed in the reverse bias mode, which it is usually in during most of its
applications, the reverse biased voltage is applied across the diode and when the Zener voltage is
equal to that of the supplied voltage, the diode starts conducting in direction of the reverse bias.
The Zener diode voltage is the particular voltage at which the depletion region vanishes
completely.
The intensity of the electric field increases across the depletion region when the reverse bias is
applied across the diode. Hence the electrons are free to move from valence band of P type
material to the conduction band of N-type material. This movement of electrons decreases the
barrier between the P-type and N-type material. During this reverse bias condition, a small
leakage current flows through the diode when it is connected in reverse biased mode. As the
, reverse voltage starts increasing and finally reaches the predetermined breakage voltage which is
represent as 𝑉𝑧 the current starts flowing through the circuit.
Breakdown in Zener diode
There are two types of reverse breakdown regions in a Zener diode:
A. Avalanche breakdown
The avalanche breakdown occurs in both the P-N junction diodes and Zener diodes at
high reverse voltage. When high reverse voltage is applied to the p-n junction diode, the
free electrons gain large amounts of energy and are accelerated to greater velocities.
Figure 2: Diagram indicating what happens during avalanche breakdown.
The free electrons moving at high speed will collides with the atoms and knock off
more electrons. These electrons are again get accelerated and collide with other atoms.
Because of this continuous collision with the atoms, a large number of free electrons are
generated. As a result, electric current in the diode increases rapidly. This sudden
increase in electric current may permanently destroys the normal diode. However, Zener
diodes may not be destroyed because they are carefully designed to operate in avalanche
breakdown region. Avalanche breakdown occurs in Zener diodes with Zener voltage (𝑉𝑧)
greater than 6V. This type of breakdown happens in normal diodes.
B. ZENER BREAKDOWN.
The Zener breakdown occurs in heavily doped p-n junction diodes because of
their narrow depletion region. When reverse biased voltage applied to the diode
is increased, the narrow depletion region generates strong electric field. When
reverse biased voltage applied to the diode reaches close to Zener voltage, the
electric field in the depletion region is strong enough to pull electrons from their
valence band.
To study the voltage current characteristics of the Zener diode under forward bias and reverse
bias conditions, so as to obtain the Zener breakdown voltage in reverse biased condition.
INTRODUCTION
A Zener diode is a semiconductor device that permits current to flow in either forward bias or
reverse bias condition. The Zener diode consists of a special, heavily doped P-N junction,
designed to conduct in the reverse direction when a certain specified voltage is reached.
Figure 1:Diagram showing a symbol of the Zener diode
The breakdown voltage of a Zener diode is carefully set by controlling the doping level during
manufacture. If the diode is heavily doped, the Zener diode will breakdown at low reverse
voltages. On the other hand, if the diode is lightly doped, the Zener diode will breakdown at high
reverse voltages. Zener diode is heavily doped than the normal P-N junction diode. Hence, it has
very small depletion region, this in turn allows more electric current to flow through the Zener
diode as compared to the P-N junction diode.
WORKING OF THE ZENER DIODE
In a Zener diode high levels of impurities are added to the semiconductor material to make it
more conductive. Due to the presence of these impurities, the depletion region of the
semiconductor becomes very thin. The intensity of the electric field is increased across the
depletion region, due to heavy doping even if a small voltage is applied.
When no biasing is applied across the Zener diode, the electrons accumulate in the valence band
of the P-type semiconductor material and no current flow occurs through the diode.
The Zener diode works just like a normal P-N junction diode when placed in the forward bias
mode. However, when placed in the reverse bias mode, which it is usually in during most of its
applications, the reverse biased voltage is applied across the diode and when the Zener voltage is
equal to that of the supplied voltage, the diode starts conducting in direction of the reverse bias.
The Zener diode voltage is the particular voltage at which the depletion region vanishes
completely.
The intensity of the electric field increases across the depletion region when the reverse bias is
applied across the diode. Hence the electrons are free to move from valence band of P type
material to the conduction band of N-type material. This movement of electrons decreases the
barrier between the P-type and N-type material. During this reverse bias condition, a small
leakage current flows through the diode when it is connected in reverse biased mode. As the
, reverse voltage starts increasing and finally reaches the predetermined breakage voltage which is
represent as 𝑉𝑧 the current starts flowing through the circuit.
Breakdown in Zener diode
There are two types of reverse breakdown regions in a Zener diode:
A. Avalanche breakdown
The avalanche breakdown occurs in both the P-N junction diodes and Zener diodes at
high reverse voltage. When high reverse voltage is applied to the p-n junction diode, the
free electrons gain large amounts of energy and are accelerated to greater velocities.
Figure 2: Diagram indicating what happens during avalanche breakdown.
The free electrons moving at high speed will collides with the atoms and knock off
more electrons. These electrons are again get accelerated and collide with other atoms.
Because of this continuous collision with the atoms, a large number of free electrons are
generated. As a result, electric current in the diode increases rapidly. This sudden
increase in electric current may permanently destroys the normal diode. However, Zener
diodes may not be destroyed because they are carefully designed to operate in avalanche
breakdown region. Avalanche breakdown occurs in Zener diodes with Zener voltage (𝑉𝑧)
greater than 6V. This type of breakdown happens in normal diodes.
B. ZENER BREAKDOWN.
The Zener breakdown occurs in heavily doped p-n junction diodes because of
their narrow depletion region. When reverse biased voltage applied to the diode
is increased, the narrow depletion region generates strong electric field. When
reverse biased voltage applied to the diode reaches close to Zener voltage, the
electric field in the depletion region is strong enough to pull electrons from their
valence band.
