The document discusses Zener diodes, which are designed to operate reliably in reverse breakdown. Zener diodes have a sharp, well-defined breakdown voltage determined by the doping concentration. There are two types of breakdown mechanisms - Zener breakdown for heavily doped PN junctions and avalanche breakdown for lightly doped junctions. Zener diodes can be used as voltage regulators since their breakdown voltage remains constant, allowing the output voltage to remain stable despite input voltage or load variations by maintaining the current through the Zener. The value of the series resistor in a Zener regulator circuit is calculated based on the input and Zener voltages and the total current.

1. Zener Diode
Miss. Sushama V. Nikam
Department of Nanoscience and Technology
Y. C. Institute of Science, Satara

2. Special Diodes
Some diodes are designed for special applications. E.g. Zener
diode, LED, photo diode, Varactor diode, etc

3. Zener Diode
A properly doped PN junction diode which has a sharp breakdown
voltage is known as Zener diode. They are designed to operate in the
breakdown region without damage.

4. In reverse bias, if reverse voltage is increased at certain point, current
increases suddenly. This is known as breakdown. The voltage at which
breakdown occurs is known as breakdown voltage.
The diode designed to operate in reverse bias with lower breakdown
voltage is called as Zener diode. It gives sharp breakdown. It operates
safely in breakdown condition. The breakdown voltage depends upon
doping concentration. More the doping concentration gives lower
breakdown.

5. The V-I Characteristics of a
Zener diode are common for any
diode when operated in forward
bias. But the reverse bias
operation of a Zener diode makes
it very important to consider. Let
us have a look at the graph.
V-I Characteristics of a Zener diode

6. The V-I characteristics of a Zener diode can be divided into two parts as follows:
(i) Forward Characteristics (ii) Reverse Characteristics
Forward Characteristics of Zener Diode
The first quadrant in the graph represents the forward characteristics of a Zener diode. From
the graph, we understand that it is almost identical to the forward characteristics of any
other P-N junction diode.
Reverse Characteristics of Zener Diode
When a reverse voltage is applied to a Zener voltage, initially a small reverse saturation
current Io flows across the diode. This current is due to thermally generated minority
carriers. As the reverse voltage is increased, at a certain value of reverse voltage, the
reverse current increases drastically and sharply. This is an indication that the breakdown has
occurred. We call this voltage breakdown voltage or Zener voltage and it is denoted by Vz.

7. At voltage Vz breakdown occurs and current increases suddenly. There are two types of mechanisms of
breakdown:
A) Zener Breakdown: This type of breakdown occurs when PN junction is heavily doped. For heavily
doped PN junction depletion region becomes very narrow. Hence applied voltage develops strong
electric field across depletion region. It is about 0.7 x 108 V/m. This strong electric field breaks
covalent bonds and electron-hole pairs are generated; and current increases suddenly. This type of
breakdown is known as Zener breakdown. It occurs at voltage less than 6V.
B) Avalanche Breakdown: This type of breakdown occurs when PN junction is lightly doped. For lightly
doped PN junction depletion region is wide. When applied reverse voltage increases, minority carriers
get accelerated. When they collide with semiconductor atoms new carriers are generated. These
carriers are further accelerated to generate more carriers. Thus with in short time large number of
carriers are generated and current suddenly increases. It is known as avalanche breakdown. This type
of breakdown occurs at voltage greater than 6V.

8. Zener Diode Specifications
1. Zener/Breakdown Voltage (Vz) – The Zener or the reverse breakdown voltage at test
current Izt(Izt is nearly ¼ of Izmax)
2. Current Iz (max) – It is the maximum current at the rated Zener Voltage (Vz – 200μA to
200 A)
3. Current Iz (min) – It is the minimum value of current required for the diode to
breakdown.
4. Power Rating – It denotes the maximum power the Zener diode can dissipate. It is given
by the product of the voltage of the diode and the current flowing through it.
5. Temperature Stability – Diodes around 5 V have the best stability
6. Voltage Tolerance – It is typically ±5%
7. Zener Resistance (Rz) – It is the resistance to the Zener diode exhibits.

9. Zener diode as voltage regulator
Regulator is a circuit which keeps the
output voltage constant independent of input
voltage variations. The important property of
Zener diode is that when it is operated in
breakdown region, the voltage across it
remain constant. Therefore Zener diode can
be used as a voltage regulator.
It consist of a series resistor Rs and Zener diode in parallel with the load resistance RL

10. Line Regulation (Variation in input voltage):
Suppose the input voltage is increased, Since the Zener is in breakdown region the
output voltage remain constant. At Vz i.e. Vo = Vz. The excess voltage dropped across the
series resistor Rs. This will cause an increase in the value of total current I, then Zener will
conduct the increase of the current while the load current remains constant. Hence output
voltage Vo remains constant irrespective of change in the input voltages.
Load Regulation (Variation in Load resistor RL):
Suppose that the input voltage is constant and load resistance is decreased, this will
cause an increase in load current . The extra current cannot come from the source because
the drop across RL will not change as the Zener is in its break down region. The additional
load current will come form decrease in Zener current Iz. In this way, the output voltage
remains constant irrespective of changes in RL .

11. To calculate the Value of Series Resistor Rs:
Voltage across Rs = Vin – Vz
Current flowing through Rs = I = Iz + IL
Vin – Vz Vin - Vz
Rs = ---------------- = --------------
I Iz + IL