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Basic Electronics (Chapter_02)
Prepared By Vijay Solanki
Unit: 02. Introduction to P-N Junction Diode
Major Learning Outcomes
1) Identify P-N junction diode, its working and
applications.
2) Identify Zener diode, it’s working and
applications
Basic Electronics By Vijay Solanki
Topics & Subtopics
2.1 Working of P-N junction diode
2.2 Working of Zener diode.
2.3 Bridge Rectifier.
2.4 ‘T’ and ‘π’ Filter circuits.
2.5 Zener diode as voltage regulator.
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1) Identify P-N junction diode, its working and applications.
P-N junction is obtained by combining P-type semiconductor with a N-type semiconductor.
When p-type and n-type materials are placed in contact with each other, the junction behaves
very differently than either type of material alone.
Specifically, current will flow readily in one direction (forward biased) but not in the other
(reverse biased), creating the basic diode.
This non-reversing behavior arises from the nature of the charge transport process in the two
types of materials.
Donor impurities (pentavalent) are introduced into one-side and acceptor impurities into the
other side of a single crystal of an intrinsic semiconductor to form a p-n diode with a junction called
depletion region (this region is depleted off the charge carriers). This region gives rise to a potential
barrier Vγ called Cut- in Voltage. This is the voltage across the diode at which it starts conducting.
The P-N junction can conduct beyond this Potential.
The P-N junction supports uni-directional current flow.
 If +ve terminal of the input supply is connected to anode (P-side) and –ve terminal of the input
supply is connected to cathode (N- side), then diode is said to be forward biased.
In this condition the height of the potential barrier at the junction is lowered by an amount
equal to given forward biasing voltage. Both the holes from p-side and electrons from n-side cross
the junction simultaneously and constitute a forward current ( injected minority current – due to
holes crossing the junction and entering N-side of the diode, due to electrons crossing the junction
and entering P-side of the diode).
Assuming current flowing through the diode to be very large, the diode can be approximated as
short-circuited switch.
If –ve terminal of the input supply is connected to anode (p-side) and +ve terminal of the input
supply is connected to cathode (n-side) then the diode is said to be reverse biased. In this condition
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an amount equal to reverse biasing voltage increases the height of the potential barrier at the
junction.
Both the holes on p-side and electrons on n-side tend to move away from the junction thereby
increasing the depleted region. However the process cannot continue indefinitely, thus a small
current called reverse saturation current continues to flow in the diode. This small current is due
to thermally generated carriers.
2.1 Working of P-N junction diode
Assuming current flowing through the diode to be negligible, the diode can be approximated as
an open circuited switch.
Forward Biased Condition:
1. Connect the PN Junction diode in forward bias i.e Anode is connected to positive of the power
supply and cathode is connected to negative of the power supply .
2. Use a Regulated power supply of range (0-30)V and a series resistance of 1kΏ.
3. For various values of forward voltage (Vf) note down the corresponding values of forward
current (If) .
Reverse biased condition:
1. Connect the PN Junction diode in Reverse bias i.e; anode is connected to negative of the power
supply and cathode is connected to positive of the power supply.
2. For various values of reverse voltage (Vr ) note down the corresponding
values of reverse current ( Ir ).
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VI characteristics of PN junction Diode
The PN junction region of a Junction Diode has the following important characteristics:
Semiconductors contain two types of mobile charge carriers, Holes and Electrons.
The holes are positively charged while the electrons negatively charged.
 A semiconductor may be doped with donor impurities such as Antimony (N-type doping), so that it
contains mobile charges which are primarily electrons.
A semiconductor may be doped with acceptor impurities such as Boron (P-type doping), so that it
contains mobile charges which are mainly holes.
The junction region itself has no charge carriers and is known as the depletion region.
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The junction (depletion) region has a physical thickness that varies with the applied voltage.
When a diode is Zero Biased no external energy source is applied and a natural Potential Barrier is
developed across a depletion layer which is approximately 0.5 to 0.7v for silicon diodes and
approximately 0.3 of a volt for germanium diodes.
When a junction diode is Forward Biased the thickness of the depletion region reduces and the
diode acts like a short circuit allowing full current to flow.
When a junction diode is Reverse Biased the thickness of the depletion region increases and the
diode acts like an open circuit blocking any current flow, (only a very small leakage current).
Applications of PN Junction Diode:
1.
2.
3.
4.
5.
6.
7.
8.
Rectifiers
Switching diodes
Zener diodes
Varactor diode(Variable resistance)
Tunnel Diode
Photo diode
Solar cell
Lasers
2) Identify Zener diode, it’s working and applications
2.2 Working of Zener diode.
The Zener Diode or "Breakdown Diode" as they are sometimes called, are basically the same as
the standard PN junction diode but are specially designed to have a low pre-determined Reverse
Breakdown Voltage that takes advantage of this high reverse voltage.
The zener diode is the simplest types of voltage regulator and the point at which a zener diode
breaks down or conducts is called the "Zener Voltage" ( Vz ).
The Zener diode is like a general-purpose signal diode consisting of a silicon PN junction. When
biased in the forward direction it behaves just like a normal signal diode passing the rated current,
but as soon as a reverse voltage applied across the zener diode exceeds the rated voltage of the
device, the diodes breakdown voltage VB is reached at which point a process called Avalanche
Breakdown occurs in the semiconductor depletion layer and a current starts to flow through the
diode to limit this increase in voltage.
The current now flowing through the zener diode increases dramatically to the maximum circuit
value (which is usually limited by a series resistor) and once archived this reverse saturation
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current remains fairly constant over a wide range of applied voltages. This breakdown voltage
point, VB is called the "zener voltage" for zener diodes and can range from less than one volt to
hundreds of volts.
The point at which the Zener voltage triggers the current to flow through the diode can be very
accurately controlled (to less than 1% tolerance) in the doping stage of the diodes semiconductor
construction giving the diode a specific Zener breakdown voltage, ( Vz ) for example, 4.3V or 7.5V.
This zener breakdown voltage on the I-V curve is almost a vertical straight line.
Zener Diode I-V Characteristics
The Zener Diode is used in its "reverse bias" or reverse breakdown mode, i.e. the diodes anode
connects to the negative supply.
 From the I-V characteristics curve above, we can see that the zener diode has a region in its
reverse bias characteristics of almost a constant negative voltage regardless of the value of the
current flowing through the diode and remains nearly constant even with large changes in current
as long as the zener diodes current remains between the breakdown current IZ(min)and the
maximum current rating IZ(max).
This ability to control itself can be used to great effect to regulate or stabilize a voltage source
against supply or load variations.
The fact that the voltage across the diode in the breakdown region is almost constant turns out to
be an important application of the zener diode as a voltage regulator.
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The function of a regulator is to provide a constant output voltage to a load connected in parallel
with it in spite of the ripples in the supply voltage or the variation in the load current and the zener
diode will continue to regulate the voltage until the diodes current falls below minimum IZ(min) value
in the reverse breakdown region.
Applications of the Zener Diode:
1. Simple Zener diode for voltage regulator
When used in a regulator circuit, the Zener diode must have the current entering it limited. If
a perfect voltage source was placed across it, then it would draw excessive current once the
breakdown voltage had been reached. To overcome this the Zener diode must be driven by a
current source. This will limit the current to the chosen value.
2. Zener diode circuit for PSU with series transistor
The very simple shunt regulator shown above is not particularly efficient and is not
practicable for many higher current applications. The solution is to utilize a Zener diode circuit
that uses a series pass transistor.
3. Zener diode circuit for overvoltage protection
Another form of Zener diode circuit is an overvoltage protection circuit. While power supplies
are normally reliable, the effects of the series pass transistor or FET can be catastrophic if it
fails by forming a short circuit. In this case the full unregulated voltage would be placed onto
the circuits using the regulated power. This could destroy all the chips being powered.
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2.3 Bridge Rectifier.
Rectifier diodes are available in various sizes, intended for different purposes.
Most rectifier diodes are made of silicon and are therefore known as silicon rectifiers. A few are
fabricated from selenium, and are called selenium rectifiers.
The simplest rectifier circuit uses just“chop off” half of the ac input cycle.
A much better scheme for changing ac to dc is to use both halves of the ac cycle. Suppose you
want to convert an ac wave to dc with positive polarity. Then you can allow the positive half of the
ac cycle to pass unchanged, and flip the negative portion of the wave upside-down, making it
positive instead. This is the principle behind full-wave rectification.
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Working Principal of Bridge rectifier:
One common full-wave circuit uses a transformer with a center-tapped secondary, as shown in
Fig. The center tap, a wire coming out of the exact middle of the secondary winding, is connected to
common ground. This produces out-of-phase waves at the ends of the winding.
These two waves can be individually half-wave rectified, cutting off the negative half of the cycle.
Because the waves are 180 degrees (half a cycle) out of phase, the output of the circuit has positive
pulses for both halves of the cycle.
In this rectifier circuit, the average dc output voltage is about 90 percent of the rms ac input
voltage. The PIV across the diodes can be as much as 2.8 times the rms input voltage.
Therefore, the diodes should have a PIV rating of at least 4.2 times the rms ac input.The bridge
circuit does not need a center-tapped transformer secondary.
This is its main practical advantage. Electrically, the bridge circuit uses the entire secondary on
both halves of the wave cycle; the center-tap circuit uses one side of the secondary for one half of
the cycle, and the other side for the other half of the cycle. For this reason, the bridge circuit makes
more efficient use of the transformer.
The main disadvantage of the bridge circuit is that it needs four diodes rather than two.Schematic
diagram of a full-wave bridge rectifier.a power supply must deliver a high current. Then, the extra
diodes—two for each half of the cycle, rather than one—dissipate more overall heat energy. When
current is used up as heat, it can’t go to the load. Therefore, center-tap circuits are preferable in
High-current applications.
The reason we have bridge rectifiers is the need for D.C., or direct current. Direct current runs
most of our home electronics. While it can be supplied by batteries, they have limited lifespan and
power. It's better in many instances to have a strong, continuous source of D.C. We can get power
from the household electrical outlet, but that's A.C., or alternating current. Fortunately, there's a
way to change A.C. into D.C., and that's by using a power supply with a bridge rectifier.
The bridge rectifier, necessary for a strong, reliable supply of D.C. power, was an essential part of
the electronics revolution, from radio and television to stereos, cell phones, and computers. Though
batteries filled this need to some extent, they also added size and weight to electronic gear.
Reducing or eliminating the need for batteries made equipment smaller, lighter, and more reliable.
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2.4 ‘T’ and ‘π’ Filter circuits.
Pi Filter
The Pi filter contains two capacitive elements and one inductive element.
It presents low impedance to both the source and the load. Because of the additional element, it
Provides better high frequency performance than the C or L configurations.
Due to the possibility of ‘ringing’, Pi filters are not recommended for switching applications.
T Filter
The T filter consists of two inductive elements and one capacitive element.
This circuit configuration presents a high impedance input from either end.
It has similar filter performance to the Pi circuit configurations. It does not have the ringing
characteristic of the Pi filter and can be used in switching applications.
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2.5 Zener diode as voltage regulator.
Zener Diodes can be used to produce a stabilised voltage output with low ripple under varying
load current conditions.
By passing a small current through the diode from a voltage source, via a suitable current limiting
resistor (RS), the zener diode will conduct sufficient current to maintain a voltage drop of Vout.
We remember from the previous tutorials that the DC output voltage from the half or full-wave
rectifiers contains ripple superimposed onto the DC voltage and that as the load value changes so to
does the average output voltage.
By connecting a simple zener stabiliser circuit as shown below across the output of the rectifier, a
more stable output voltage can be produced.
The resistor, RS is connected in series with the zener diode to limit the current flow through the
diode with the voltage source, VS being connected across the combination. The stabilised output
voltage Voutis taken from across the zener diode. The zener diode is connected with its cathode
terminal connected to the positive rail of the DC supply so it is reverse biased and will be operating
in its breakdown condition. Resistor RS is selected so to limit the maximum current flowing in the
circuit.
With no load connected to the circuit, the load current will be zero, ( IL = 0 ), and all the circuit
current passes through the zener diode which inturn dissipates its maximum power. Also a small
value of the series resistor RS will result in a greater diode current when the load resistance RL is
connected and large as this will increase the power dissipation requirement of the diode so care
must be taken when selecting the appropriate value of series resistance so that the zeners
maximum power rating is not exceeded under this no-load or high-impedance condition.
The load is connected in parallel with the zener diode, so the voltage across RL is always the same
as the zener voltage, ( VR = VZ ). There is a minimum zener current for which the stabilization of the
voltage is effective and the zener current must stay above this value operating under load within its
breakdown region at all times. The upper limit of current is of course dependant upon the power
rating of the device. The supply voltage VS must be greater than VZ.
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One small problem with zener diode stabiliser circuits is that the diode can sometimes generate
electrical noise on top of the DC supply as it tries to stabilise the voltage.
Normally this is not a problem for most applications but the addition of a large value decoupling
capacitor across the zeners output may be required to give additional smoothing.
A zener diode is always operated in its reverse biased condition.
A voltage regulator circuit can be designed using a zener diode to maintain a constant DC output
voltage across the load in spite of variations in the input voltage or changes in the load current.
The zener voltage regulator consists of a current limiting resistor RS connected in series with the
input voltage VS with the zener diode connected in parallel with the load RL in this reverse biased
condition.
The stabilized output voltage is always selected to be the same as the breakdown voltage VZ of
the diode.
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