N Junction Diode

A PN-junction diode is formed when a p-type semiconductor is fused to an n-type semiconductor

creating a potential barrier voltage across the diode junction

The effect described in the previous tutorial is achieved without any external voltage being
applied to the actual PN junction resulting in the junction being in a state of equilibrium.
However, if we were to make electrical connections at the ends of both the N-type and the P-type
materials and then connect them to a battery source, an additional energy source now exists to
overcome the potential barrier.
The effect of adding this additional energy source results in the free electrons being able to cross
the depletion region from one side to the other. The behaviour of the PN junction with regards to
the potential barrier’s width produces an asymmetrical conducting two terminal device, better
known as the PN Junction Diode.
A PN Junction Diode is one of the simplest semiconductor devices around, and which has the
characteristic of passing current in only one direction only. However, unlike a resistor, a diode
does not behave linearly with respect to the applied voltage as the diode has an exponential
current-voltage ( I-V ) relationship and therefore we can not described its operation by simply
using an equation such as Ohm’s law.
If a suitable positive voltage (forward bias) is applied between the two ends of the PN junction, it
can supply free electrons and holes with the extra energy they require to cross the junction as the
width of the depletion layer around the PN junction is decreased.
By applying a negative voltage (reverse bias) results in the free charges being pulled away from
the junction resulting in the depletion layer width being increased. This has the effect of
increasing or decreasing the effective resistance of the junction itself allowing or blocking
current flow through the diode.
Then the depletion layer widens with an increase in the application of a reverse voltage and
narrows with an increase in the application of a forward voltage. This is due to the differences in
the electrical properties on the two sides of the PN junction resulting in physical changes taking
place. One of the results produces rectification as seen in the PN junction diodes static I-V
(current-voltage) characteristics. Rectification is shown by an asymmetrical current flow when
the polarity of bias voltage is altered as shown below.

Junction Diode Symbol and Static I-V Characteristics

But before we can use the PN junction as a practical device or as a rectifying device we need to
firstly bias the junction, ie connect a voltage potential across it. On the voltage axis above,
“Reverse Bias” refers to an external voltage potential which increases the potential barrier. An
external voltage which decreases the potential barrier is said to act in the “Forward Bias”
direction.
There are two operating regions and three possible “biasing” conditions for the standard Junction
Diode and these are:
 1. Zero Bias – No external voltage potential is applied to the PN junction diode.
 2. Reverse Bias – The voltage potential is connected negative, (-ve) to the P-type material
and positive, (+ve) to the N-type material across the diode which has the effect
of Increasing the PN junction diode’s width.
 3. Forward Bias – The voltage potential is connected positive, (+ve) to the P-type material
and negative, (-ve) to the N-type material across the diode which has the effect
of Decreasing the PN junction diodes width.

Zero Biased Junction Diode

When a diode is connected in a Zero Bias condition, no external potential energy is applied to the
PN junction. However if the diodes terminals are shorted together, a few holes (majority carriers)
in the P-type material with enough energy to overcome the potential barrier will move across the
junction against this barrier potential. This is known as the “Forward Current” and is referenced
as IF
Likewise, holes generated in the N-type material (minority carriers), find this situation
favourable and move across the junction in the opposite direction. This is known as the “Reverse
Current” and is referenced as IR. This transfer of electrons and holes back and forth across the
PN junction is known as diffusion, as shown below.

Zero Biased PN Junction Diode

The potential barrier that now exists discourages the diffusion of any more majority carriers
across the junction. However, the potential barrier helps minority carriers (few free electrons in
the P-region and few holes in the N-region) to drift across the junction.
Then an “Equilibrium” or balance will be established when the majority carriers are equal and
both moving in opposite directions, so that the net result is zero current flowing in the circuit.
When this occurs the junction is said to be in a state of “Dynamic Equilibrium“.
The minority carriers are constantly generated due to thermal energy so this state of equilibrium
can be broken by raising the temperature of the PN junction causing an increase in the generation
of minority carriers, thereby resulting in an increase in leakage current but an electric current
cannot flow since no circuit has been connected to the PN junction.

Reverse Biased PN Junction Diode

When a diode is connected in a Reverse Bias condition, a positive voltage is applied to the N-
type material and a negative voltage is applied to the P-type material.
The positive voltage applied to the N-type material attracts electrons towards the positive
electrode and away from the junction, while the holes in the P-type end are also attracted away
from the junction towards the negative electrode.
The net result is that the depletion layer grows wider due to a lack of electrons and holes and
presents a high impedance path, almost an insulator. The result is that a high potential barrier is
created thus preventing current from flowing through the semiconductor material.

Increase in the Depletion Layer due to Reverse Bias

This condition represents a high resistance value to the PN junction and practically zero current
flows through the junction diode with an increase in bias voltage. However, a very small leakage
current does flow through the junction which can be measured in micro-amperes, ( μA ).
One final point, if the reverse bias voltage Vr applied to the diode is increased to a sufficiently
high enough value, it will cause the diode’s PN junction to overheat and fail due to the avalanche
effect around the junction. This may cause the diode to become shorted and will result in the
flow of maximum circuit current, and this shown as a step downward slope in the reverse static
characteristics curve below.

Reverse Characteristics Curve for a Junction Diode

Sometimes this avalanche effect has practical applications in voltage stabilising circuits where a
series limiting resistor is used with the diode to limit this reverse breakdown current to a preset
maximum value thereby producing a fixed voltage output across the diode. These types of diodes
are commonly known as Zener Diodes and are discussed in a later tutorial.

Forward Biased PN Junction Diode

When a diode is connected in a Forward Bias condition, a negative voltage is applied to the N-
type material and a positive voltage is applied to the P-type material. If this external voltage
becomes greater than the value of the potential barrier, approx. 0.7 volts for silicon and 0.3 volts
for germanium, the potential barriers opposition will be overcome and current will start to flow.
This is because the negative voltage pushes or repels electrons towards the junction giving them
the energy to cross over and combine with the holes being pushed in the opposite direction
towards the junction by the positive voltage. This results in a characteristics curve of zero current
flowing up to this voltage point, called the “knee” on the static curves and then a high current
flow through the diode with little increase in the external voltage as shown below.

Forward Characteristics Curve for a Junction Diode

The application of a forward biasing voltage on the junction diode results in the depletion layer
becoming very thin and narrow which represents a low impedance path through the junction
thereby allowing high currents to flow. The point at which this sudden increase in current takes
place is represented on the static I-V characteristics curve above as the “knee” point.

Reduction in the Depletion Layer due to Forward Bias

This condition represents the low resistance path through the PN junction allowing very large
currents to flow through the diode with only a small increase in bias voltage. The actual potential
difference across the junction or diode is kept constant by the action of the depletion layer at
approximately 0.3v for germanium and approximately 0.7v for silicon junction diodes.
Since the diode can conduct “infinite” current above this knee point as it effectively becomes a
short circuit, therefore resistors are used in series with the diode to limit its current flow.
Exceeding its maximum forward current specification causes the device to dissipate more power
in the form of heat than it was designed for resulting in a very quick failure of the device.
Junction Diode Summary
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.
 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).
We have also seen above that the diode is two terminal non-linear device whose I-V
characteristic are polarity dependent as depending upon the polarity of the applied
voltage, VD the diode is either Forward Biased, VD > 0 or Reverse Biased, VD < 0. Either way
we can model these current-voltage characteristics for both an ideal diode and for a real silicon
diode as shown:

Junction Diode Ideal and Real Characteristics

In the next tutorial about diodes, we will look at the small signal diode sometimes called a
switching diode which is used in general electronic circuits. As its name implies, the signal diode
is designed for low-voltage or high frequency signal applications such as in radio or digital
switching circuits.
Signal diodes, such as the 1N4148 only pass very small electrical currents as opposed to the
high-current mains rectification diodes in which silicon diodes are usually used. Also in the next
tutorial we will examine the Signal Diode static current-voltage characteristics curve and
parameters.

Rectification

What is rectification?

Full Wave Rectification

A Full wave rectifier is a circuit arrangement which makes use of both half cycles of input
alternating current (AC) and converts them to direct current (DC). In our tutorial on Half wave
rectifiers, we have seen that a half wave rectifier makes use of only one-half cycle of the input
alternating current. Thus a full wave rectifier is much more efficient (double+) than a half wave
rectifier. This process of converting both half cycles of the input supply (alternating current) to
direct current (DC) is termed full wave rectification.

Full wave rectifier can be constructed in 2 ways. The first method makes use of a centre tapped
transformer and 2 diodes. This arrangement is known as Center Tapped Full Wave Rectifier.

The second method uses a normal transformer with 4 diodes arranged as a bridge. This
arrangement is known as a Bridge Rectifier.
Full Wave Rectifier Theory

To understand full wave bridge rectifier theory perfectly, you need to learn half wave rectifier
first. In the tutorial of half wave rectifier, we have clearly explained the basic working of a
rectifier. In addition, we have also explained the theory behind a pn junction and
the characteristics of a pn junction diode.

Full Wave Rectifier – Working & Operation

The working & operation of a full wave bridge rectifier is pretty simple. The circuit diagrams
and waveforms we have given below will help you understand the operation of a bridge rectifier
perfectly. In the circuit diagram, 4 diodes are arranged in the form of a bridge. The transformer
secondary is connected to two diametrically opposite points of the bridge at points A & C. The
load resistance RL is connected to bridge through points B and D.
 Full Wave Bridge Rectifier – Circuit Diagram with Input and Output Wave Forms

During the first half cycle

During the first half cycle of the input voltage, the upper end of the transformer secondary
winding is positive with respect to the lower end. Thus during the first half cycle diodes D1 and
D3 are forward biased and current flows through arm AB, enters the load resistance RL, and
returns back flowing through arm DC. During this half of each input cycle, the diodes D2 and
D4 are reverse biased and current is not allowed to flow in arms AD and BC. The flow of current
is indicated by solid arrows in the figure above. We have developed another diagram below to
help you understand the current flow quickly. See the diagram below – the green arrows indicate
the beginning of current flow from the source (transformer secondary) to the load resistance. The
red arrows indicate the return path of current from load resistance to the source, thus completing
the circuit.

Flow of current in Bridge Rectifier

During the second half cycle

During the second half cycle of the input voltage, the lower end of the transformer secondary
winding is positive with respect to the upper end. Thus diodes D2 and D4become forward biased
and current flows through arm CB, enters the load resistance RL, and returns back to the source
flowing through arm DA. The flow of current has been shown by dotted arrows in the figure.
Thus the direction of flow of current through the load resistance RL remains the same during
both half cycles of the input supply voltage. See the diagram below – the green arrows indicate
the beginning of current flow from the source (transformer secondary) to the load resistance. The
red arrows indicate the return path of current from load resistance to the source, thus completing
the circuit.
 Path of current in 2nd Half Cycle

Peak Inverse Voltage of a Full wave bridge rectifier:

Let’s analyse peak inverse voltage (PIV) of a full wave bridge rectifier using the circuit diagram.
At any instant when the transformer secondary voltage attains positive peak value Vmax, diodes
D1 and D3 will be forward biased (conducting) and the diodes D2 and D4 will be reverse biased
(non conducting). If we consider ideal diodes in bridge, the forward biased diodes D1 and D3
will have zero resistance. This means voltage drop across the conducting diodes will be zero.
This will result in the entire transformer secondary voltage being developed across load
resistance RL.

Thus PIV of a bridge rectifier = Vmax (max of secondary voltage)

Bridge Rectifier Circuit Analysis

The only difference in the analysis between full wave and centre tap rectifier is that

1. In a bridge rectifier circuit, two diodes conduct during each half cycle and the forward resistance
becomes double (2RF).
2. In a bridge rectifier circuit, Vsmax is the maximum voltage across the transformer secondary
winding whereas in a centre tap rectifier Vsmax represents that maximum voltage across each
half of the secondary winding.

The different parameters are explained with equations below:

1. Peak Current

The instantaneous value of the voltage applied to the rectifier is given as

vs = Vsmax Sin wt

If the diode is assumed to have a forward resistance of RF ohms and a reverse resistance equal to
infinity, the current flowing through the load resistance is given as

i1 = Imax Sin wt and i2 = 0 for the first half cycle

and i1 = 0 and i2 = Imax Sin wt for second half cycle

The total current flowing through the load resistance RL, being the sum of currents i1 and i2 is
given as

i = i1 + i2 = Imax Sin wt for the whole cycle.

Where the peak value of the current flowing through the load resistance RL is given as

Imax = Vsmax/(2RF + RL)

2. Output Current

Since the current is the same through the load resistance RL in the two halves of the ac cycle,
magnitude od dc current Idc, which is equal to the average value of ac current, can be obtained
by integrating the current i1 between 0 and pi or current i2 between pi and 2pi.

Output Current of Full Wave Rectifier

3. DC Output Voltage

Average or dc value of voltage across the load is given as

DC Output Voltage of Full Wave Rectifier

4. Root Mean Square (RMS) Value of Current

RMS or effective value of current flowing through the load resistance R L is given as
 RMS Value of Current of Full Wave Rectifier

5. Root Mean Square (RMS) Value of Output Voltage

RMS value of voltage across the load is given as

RMS Value of Output Voltage of Full Wave Rectifier

6. Rectification Efficiency

Power delivered to load,

Rectification Efficiency of Full Wave Rectifier

7. Ripple Factor

Form factor of the rectified output voltage of a full wave rectifier is given as

Ripple Factor of Full Wave Rectifier

So, ripple factor, γ = 1.112 – 1) = 0.482

8. Regulation

The dc output voltage is given as

 Regulation of Full Wave Rectifier

Merits and Demerits of Full-wave Rectifier Over Half-Wave Rectifier

Merits – let us talk about the advantages of full wave bridge rectifier over half wave version first.
I can think about 4 specific merits at this point.

 Efficiency is double for a full wave bridge rectifier. The reason is that, a half wave rectifier
makes use of only one half of the input signal. A bridge rectifier makes use of both halves and
hence double efficiency
 The residual ac ripples (before filtering) is very low in the output of a bridge rectifier. The same
ripple percentage is very high in half wave rectifier. A simple filter is enough to get a constant dc
voltage from the bridge rectifier.
 We know the efficiency of FW bridge is double than HW rectifier. This means higher output
voltage, Higher transformer utilization factor (TUF) and higher output power.

Demerits – Full-wave rectifier needs more circuit elements and is costlier.

Merits and Demerits of Bridge Rectifier Over Center-Tap Rectifier.

A centre tap rectifier is always a difficult one to implement because of the special transformer
involved. A centre tapped transformer is costly as well. One key difference between center tap &
bridge rectifier is in the number of diodes involved in construction. A center tap full wave
rectifier needs only 2 diodes whereas a bridge rectifier needs 4 diodes. But silicon diodes being
cheaper than a center tap transformer, a bridge rectifier is much-preferred solution in a DC power
supply. Following are the advantages of bridge rectifier over a center tap rectifier.

 A bridge rectifier can be constructed with or without a transformer. If a transformer is involved,

any ordinary step down/step up transformer will do the job. This luxury is not available in a
center tap rectifier. Here the design of rectifier is dependent on the center tap transformer, which
can not be replaced.
 Bridge rectifier is suited for high voltage applications. The reason is the high peak inverse
voltage (PIV) of bridge rectifier when compared to the PIV of a center tap rectifier.
 Transformer utilization factor (TUF) is higher for bridge rectifier.

Demerits of Bridge rectifier over center tap rectifier

The significant disadvantage of a bridge rectifier over center tap is the involvement of 4 diodes
in the construction of bridge rectifier. In a bridge rectifier, 2 diodes conduct simultaneously on a
half cycle of input. A center tap rectifier has only 1 diode conducting on one-half cycle. This
increases the net voltage drop across diodes in a bridge rectifier (it is double to the value of
center tap).

Applications of Full wave Bridge rectifier

Full wave rectifier finds uses in the construction of constant dc voltage power supplies,
especially in general power supplies. A bridge rectifier with an efficient filter is ideal for any
type of general power supply applications like charging a battery, powering a dc device (like a
motor, led etc) etc. However, for an audio application, a general power supply may not be
enough. This is because of the residual ripple factor in a bridge rectifier. There are limitations to
filtering ripples. For audio applications, specially built power supplies (using IC regulators) may
be ideal.

Full Wave Bridge Rectifier with Capacitor Filter

The output voltage of the full wave rectifier is not constant, it is always pulsating. But this
cannot be used in real life applications. In other words, we desire a DC power supply with a
constant output voltage. In order to achieve a smooth and constant voltage a filter with a
capacitor or an inductor is used. The circuit diagram below shows a half wave rectifier with
capacitor filter.
 Full Wave Rectifier – with Capacitor Filter

Ripple factor in a bridge rectifier

Ripple factor is a ratio of the residual ac component to dc component in the output voltage.
Ripple factor in a bridge rectifier is half than that of a half wave rectifier.