CHAPTER 2: P-N JUNCTION

BASIC STRUCTURE OF PN JUNCTION

In a piece of sc, if one half is doped by p type impurity and the other half is doped by
n type impurity, a PN junction is formed. The plane dividing the two halves or zones is called
PN junction. As shown in the fig the n type material has high concentration of free electrons,
while p type material has high concentration of holes. Therefore at the junction there is a
tendency of free electrons to diffuse over to the P side and the holes to the N side. This process
is called diffusion. As the free electrons move across the junction from N type to P type, the
donor atoms become positively charged. Hence a positive charge is built on the N-side of the
junction. The free electrons that cross the junction uncover the negative acceptor ions by filing
the holes. Therefore a negative charge is developed on the p –side of the junction..This net
negative charge on the p side prevents further diffusion of electrons into the p side. Similarly
the net positive charge on the N side repels the hole crossing from p side to N side. Thus a
barrier sis set up near the junction which prevents the further movement of charge carriers i.e.
electrons and holes. As a consequence of induced electric field across the depletion layer, an
electrostatic potential difference is established between P and N regions, which are called the
potential barrier, junction barrier, diffusion potential or contact potential, Vo. The magnitude
of the contact potential Vo varies with doping levels and temperature. Vo is 0.3V for Ge and
0.72 V for Si.

Fig 2.1: Symbol of PN Junction Diode

The electrostatic field across the junction caused by the positively charged N-Type region tends to
drive the holes away from the junction and negatively charged p type regions tend to drive the
electrons away from the junction. The majority holes diffusing out of the P region leave behind
negatively charged acceptor atoms bound to the lattice, thus exposing a negatives pace charge in a
previously neutral region. Similarly electrons diffusing from the N region expose positively
ionized donor atoms and a double space charge builds up at the junction as shown in the fig. 2.2 a

Fig 2.2 a

It is noticed that the space charge layers are of opposite sign to the majority carriers
diffusing into them, which tends to reduce the diffusion rate. Thus the double space of the
layer causes an electric field to be set up across the junction directed from N to P regions,
which is in such a direction to inhibit the diffusion of majority electrons and holes as
illustrated in fig 2.2 b. The shape of the charge density, ρ, depends upon how diode id doped.
Thus the junction region is depleted of mobile charge carriers. Hence it is called depletion
layer, space region, and transition region. The depletion region is of the order of 0.5µm thick.
There are no mobile carriers in this narrow depletion region. Hence no current flows across
the junction and the system is in equilibrium. To the left of this depletion layer, the carrier
concentration is p= NA and to its right it is n= ND.
 Fig 2.2 b
FORWARD BIASED 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

Fig 2.3 a: Diode Forward Characteristics

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 therebyallowing 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.
 Forward Biased Junction Diode showing a Reduction in the Depletion Layer

Fig 2.3 b: Diode 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.

REVERSE BIASED 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.
 Reverse Biased Junction Diode showing an Increase in the Depletion

Fig 2.4 a: Diode 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
microamperes, (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 PN junction to overheat and
fail due to the avalanche effect around the junction. This maycause 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

Fig 2.4 b: Diode Reverse Characteristics

Sometimes this avalanche effect has practical applications in voltage stabilizing 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 commonlyknown as Zener Diodes
 Fig 2.5: Diode Characteristics

Temperature Effects on Diode

Temperature can have a marked effect on the characteristics of a silicon

semiconductor diode as shown in Fig. 11 It has been found experimentally that the reverse
saturation current Io will just about double in magnitude for every 10°C increase in
temperature.
 Fig 2.6 Variation in Diode Characteristics with temperature change

It is not uncommon for a germanium diode with an Io in the order of 1 or 2 A at 25°C to have
a leakage current of 100 A - 0.1 mA at a temperature of 100°C. Typical values of Io for silicon
are much lower than that of germanium for similar power and current levels. The result is that
even at high temperatures the levels of Io for silicon diodes do not reach the same high levels
obtained. For germanium—a very important reason that silicon devices enjoy a significantly
higher level of development and utilization in design. Fundamentally, the open-circuit
equivalent in the reverse bias region is better realized at any temperature with silicon than
with germanium. The increasing levels of Io with temperature account for the lower levels of
threshold voltage, as shown in Fig. 1.11. Simply increase the level of I o in and not rise in diode
current. Of course, the level of TK also will be increase, but the increasing level of I o will
overpower the smaller percent change in TK. As the temperature increases the forward
characteristics are actually becoming more “ideal,”

IDEAL VERSUS PRACTICAL RESISTANCE LEVELS

DC or Static Resistance

The application of a dc voltage to a circuit containing a semiconductor diode will

result in an operating point on the characteristic curve that will not change with time. The
resistance of the diode at the operating point can be found simply by finding the corresponding
levels of VD and ID as shown in Fig. 2.7 and applying the following Equation:

The dc resistance levels at the knee and below will be greater than the resistance levels
obtained for the vertical rise section of the characteristics. The resistance levels in the
reverse-bias region will naturally be quite high. Since ohmmeters typically employ a
relatively constant-current source, the resistance determined will be at a preset current level
(typically, a few mill amperes).
Fig 2.7 Determining the dc resistance of a diode at a particular operating point.
AC or Dynamic Resistance

DC resistance of a diode is independent of the shape of the characteristic in the region surrounding the point
of interest. If a sinusoidal rather than dc input is applied, the situation will change completely. The varying input
will move the instantaneous operating point up and down a region of the characteristics and thus defines a specific
change in current and voltage as shown in Fig. 2.7. With no applied varying signal, the point of operation would be
the Q- point appearing on Fig. 2.7 determined by the applied dc levels. The designation Q-point is derived from the
word quiescent, which means “still or unvarying.” A straight-line drawn tangent to the curve through the Q-point as
shown in Fig. 2.7 will define a particular change in voltage and current that can be used to determine the ac or
dynamic resistance for this region of the diode characteristics. In equation form,

Where Δ Signifies a finite change in the quantity

 Fig 2.7 : Determining the ac resistance of a diode a t a particular operating point.

Energy Band Diagram and Depletion Layer of a PN Junction at equilibrium

• First draw a horizontal line for EF

because there is only one Fermi
level at equilibrium as shown in fig
a.

• Figure b shows that far from the

junction, we simply have an N- type
semiconductor on one side (with Ec
close to EF), and a P-type
semiconductor on the other side (with
Ev close to EF).

• Finally, in Fig.c we draw an arbitrary (for

now) smooth curve to link the Ec from
 the N layer to the P layer. Ev of course
follows Ec, being below Ec by a
constant Eg.

• Figure d shows that a PN junction can be

divided into three layers: the neutral N
layer, the neutral P layer, and a depletion
layer in the middle. In the middle layer,
EF is close to neither Ev nor Ec.
Therefore, both the electron and hole
concentrations are quite small. For
mathematical simplicity, it is assumed
that

• The term depletion layer means that the

layer is depleted of electrons and holes.
Fig 31.9
From band diagram:

• A potential barrier V0 Or an energy hill of height eV0 at electrons in the conduction band of n
region face an energy hill namely conduction hill.

• Electrons approaching the junction region cannot surmount the Conduction hill unless they
have a minimum energy of eV0.

• for example electron mark (1) in figure fails to climb the conduction hill. Occasionally
a few of the electrons that have kinetic energy equal to greater than eV0 overcome the
conduction hill and go Into P region.

• On the other hand the electrons near the junction in P region can
roll down the conduction hill effortlessly and pass into n-region.

• For example the electron mark (2) in the figure can roll down easily the conduction hill.
The component of current due to such migration of electrons are in opposite
directions and balances each other
• As the direction of increasing energy is downward for the holes in the valence band of P region encounter
an energy hill, namely valence hill.

• The holes on the P side cannot go into and n region unless they have a minimum energy of eV0.

• The whole mark 3 in the figure fails to surmount the balance in where is a few holes having kinetic energy
equal to a greater than eV0 succeeds in going into n-region.

• On the other hand holes near the junction on the n-side can readily float up the hill irrespective of the energy.

• For example the hole marked 4 in figure floats up the Valence hill. The two
components of current due to opposite flow of holes balance each other.

• Thus, the current due to occasional diffusion of majority carriers is balanced by the occasional drift of majority
carriers and the current across the p-n junction is zero.
CALCULATION OF INTERNAL POTENTIAL BARRIER V0
PN Forward Bias Characteristics

• 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.
Energy band diagram of PN diode in forward bias
From Energy band diagram

• In forward bias –ve voltage is applied to n side that will raise the Efn (Fermi level on n side) and lower the
Efp on p side. Thus Fermi level get separated by an amount of energy eVF

• Height of the potential barrier are reduced by an amount of energy eVF to a value of e(Vo-VF)

• Hence the energy required by the majority carriers to move into opposite region is reduced to e(Vo-VF) does
the components Jhp and Jen increases.

• Therefore, electrons slide down the conduction hill from P side to n side, while holes float up the valence
heel from n side to P side.

• The small drift current density components Jhp and Jen due to minority carriers do not change due to forward
bias.
PN JUNCTION UNDER REVERSE BIAS CONDITION

Fig 2: Diode Reverse Bias

 PN JUNCTION UNDER REVERSE BIAS CONDITION

• 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 Ntype 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.

• 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.

Energy band diagram of PN diode in reverse bias

 PN JUNCTION UNDER REVERSE BIAS CONDITION

• However, a very small leakage

current does flow through the
junction which can be measured in
microamperes, (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 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
Complete VI CHARACTERISTICS
Parameters ofDiode

Forward Voltage drop-VF

The diode forward voltage is the drop in electrical voltage that occurs when electrical current is conducted through a diode.
Diodes are two-lead semiconductor devices that conduct an electrical signal in one direction but not the other.

• Maximum Forward current-

It is the maximum current that can flow in a diode when it is forward biased.

• Reverse saturation current-

This is the current flowing through the diode during reverse bias due to the flow of minority charge carriers.

• Reverse breakdown voltage-

When the diode reverse voltage (VR) is sufficiently increased, the device goes to reverse breakdown & it can destroy a diode
unless the current is limited by a resistor.
Temperature effects on diode

• An increased temperature will result in a large number of broken covalent bonds increasing the large number
of majority and minority carriers. This amounts to a diode current larger than its previous diode current. The
above phenomenon applies both to forward and reverse current.

• It may be noted that the forward characteristics shifts upwards with increase in temperature. On the other
hand, the reverse characteristics shifts downwards with the increase in temperature.

• The effect of increased temperature on the characteristics curve of a PN junction diode is as shown in figure.
 Diode Equation
When a diode is forward b biased the potential barrier is lowered by amount of energy eVf and the probability of majority carriers crossing the junction
𝑒𝑉𝐹 𝑒𝑉𝐹
is increased by a factor 𝑒 𝑘𝑇 the diffusion current density increases by a factor 𝑒 𝑘𝑇
Thus, the diffusion current density components J*hp and J*en in a forward biased diode
Problem 2: Find the static resistance of the diode whose characteristics is shown in Fig. when the forward

current is 80mA & reverse voltage is 40V.Cut-in-voltage is 0.35V

•
 TUNNEL DIODE (Esaki Diode)

 It was introduced by Leo Esaki in 1958.

 Heavily-doped p-n junction

o Impurity concentration is 1 part in 10^3 as compared to 1 part in 10^8 in p-n junction diode

 Width of the depletion layer is very small (about 100 A).

 It is generally made up of Ge and GaAs.

 It shows tunneling phenomenon.

 Circuit symbol of tunnel diode is :

What is Tunneling?
Classically, carrier must have energy at least equal to potential-barrier height to cross the junction .

 But according to Quantum mechanics there is finite probability that it can penetrate through the barrier for a
thin width.
 This phenomenon is called
tunneling and hence the Esaki
Diode is know as Tunnel diode.
CHARACTERISTIC OF TUNNEL DIODE I
Ip:- Peak Current

Iv :- Valley Current
- Ve Resistance

Forward Current
Region
Vp:- Peak Voltage

Vv:- Valley Voltage I

Vf:- Peak Forward voltage

Revers V V
Vv
e
voltage Forward Voltage
 ENERGY BAND DIAGRAM

Energy-band diagram of pn junction in thermal equilibrium in which both the n and p region
are degenerately doped.
AT ZERO BIAS

Simplified energy-band diagram and I-V characteristics of the tunnel diode at zero bias.

-Zero current on the I-V diagram;

-All energy states are filled below EF on both sides of the junction;
 AT SMALL FORWARD VOLTAGE

Simplified energy-band diagram and I-V characteristics of the tunnel diode at a slight forward bias.

-Electrons in the conduction band of the n region are directly opposite to the empty states in the valence band of the p region.
 AT MAXIMUM TUNNELING CURENT

Simplified energy-band diagraam and I-V characteristics of the tunnel diode at a forward bias producing maximum tunneling
current.
-The maximum number of electrons in the n region are opposite to the maximum number of empty states in the p region.
- Hence tunneling current is maximum.
 AT DECREASING CURRENT REGION

Simplified energy-band diagram and I-V characteristics of the tunnel diode at a higher forward bias producing less
tunneling current.

-The forward-bias voltage increases so the number of electrons on the n side, directly opposite empty states on the p side
decreases. Hence the tunneling current decreases.
 AT HIGHER FORWARD VOLTAG

Simplified energy-band diagram and I-V characteristics of the tunnel diode at a forward bias for which the
diffusion current dominates.
-No electrons on the n side are directly opposite to the empty states on the p side.
- The tunneling current is zero.
-The normal ideal diffusion current exists in the device.
 AT REVERSE BIAS VOLTAGE

Electrons in the valence band on the p side are directly opposite to empty states in the conduction band on the n side.
-Electrons tunnel directly from the p region into the n region.
- The reverse-bias current increases monotonically and rapidly with reverse-bias voltage.

Applications of tunnel diode: Tunnel diodes are used in a variety of applications, including logic memory storage,
relaxation oscillator circuits, ultra-high-speed switches, and FM receivers.
 Schottky diode

o Schottky diode is a metal-semiconductor junction diode that has less forward voltage drop than the P-N
junction diode and can be used in high- speed switching applications.

o In schottky diode, metals such as aluminum or platinum replace the p- type semiconductor. The schottky
diode is named after German physicist Walter H. Schottky.

o Schottky diode is also known as schottky barrier diode, surface barrier diode, majority carrier device, hot-
electron diode, or hot carrier diode. Schottky diodes are widely used in radio frequency (RF) applications.

o The voltage needed to turn on the schottky diode is same as that of a germanium diode. But germanium
diodes are rarely used because the switching speed of germanium diodes is very low as compared to the
schottky diodes.
Symbol of schottky diode

The symbol of schottky diode is shown in the below figure. In Schottky diode, the metal acts as the anode
and n-type semiconductor acts as the cathode.
Metal-semiconductor (M-S) junction

Metal-semiconductor (M-S) junction is a type of junction formed between a metal and an n-type semiconductor
when the metal is joined with the n-type semiconductor. Metal-semiconductor junction is also sometimes referred
to as M-S junction.

The metal-semiconductor junction can be either non-rectifying or rectifying. The non-rectifying metal-
Semiconductor junction is called ohmic contact. The rectifying metal-semiconductor junction is called non-
ohmic contact.

• A metal-semiconductor junction formed between a metal and n-type semiconductor creates a barrier or depletion
layer known as a schottky barrier.

• When sufficient voltage is applied to the schottky diode, current starts flowing in the forward direction. Because
of this current flow, a small voltage loss occurs across the terminals of the schottky diode. This voltage loss is
known as voltage drop.

• A silicon diode has a voltage drop of 0.6 to 0.7 volts, while a schottky diode has a voltage drop of 0.2 to 0.3 volts.
Voltage loss or voltage drop is the amount of voltage wasted to turn on a diode.

• In silicon diode, 0.6 to 0.7 volts is wasted to turn on the diode, whereas in schottky diode, 0.2 to 0.3 volts is wasted
to turn on the diode. Therefore, the schottky diode consumes less voltage to turn on.
What is a schottky barrier?

Schottky barrier is a depletion layer formed at the junction of a metal and n-type semiconductor. In
simple words, schottky barrier is the potential energy barrier formed at the metal-semiconductor
junction. The electrons have to overcome this potential energy barrier to flow across the diode.

.
• One of the most important characteristics of a schottky barrier is the schottky barrier height. The value of
this barrier height depends on the combination of semiconductor and metal.

• The schottky barrier height of ohmic contact (non-rectifying barrier) is very low( hence depletion layer
is absent)

• whereas the schottky barrier height of non-ohmic contact (rectifying barrier) is high.(there is a
depletion region)

• The rectifying schottky barrier is formed when a metal is in contact with the lightly doped semiconductor,
whereas the non-rectifying barrier is formed when a metal is in contact with the heavily doped semiconductor.

• The ohmic contact has a linear current-voltage (I-V) curve whereas the non-ohmic contact has a non- linear
current-voltage (I-V) curve.
 How Schottky diode works?

• Unbiased schottky diode

When the metal is joined with the n-type semiconductor, the conduction band electrons (free electrons) in the n-type
semiconductor will move from n-type semiconductor to metal to establish an equilibrium state.

We know that when a neutral atom loses an electron it becomes a positive ion similarly when a neutral atom gains
an extra electron it becomes a negative ion.

The conduction band electrons or free electrons that are crossing the junction will provide extra electrons to the atoms
in the metal. As a result, the atoms at the metal junction gains extra electrons and the atoms at the n-side junction
lose electrons.
• The atoms that lose electrons at the n-side junction will become positive ions whereas the atoms
that gain extra electrons at the metal junction will become negative ions. Thus, positive ions are
created the n-side junction and negative ions are created at the metal junction. These positive and
negative ions are nothing but the depletion region.

• Since the metal has a sea of free electrons, the width over which these electrons move into the metal
is negligibly thin as compared to the width inside the n-type semiconductor. So the built-in-potential
or built-in-voltage is primarily present inside the n-type semiconductor. The built-in-voltage is the
barrier seen by the conduction band electrons of the n-type semiconductor when trying to move into
the metal.

• To overcome this barrier, the free electrons need energy greater than the built-in- voltage. In
unbiased schottky diode, only a small number of electrons will flow from n- type semiconductor to
metal. The built-in-voltage prevents further electron flow from the semiconductor conduction band
into the metal.

• The transfer of free electrons from the n-type semiconductor into metal results in energy band
bending near the contact.
Forward biased schottky diode

• If the positive terminal of the battery is connected to the metal and the negative terminal of the battery is
connected to the n-type semiconductor, the schottky diode is said to be forward biased.

• When a forward bias voltage is applied to the schottky diode, a large number of free electrons are
generated in the n-type semiconductor and metal. However, the free electrons in n-type semiconductor
and metal cannot cross the junction unless the applied voltage is greater than 0.2 volts.
• If the applied voltage is greater than 0.2 volts, the free electrons gain enough energy and overcomes the
built-in-voltage of the depletion region. As a result, electric current starts flowing through the schottky
diode.

• If the applied voltage is continuously increased, the depletion region becomes very thin and finally
disappears.
 Reverse bias schottky diode

• If the negative terminal of the battery is connected to the metal and the positive terminal of the battery
is connected to the n-type semiconductor, the schottky diode is said to be reverse biased.

• When a reverse bias voltage is applied to the schottky diode, the depletion width increases. As a
result, the electric current stops flowing. However, a small leakage current flows due to the thermally
excited electrons in the metal

• If the reverse bias voltage is continuously increased, the electric current gradually increases due to the
weak barrier.

• If the reverse bias voltage is largely increased, a sudden rise in electric current takes place. This sudden
rise in electric current causes depletion region to break down which may permanently damage the
device.

• .
V-I characteristics of Schottky diode

• The V-I (Voltage-Current) characteristics of schottky diode is shown in the below figure. The
vertical line in the below figure represents the current flow in the schottky diode and the horizontal
line represents the voltage applied across the schottky diode.

• The V-I characteristics of schottky diode is almost similar to the P-N junction diode. However, the
forward voltage drop of schottky diode is very low as compared to the P-N junction diode.
• The forward voltage drop of schottky diode is 0.2 to 0.3 volts whereas the forward voltage
drop of silicon P-N junction diode is 0.6 to 0.7 volts.

• If the forward bias voltage is greater than 0.2 or 0.3 volts, electric current starts flowing
through the schottky diode.

• In schottky diode, the reverse saturation current occurs at a very low voltage as
compared to the silicon diode.
Difference between schottky diode and P-N junction diode

The main difference between schottky diode and p-n junction diode is as follows:

• In schottky diode, the free electrons carry most of the electric current. Holes carry
negligible electric current. So schottky diode is a unipolar device. In P-N junction
diode, both free electrons and holes carry electric current. So P-N junction diode
is a bipolar device.

• The reverse breakdown voltage of a schottky diode is very small as compared to

the p-n junction diode.

• In schottky diode, the depletion region is absent or negligible, whereas in p-n

junction diode the depletion region is present.

• The turn-on voltage for a schottky diode is very low as compared to the p-n
junction diode.

• In schottky diode, electrons are the majority carriers in both metal and
semiconductor. In P-N junction diode, electrons are the majority carriers in n-
region and holes are the majority carriers in p-region.
Advantages of schottky diode

• Low junction capacitance

• Fast reverse recovery time

• High current density

• Low forward voltage drop or low turn on voltage

• High efficiency
• Schottky diodes operate at high frequencies.
• Schottky diode produces less unwanted noise than P-N junction diode.
Disadvantages of schottky diode

• Large reverse saturation current

Applications of schottky diodes

• Schottky diodes are used as general-purpose rectifiers.

• Schottky diodes are used in radio frequency (RF) applications.
• Schottky diodes are widely used in power supplies.
• Schottky diodes are used to detect signals.
• Schottky diodes are used in logic circuits.
 LIGHT EMITTING DIODE (LED)

• LED is a specially made forward biased P-N junction diode, which emits light due to electron-hole
recombination, when energized.
• This works on the principle of electroluminescence, the process in which electrical energy is converted
into light energy.
• Henry Joseph Round was the first person to observe the phenomenon of
electroluminescence in 1907 and invent the first LED.
• It is operated only in forward bias.
• The symbol of LED is shown in below Fig

Working of LED

• When Light Emitting Diode (LED) is forward biased, the free electrons from N-side and the holes from P-
side move towards the junction as shown in fig.2.(a)
• Like ordinary diode, the forward current is negligible up to a certain value of forward applied voltage due to
the potential barrier across the PN-junction.
• When free electrons reach the junction, they overcome the potential barrier and recombine with the holes.
• In the similar way, holes from p-side recombine with electrons in the depletion region. Because of the
recombination of free electrons and holes in the depletion region, the width of depletion region decreases.
• The recombination of free electrons and holes leads to generation of light.
• The wavelength of light emitted and its color depends on Energy Gap(Eg) of
material used in making of LE
• The wavelength of light emitted

• For light to be emitted in visible region, Energy gap Eg should be between

1.77eV to 3.11eV.
• Silicon and Germanium have bandgap energies that correspond to infrared light which is not visible. Energy
gap Eg of Silicon is 1.1eV and that of Germanium is 0.72eV. They are known as indirect- bandgap
semiconductors. This means, it is not possible for electrons and holes to recombine directly and form a photon.
Instead, you get a phonon (type of lattice vibration) which is effectively heat.
• An LED requires a direct- band gap semiconductors having Energy gap ≥
1.8eV.This can happen very efficiently in materials like Gallium Arsenide Phosphide (GaAsP) Eg = 1.9eV and
Gallium Phosphide (GaP) Eg = 2.26eV.The Energy released in these materials produces intense visible light.
The V-I characteristics of LED

• The V-I characteristics of LED is shown in fig. 2(b).

• When the voltage applied overcomes the Energy gap(potential barrier), current increases rapidly as
more number of charge carriers cross the PN junction. This voltage above which the diode starts
conducting is called cut-in voltage of LED.
• The voltage after cut-in value remains almost constant once LED starts conducting like ordinary diode,
but current increases rapidly.
 APPLICATIONS OF LIGHT EMITTING DIODE

• LED are commonly used as indicator in electronic circuits.

• They are used in electronic calculators to display the numbers.
• They are used in digital meters & electronic clocks.
• Infrared LEDs can be used as a source in optical fiber communications.
• LEDs are used in Burglar alarms systems, Calculators, Picture phones, Traffic signals, Digital computers,
Microprocessors, Digital watches, Automotive heat lamps, Camera flashes and Aviation lighting.
 Varactor diode

Varactor diode is a p-n junction which operates only in reverse bias, its capacitance is varied by varying the reverse
voltage . The term varactor is originated from a variable capacitor. It is also sometimes referred to as varicap diode,
tuning diode, or variable capacitance diode. The varactor diode is manufactured in such as way that it shows better
transition capacitance property than the ordinary diodes.
Varactor diode symbol :The symbol of the varactor diode is almost similar to the normal p-n junction diode. Two parallel
lines at the cathode side represents two conductive plates and the space between these two parallel lines represents
dielectric

Unbiased varactor diode

In the n-type semiconductor, a large number of free electrons are present and in the p-type semiconductor, a large
number of holes are present. The free electrons and holes always try to move from a higher concentration region to
a lower concentration region. Therefore, the free electrons always try to move from n- region to p-region similarly
holes always try to move from p-region to n-region.

When no voltage is applied, a large number of free electrons in the n-region get repelled from each other and move
towards the p-region. When the free electrons reach p-n junction, they experience an attractive force from the holes in
the p- region. As a result, the free electrons cross the p-n junction. In the similar way, holes also cross the p-n junction.
Because of the flow of these charge carriers, a tiny current flows across diode for some period.
During this process, some neutral atoms near the junction at n-side lose electrons and become positively charged
atoms (positive ions) similarly some neutral atoms near the junction at p-side gains extra electrons and become
negatively charged atoms (negative ions). These positive and negative ions created at the p-n junction create the
depletion region. This depletion region prevents further current flow across the p-n junction. The width of depletion
region depends on the number of impurities added (amount of doping). A heavily doped varactor diode has a thin
depletion layer whereas a lightly doped varactor diode has a wide depletion layer.
We know that an insulator or a dielectric does not allow electric current through it. The depletion region also does not
allow electric current through it. So the depletion region acts like a dielectric of a capacitor.
The electrodes or conductive plates easily allow electric
current through them. The p-type and n-type semiconductor
also easily allow electric current through them. So the p-type
and n-type semiconductor acts like the electrodes or
conductive plates of the capacitor. Thus, varactor diode
behaves like a normal capacitor. In an unbiased varactor
diode, the depletion width is small. So the capacitance
(charge storage) is very large.
 Operation of varactor diode

The varactor diode should always be operated in reverse bias. (A varactor diode is designed to store electric charge
not to conduct electric current).

When a reverse bias voltage is applied, the electrons from n-region and holes from p-region move away from the
junction. As a result, the width of depletion region increases and the capacitance decreases. However, if the applied
reverse bias voltage is very low the capacitance will be very large. So the reverse bias voltage should be kept at a
minimum to achieve large storage charge. Thus, capacitance or transition capacitance can be varied by varying the
voltage.
Applications of varactor diode

• Varactor diode is used in frequency multipliers.

• Varactor diode is used in parametric amplifiers.

• Varactor diode is used in voltage-controlled oscillators