Page 1
o
MODULE-I
DIODE CIRCUITS
P-N junction diode, I-V characteristics of a diode; review of half-wave and full-wave rectifiers, clamping
and clipping circuits. Input output characteristics of BJT in CB, CE, CC configurations, biasing circuits,
Load line analysis, common emitter, common base and common collector amplifiers; Small signal
equivalent circuits.
INTRODUCTON
Based on the electrical conductivity all the materials in nature are classified as insulators,
semiconductors, and conductors.
Insulator: An insulator is a material that offers a very low level (or negligible) of conductivity when
voltage is applied. Eg: Paper, Mica, glass, quartz. Typical resistivity level of an insulator is of the order
of 10
10
to 10
12
?-cm. The energy band structure of an insulator is shown in the fig.1.1. Band structure of
a material defines the band of energy levels that an electron can occupy. Valance band is the range of
electron energy where the electron remain bended too the atom and do not contribute to the electric
current. Conduction bend is the range of electron energies higher than valance band where electrons are
free to accelerate under the influence of external voltage source resulting in the flow of charge.
The energy band between the valance band and conduction band is called as forbidden band gap.
It is the energy required by an electron to move from balance band to conduction band i.e. the energy
required for a valance electron to become a free electron.
1 eV = 1.6 x 10
-19
J
For an insulator, as shown in the fig.1.1 there is a large forbidden band gap of greater than 5Ev. Because
of this large gap there a very few electrons in the CB and hence the conductivity of insulator is poor.
Even an increase in temperature or applied electric field is insufficient to transfer electrons from VB to
CB.
Insulator Semiconductor Conductor
FiG:1.1 Energy band diagrams insulator, semiconductor and conductor
VB
Forbidden band
gap Eo ˜6eV
CB
VB
Eo =˜6eV
CB
CB
VB
Page 2
o
MODULE-I
DIODE CIRCUITS
P-N junction diode, I-V characteristics of a diode; review of half-wave and full-wave rectifiers, clamping
and clipping circuits. Input output characteristics of BJT in CB, CE, CC configurations, biasing circuits,
Load line analysis, common emitter, common base and common collector amplifiers; Small signal
equivalent circuits.
INTRODUCTON
Based on the electrical conductivity all the materials in nature are classified as insulators,
semiconductors, and conductors.
Insulator: An insulator is a material that offers a very low level (or negligible) of conductivity when
voltage is applied. Eg: Paper, Mica, glass, quartz. Typical resistivity level of an insulator is of the order
of 10
10
to 10
12
?-cm. The energy band structure of an insulator is shown in the fig.1.1. Band structure of
a material defines the band of energy levels that an electron can occupy. Valance band is the range of
electron energy where the electron remain bended too the atom and do not contribute to the electric
current. Conduction bend is the range of electron energies higher than valance band where electrons are
free to accelerate under the influence of external voltage source resulting in the flow of charge.
The energy band between the valance band and conduction band is called as forbidden band gap.
It is the energy required by an electron to move from balance band to conduction band i.e. the energy
required for a valance electron to become a free electron.
1 eV = 1.6 x 10
-19
J
For an insulator, as shown in the fig.1.1 there is a large forbidden band gap of greater than 5Ev. Because
of this large gap there a very few electrons in the CB and hence the conductivity of insulator is poor.
Even an increase in temperature or applied electric field is insufficient to transfer electrons from VB to
CB.
Insulator Semiconductor Conductor
FiG:1.1 Energy band diagrams insulator, semiconductor and conductor
VB
Forbidden band
gap Eo ˜6eV
CB
VB
Eo =˜6eV
CB
CB
VB
Conductors: A conductor is a material which supports a generous flow of charge when a voltage is
applied across its terminals. i.e. it has very high conductivity. Eg: Copper, Aluminum, Silver, Gold. The
resistivity of a conductor is in the order of 10
-4
and 10
-6
?-cm. The Valance and conduction bands
overlap (fig1.1) and there is no energy gap for the electrons to move from valance band to conduction
band. This implies that there are free electrons in CB even at absolute zero temperature (0K). Therefore
at room temperature when electric field is applied large current flows through the conductor.
Semiconductor: A semiconductor is a material that has its conductivity somewhere between the
insulator and conductor. The resistivity level is in the range of 10 and 10
4
?-cm. Two of the most
commonly used are Silicon (Si=14 atomic no.) and germanium (Ge=32 atomic no.). Both have 4 valance
electrons. The forbidden band gap is in the order of 1eV. For eg., the band gap energy for Si, Ge and
GaAs is 1.21, 0.785 and 1.42 eV, respectively at absolute zero temperature (0K). At 0K and at low
temperatures, the valance band electrons do not have sufficient energy to move from V to CB. Thus
semiconductors act a insulators at 0K. as the temperature increases, a large number of valance electrons
acquire sufficient energy to leave the VB, cross the forbidden bandgap and reach CB. These are now
free electrons as they can move freely under the influence of electric field. At room temperature there
are sufficient electrons in the CB and hence the semiconductor is capable of conducting some current at
room temperature.
Inversely related to the conductivity of a material is its resistance to the flow of charge or
current. Typical resistivity values for various materials’ are given as follows.
Semiconductor Types
A pure form of semiconductors is called as intrinsic semiconductor. Conduction in
intrinsic sc is either due to thermal excitation or crystal defects. Si and Ge are the two most important
semiconductors used. Other examples include Gallium arsenide GaAs, Indium Antimonide (InSb) etc.
Let us consider the structure of Si. A Si atomic no. is 14 and it has 4 valance electrons. These 4
electrons are shared by four neighboring atoms in the crystal structure by means of covalent bond. Fig.
1.2a shows the crystal structure of Si at absolute zero temperature (0K). Hence a pure SC acts has poor
conductivity (due to lack of free electrons) at low or absolute zero temperature.
The absence of electrons in covalent bond is represented by a small circle usually referred to as
hole which is of positive charge. Even a hole serves as carrier of electricity in a manner similar to that
of free electron. In a pure semiconductor, the number of holes is equal to the number of free electrons.
Page 3
o
MODULE-I
DIODE CIRCUITS
P-N junction diode, I-V characteristics of a diode; review of half-wave and full-wave rectifiers, clamping
and clipping circuits. Input output characteristics of BJT in CB, CE, CC configurations, biasing circuits,
Load line analysis, common emitter, common base and common collector amplifiers; Small signal
equivalent circuits.
INTRODUCTON
Based on the electrical conductivity all the materials in nature are classified as insulators,
semiconductors, and conductors.
Insulator: An insulator is a material that offers a very low level (or negligible) of conductivity when
voltage is applied. Eg: Paper, Mica, glass, quartz. Typical resistivity level of an insulator is of the order
of 10
10
to 10
12
?-cm. The energy band structure of an insulator is shown in the fig.1.1. Band structure of
a material defines the band of energy levels that an electron can occupy. Valance band is the range of
electron energy where the electron remain bended too the atom and do not contribute to the electric
current. Conduction bend is the range of electron energies higher than valance band where electrons are
free to accelerate under the influence of external voltage source resulting in the flow of charge.
The energy band between the valance band and conduction band is called as forbidden band gap.
It is the energy required by an electron to move from balance band to conduction band i.e. the energy
required for a valance electron to become a free electron.
1 eV = 1.6 x 10
-19
J
For an insulator, as shown in the fig.1.1 there is a large forbidden band gap of greater than 5Ev. Because
of this large gap there a very few electrons in the CB and hence the conductivity of insulator is poor.
Even an increase in temperature or applied electric field is insufficient to transfer electrons from VB to
CB.
Insulator Semiconductor Conductor
FiG:1.1 Energy band diagrams insulator, semiconductor and conductor
VB
Forbidden band
gap Eo ˜6eV
CB
VB
Eo =˜6eV
CB
CB
VB
Conductors: A conductor is a material which supports a generous flow of charge when a voltage is
applied across its terminals. i.e. it has very high conductivity. Eg: Copper, Aluminum, Silver, Gold. The
resistivity of a conductor is in the order of 10
-4
and 10
-6
?-cm. The Valance and conduction bands
overlap (fig1.1) and there is no energy gap for the electrons to move from valance band to conduction
band. This implies that there are free electrons in CB even at absolute zero temperature (0K). Therefore
at room temperature when electric field is applied large current flows through the conductor.
Semiconductor: A semiconductor is a material that has its conductivity somewhere between the
insulator and conductor. The resistivity level is in the range of 10 and 10
4
?-cm. Two of the most
commonly used are Silicon (Si=14 atomic no.) and germanium (Ge=32 atomic no.). Both have 4 valance
electrons. The forbidden band gap is in the order of 1eV. For eg., the band gap energy for Si, Ge and
GaAs is 1.21, 0.785 and 1.42 eV, respectively at absolute zero temperature (0K). At 0K and at low
temperatures, the valance band electrons do not have sufficient energy to move from V to CB. Thus
semiconductors act a insulators at 0K. as the temperature increases, a large number of valance electrons
acquire sufficient energy to leave the VB, cross the forbidden bandgap and reach CB. These are now
free electrons as they can move freely under the influence of electric field. At room temperature there
are sufficient electrons in the CB and hence the semiconductor is capable of conducting some current at
room temperature.
Inversely related to the conductivity of a material is its resistance to the flow of charge or
current. Typical resistivity values for various materials’ are given as follows.
Semiconductor Types
A pure form of semiconductors is called as intrinsic semiconductor. Conduction in
intrinsic sc is either due to thermal excitation or crystal defects. Si and Ge are the two most important
semiconductors used. Other examples include Gallium arsenide GaAs, Indium Antimonide (InSb) etc.
Let us consider the structure of Si. A Si atomic no. is 14 and it has 4 valance electrons. These 4
electrons are shared by four neighboring atoms in the crystal structure by means of covalent bond. Fig.
1.2a shows the crystal structure of Si at absolute zero temperature (0K). Hence a pure SC acts has poor
conductivity (due to lack of free electrons) at low or absolute zero temperature.
The absence of electrons in covalent bond is represented by a small circle usually referred to as
hole which is of positive charge. Even a hole serves as carrier of electricity in a manner similar to that
of free electron. In a pure semiconductor, the number of holes is equal to the number of free electrons.
EXTRINSIC SEMICONDUCTOR
Intrinsic semiconductor has very limited applications as they conduct very small amounts of
current at room temperature. The current conduction capability of intrinsic semiconductor can be
increased significantly by adding a small amounts impurity to the intrinsic semiconductor. By adding
impurities it becomes impure or extrinsic semiconductor. This process of adding impurities is called as
doping. The amount of impurity added is 1 part in 10
6
atoms.
N type semiconductor: If the added impurity is a pentavalent atom then the resultant semiconductor is
called N-type semiconductor. Examples of pentavalent impurities are Phosphorus, Arsenic, Bismuth,
Antimony etc.
P type semiconductor: If the added impurity is a trivalent atom then the resultant semiconductor is
called P-type semiconductor. Examples of trivalent impurities are Boron, Gallium , indium etc. Thus in
P type sc , holes are majority carriers and electrons are minority carriers. Since each trivalent impurity
atoms are capable accepting an electron, these are called as acceptor atoms. The following fig 1.5b
shows the pictorial representation of P type sc
? The conductivity of N type sc is greater than that of P type sc as the mobility of electron is
greater than that of hole.
? For the same level of doping in N type sc and P type sc, the conductivity of an N type sc
is around twice that of a P type sc.
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 cannot 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.
Page 4
o
MODULE-I
DIODE CIRCUITS
P-N junction diode, I-V characteristics of a diode; review of half-wave and full-wave rectifiers, clamping
and clipping circuits. Input output characteristics of BJT in CB, CE, CC configurations, biasing circuits,
Load line analysis, common emitter, common base and common collector amplifiers; Small signal
equivalent circuits.
INTRODUCTON
Based on the electrical conductivity all the materials in nature are classified as insulators,
semiconductors, and conductors.
Insulator: An insulator is a material that offers a very low level (or negligible) of conductivity when
voltage is applied. Eg: Paper, Mica, glass, quartz. Typical resistivity level of an insulator is of the order
of 10
10
to 10
12
?-cm. The energy band structure of an insulator is shown in the fig.1.1. Band structure of
a material defines the band of energy levels that an electron can occupy. Valance band is the range of
electron energy where the electron remain bended too the atom and do not contribute to the electric
current. Conduction bend is the range of electron energies higher than valance band where electrons are
free to accelerate under the influence of external voltage source resulting in the flow of charge.
The energy band between the valance band and conduction band is called as forbidden band gap.
It is the energy required by an electron to move from balance band to conduction band i.e. the energy
required for a valance electron to become a free electron.
1 eV = 1.6 x 10
-19
J
For an insulator, as shown in the fig.1.1 there is a large forbidden band gap of greater than 5Ev. Because
of this large gap there a very few electrons in the CB and hence the conductivity of insulator is poor.
Even an increase in temperature or applied electric field is insufficient to transfer electrons from VB to
CB.
Insulator Semiconductor Conductor
FiG:1.1 Energy band diagrams insulator, semiconductor and conductor
VB
Forbidden band
gap Eo ˜6eV
CB
VB
Eo =˜6eV
CB
CB
VB
Conductors: A conductor is a material which supports a generous flow of charge when a voltage is
applied across its terminals. i.e. it has very high conductivity. Eg: Copper, Aluminum, Silver, Gold. The
resistivity of a conductor is in the order of 10
-4
and 10
-6
?-cm. The Valance and conduction bands
overlap (fig1.1) and there is no energy gap for the electrons to move from valance band to conduction
band. This implies that there are free electrons in CB even at absolute zero temperature (0K). Therefore
at room temperature when electric field is applied large current flows through the conductor.
Semiconductor: A semiconductor is a material that has its conductivity somewhere between the
insulator and conductor. The resistivity level is in the range of 10 and 10
4
?-cm. Two of the most
commonly used are Silicon (Si=14 atomic no.) and germanium (Ge=32 atomic no.). Both have 4 valance
electrons. The forbidden band gap is in the order of 1eV. For eg., the band gap energy for Si, Ge and
GaAs is 1.21, 0.785 and 1.42 eV, respectively at absolute zero temperature (0K). At 0K and at low
temperatures, the valance band electrons do not have sufficient energy to move from V to CB. Thus
semiconductors act a insulators at 0K. as the temperature increases, a large number of valance electrons
acquire sufficient energy to leave the VB, cross the forbidden bandgap and reach CB. These are now
free electrons as they can move freely under the influence of electric field. At room temperature there
are sufficient electrons in the CB and hence the semiconductor is capable of conducting some current at
room temperature.
Inversely related to the conductivity of a material is its resistance to the flow of charge or
current. Typical resistivity values for various materials’ are given as follows.
Semiconductor Types
A pure form of semiconductors is called as intrinsic semiconductor. Conduction in
intrinsic sc is either due to thermal excitation or crystal defects. Si and Ge are the two most important
semiconductors used. Other examples include Gallium arsenide GaAs, Indium Antimonide (InSb) etc.
Let us consider the structure of Si. A Si atomic no. is 14 and it has 4 valance electrons. These 4
electrons are shared by four neighboring atoms in the crystal structure by means of covalent bond. Fig.
1.2a shows the crystal structure of Si at absolute zero temperature (0K). Hence a pure SC acts has poor
conductivity (due to lack of free electrons) at low or absolute zero temperature.
The absence of electrons in covalent bond is represented by a small circle usually referred to as
hole which is of positive charge. Even a hole serves as carrier of electricity in a manner similar to that
of free electron. In a pure semiconductor, the number of holes is equal to the number of free electrons.
EXTRINSIC SEMICONDUCTOR
Intrinsic semiconductor has very limited applications as they conduct very small amounts of
current at room temperature. The current conduction capability of intrinsic semiconductor can be
increased significantly by adding a small amounts impurity to the intrinsic semiconductor. By adding
impurities it becomes impure or extrinsic semiconductor. This process of adding impurities is called as
doping. The amount of impurity added is 1 part in 10
6
atoms.
N type semiconductor: If the added impurity is a pentavalent atom then the resultant semiconductor is
called N-type semiconductor. Examples of pentavalent impurities are Phosphorus, Arsenic, Bismuth,
Antimony etc.
P type semiconductor: If the added impurity is a trivalent atom then the resultant semiconductor is
called P-type semiconductor. Examples of trivalent impurities are Boron, Gallium , indium etc. Thus in
P type sc , holes are majority carriers and electrons are minority carriers. Since each trivalent impurity
atoms are capable accepting an electron, these are called as acceptor atoms. The following fig 1.5b
shows the pictorial representation of P type sc
? The conductivity of N type sc is greater than that of P type sc as the mobility of electron is
greater than that of hole.
? For the same level of doping in N type sc and P type sc, the conductivity of an N type sc
is around twice that of a P type sc.
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 cannot 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.
Page 5
o
MODULE-I
DIODE CIRCUITS
P-N junction diode, I-V characteristics of a diode; review of half-wave and full-wave rectifiers, clamping
and clipping circuits. Input output characteristics of BJT in CB, CE, CC configurations, biasing circuits,
Load line analysis, common emitter, common base and common collector amplifiers; Small signal
equivalent circuits.
INTRODUCTON
Based on the electrical conductivity all the materials in nature are classified as insulators,
semiconductors, and conductors.
Insulator: An insulator is a material that offers a very low level (or negligible) of conductivity when
voltage is applied. Eg: Paper, Mica, glass, quartz. Typical resistivity level of an insulator is of the order
of 10
10
to 10
12
?-cm. The energy band structure of an insulator is shown in the fig.1.1. Band structure of
a material defines the band of energy levels that an electron can occupy. Valance band is the range of
electron energy where the electron remain bended too the atom and do not contribute to the electric
current. Conduction bend is the range of electron energies higher than valance band where electrons are
free to accelerate under the influence of external voltage source resulting in the flow of charge.
The energy band between the valance band and conduction band is called as forbidden band gap.
It is the energy required by an electron to move from balance band to conduction band i.e. the energy
required for a valance electron to become a free electron.
1 eV = 1.6 x 10
-19
J
For an insulator, as shown in the fig.1.1 there is a large forbidden band gap of greater than 5Ev. Because
of this large gap there a very few electrons in the CB and hence the conductivity of insulator is poor.
Even an increase in temperature or applied electric field is insufficient to transfer electrons from VB to
CB.
Insulator Semiconductor Conductor
FiG:1.1 Energy band diagrams insulator, semiconductor and conductor
VB
Forbidden band
gap Eo ˜6eV
CB
VB
Eo =˜6eV
CB
CB
VB
Conductors: A conductor is a material which supports a generous flow of charge when a voltage is
applied across its terminals. i.e. it has very high conductivity. Eg: Copper, Aluminum, Silver, Gold. The
resistivity of a conductor is in the order of 10
-4
and 10
-6
?-cm. The Valance and conduction bands
overlap (fig1.1) and there is no energy gap for the electrons to move from valance band to conduction
band. This implies that there are free electrons in CB even at absolute zero temperature (0K). Therefore
at room temperature when electric field is applied large current flows through the conductor.
Semiconductor: A semiconductor is a material that has its conductivity somewhere between the
insulator and conductor. The resistivity level is in the range of 10 and 10
4
?-cm. Two of the most
commonly used are Silicon (Si=14 atomic no.) and germanium (Ge=32 atomic no.). Both have 4 valance
electrons. The forbidden band gap is in the order of 1eV. For eg., the band gap energy for Si, Ge and
GaAs is 1.21, 0.785 and 1.42 eV, respectively at absolute zero temperature (0K). At 0K and at low
temperatures, the valance band electrons do not have sufficient energy to move from V to CB. Thus
semiconductors act a insulators at 0K. as the temperature increases, a large number of valance electrons
acquire sufficient energy to leave the VB, cross the forbidden bandgap and reach CB. These are now
free electrons as they can move freely under the influence of electric field. At room temperature there
are sufficient electrons in the CB and hence the semiconductor is capable of conducting some current at
room temperature.
Inversely related to the conductivity of a material is its resistance to the flow of charge or
current. Typical resistivity values for various materials’ are given as follows.
Semiconductor Types
A pure form of semiconductors is called as intrinsic semiconductor. Conduction in
intrinsic sc is either due to thermal excitation or crystal defects. Si and Ge are the two most important
semiconductors used. Other examples include Gallium arsenide GaAs, Indium Antimonide (InSb) etc.
Let us consider the structure of Si. A Si atomic no. is 14 and it has 4 valance electrons. These 4
electrons are shared by four neighboring atoms in the crystal structure by means of covalent bond. Fig.
1.2a shows the crystal structure of Si at absolute zero temperature (0K). Hence a pure SC acts has poor
conductivity (due to lack of free electrons) at low or absolute zero temperature.
The absence of electrons in covalent bond is represented by a small circle usually referred to as
hole which is of positive charge. Even a hole serves as carrier of electricity in a manner similar to that
of free electron. In a pure semiconductor, the number of holes is equal to the number of free electrons.
EXTRINSIC SEMICONDUCTOR
Intrinsic semiconductor has very limited applications as they conduct very small amounts of
current at room temperature. The current conduction capability of intrinsic semiconductor can be
increased significantly by adding a small amounts impurity to the intrinsic semiconductor. By adding
impurities it becomes impure or extrinsic semiconductor. This process of adding impurities is called as
doping. The amount of impurity added is 1 part in 10
6
atoms.
N type semiconductor: If the added impurity is a pentavalent atom then the resultant semiconductor is
called N-type semiconductor. Examples of pentavalent impurities are Phosphorus, Arsenic, Bismuth,
Antimony etc.
P type semiconductor: If the added impurity is a trivalent atom then the resultant semiconductor is
called P-type semiconductor. Examples of trivalent impurities are Boron, Gallium , indium etc. Thus in
P type sc , holes are majority carriers and electrons are minority carriers. Since each trivalent impurity
atoms are capable accepting an electron, these are called as acceptor atoms. The following fig 1.5b
shows the pictorial representation of P type sc
? The conductivity of N type sc is greater than that of P type sc as the mobility of electron is
greater than that of hole.
? For the same level of doping in N type sc and P type sc, the conductivity of an N type sc
is around twice that of a P type sc.
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 cannot 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.
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
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