Detailed Notes: Diode Circuits | Analog Circuits - Electronics and Communication Engineering (ECE) PDF Download

 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