Electronic Devices - Chapter Notes, Class 12 Physics PDF Download

 Page 1


On the basis of electrical conductivity ( ?) and resistivity ( ?), the solids are classified into three categories.
(1) Condutors : Those solids which have high conductivity and low resistivity even at low temperature.
(2) Insulators : Those solids which have very low conductivity and high resistivity even at very high
temperature.
(3) Semiconductors : Those solids which have low conductivity and high resistivity than conductors at
room temperature.
Energy Band Theory in Solids : According to Bohr's Model the electrons in an atom have definite descrete
energy levels. If large number of atoms are brought closer together to form a crystal they begin to influence
each other. Due to this interatomic interaction there is modification in the energy level of electrons in the outer
shells. Si confugration 1s
2
, 2s
2
, 2p
6
, 3s
2
, 3p
2
 / Ge 1s
2
, 2s
2
, 2p
6
, 3s
2
, 3p
6
, 4s
2
, 3d
10
 4p
2
. In Si and Ge four energy
levels as filled and four are empty.
(1) If the interatomic spacing of Si atom is [r = d] very large, there is no interatomic interaction. Each atom
has descrete energy levels.
(2) When the interatomic spacing (r = c) then interaction between outermost shell electrons [3s
2
 and 3p
2
]
with neighbouring silicon atoms begins. As a result the splitting of these energy levels starts and there is no
change in the energy levels of electrons of the inner shells.
(3) When interatomic spacing is (r = b) then the interaction becomes strong and spreading of energy levels
of 3s and 3p levels reduce the gap between them. So the energy levels change into energy band.
(4) When the mteratomic spacing becomes (r = a) then 4 N filled energy levels is separated from 4 N
unfilled energy levels by a gap called as energy gap (Eg). The lower completely filled band is called valence
band and the upper unfilled band is conduction band. The energy gap is small [< 3 eV].
Distinction between conductors, semiconductors and insulators on the basis of energy band theory.
Conductors :
(1) If C.B is empty then V.B should  be partially filled.
Eg
C.B
V .B
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Page 2


On the basis of electrical conductivity ( ?) and resistivity ( ?), the solids are classified into three categories.
(1) Condutors : Those solids which have high conductivity and low resistivity even at low temperature.
(2) Insulators : Those solids which have very low conductivity and high resistivity even at very high
temperature.
(3) Semiconductors : Those solids which have low conductivity and high resistivity than conductors at
room temperature.
Energy Band Theory in Solids : According to Bohr's Model the electrons in an atom have definite descrete
energy levels. If large number of atoms are brought closer together to form a crystal they begin to influence
each other. Due to this interatomic interaction there is modification in the energy level of electrons in the outer
shells. Si confugration 1s
2
, 2s
2
, 2p
6
, 3s
2
, 3p
2
 / Ge 1s
2
, 2s
2
, 2p
6
, 3s
2
, 3p
6
, 4s
2
, 3d
10
 4p
2
. In Si and Ge four energy
levels as filled and four are empty.
(1) If the interatomic spacing of Si atom is [r = d] very large, there is no interatomic interaction. Each atom
has descrete energy levels.
(2) When the interatomic spacing (r = c) then interaction between outermost shell electrons [3s
2
 and 3p
2
]
with neighbouring silicon atoms begins. As a result the splitting of these energy levels starts and there is no
change in the energy levels of electrons of the inner shells.
(3) When interatomic spacing is (r = b) then the interaction becomes strong and spreading of energy levels
of 3s and 3p levels reduce the gap between them. So the energy levels change into energy band.
(4) When the mteratomic spacing becomes (r = a) then 4 N filled energy levels is separated from 4 N
unfilled energy levels by a gap called as energy gap (Eg). The lower completely filled band is called valence
band and the upper unfilled band is conduction band. The energy gap is small [< 3 eV].
Distinction between conductors, semiconductors and insulators on the basis of energy band theory.
Conductors :
(1) If C.B is empty then V.B should  be partially filled.
Eg
C.B
V .B
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(2) If V.B is filled then C.B should be partially filled.           
Eg
C.B
V .B
(3) The C.B and V.B overlap and there is no energy gap.
V .B
C.B
Insulators : V.B is completely filled and C.B is empty. The energy gap is large > 3eV. Due to large energy gap
electron are not able to go from V.B. to C.B. even if it is placed at high temperature. Hence electrical conduction
is these materials is impossible.
Semiconductors : Valence band is totally filled and conduction band is empty. But energy gap between C.B
and V .B. is small. The energy gap for Si is 1.1 eV and for Ge is 0.72 eV. At low temperature electrons are not
able to cross this small energy gap and C.B. remains completely empty. Thus a semiconductor behave as
insulator. However at room temperature some electrons in the V.B. acquire thermal energy greater than
energy gap and jump over to the C.B. where they are free to move under the influence of a small electric field.
As a result semiconductors acquire small conductivity.
Intrinsic Semiconductors : A pure semiconductor which is free of any impurity is called intrinsic semiconductor.
Ge and Si are the examples of intrinsic semiconductor.
Bond Model : Both these atoms have four valence electrons. The four valence electrons form four covalent
bonds by sharing the electrons with neighbouring four Ge atoms. Thus no free electron exist at low temperature
to conduct electricity.
At room temperature some of the electrons break away from covalent bond. The empty space or vacancy left
is called hole. When an external electric field is applied these free electrons and holes move in opposite
directions and constitute a current flow through intrinsic semiconductor.
Here ne = nh = ni
where ni is called intrinsic carrier concentration.
Page 3


On the basis of electrical conductivity ( ?) and resistivity ( ?), the solids are classified into three categories.
(1) Condutors : Those solids which have high conductivity and low resistivity even at low temperature.
(2) Insulators : Those solids which have very low conductivity and high resistivity even at very high
temperature.
(3) Semiconductors : Those solids which have low conductivity and high resistivity than conductors at
room temperature.
Energy Band Theory in Solids : According to Bohr's Model the electrons in an atom have definite descrete
energy levels. If large number of atoms are brought closer together to form a crystal they begin to influence
each other. Due to this interatomic interaction there is modification in the energy level of electrons in the outer
shells. Si confugration 1s
2
, 2s
2
, 2p
6
, 3s
2
, 3p
2
 / Ge 1s
2
, 2s
2
, 2p
6
, 3s
2
, 3p
6
, 4s
2
, 3d
10
 4p
2
. In Si and Ge four energy
levels as filled and four are empty.
(1) If the interatomic spacing of Si atom is [r = d] very large, there is no interatomic interaction. Each atom
has descrete energy levels.
(2) When the interatomic spacing (r = c) then interaction between outermost shell electrons [3s
2
 and 3p
2
]
with neighbouring silicon atoms begins. As a result the splitting of these energy levels starts and there is no
change in the energy levels of electrons of the inner shells.
(3) When interatomic spacing is (r = b) then the interaction becomes strong and spreading of energy levels
of 3s and 3p levels reduce the gap between them. So the energy levels change into energy band.
(4) When the mteratomic spacing becomes (r = a) then 4 N filled energy levels is separated from 4 N
unfilled energy levels by a gap called as energy gap (Eg). The lower completely filled band is called valence
band and the upper unfilled band is conduction band. The energy gap is small [< 3 eV].
Distinction between conductors, semiconductors and insulators on the basis of energy band theory.
Conductors :
(1) If C.B is empty then V.B should  be partially filled.
Eg
C.B
V .B
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(2) If V.B is filled then C.B should be partially filled.           
Eg
C.B
V .B
(3) The C.B and V.B overlap and there is no energy gap.
V .B
C.B
Insulators : V.B is completely filled and C.B is empty. The energy gap is large > 3eV. Due to large energy gap
electron are not able to go from V.B. to C.B. even if it is placed at high temperature. Hence electrical conduction
is these materials is impossible.
Semiconductors : Valence band is totally filled and conduction band is empty. But energy gap between C.B
and V .B. is small. The energy gap for Si is 1.1 eV and for Ge is 0.72 eV. At low temperature electrons are not
able to cross this small energy gap and C.B. remains completely empty. Thus a semiconductor behave as
insulator. However at room temperature some electrons in the V.B. acquire thermal energy greater than
energy gap and jump over to the C.B. where they are free to move under the influence of a small electric field.
As a result semiconductors acquire small conductivity.
Intrinsic Semiconductors : A pure semiconductor which is free of any impurity is called intrinsic semiconductor.
Ge and Si are the examples of intrinsic semiconductor.
Bond Model : Both these atoms have four valence electrons. The four valence electrons form four covalent
bonds by sharing the electrons with neighbouring four Ge atoms. Thus no free electron exist at low temperature
to conduct electricity.
At room temperature some of the electrons break away from covalent bond. The empty space or vacancy left
is called hole. When an external electric field is applied these free electrons and holes move in opposite
directions and constitute a current flow through intrinsic semiconductor.
Here ne = nh = ni
where ni is called intrinsic carrier concentration.
3 / 2015
Energy bond theory : The eneregy gap is 1eV in intrinsic semiconductor. At low temperature the semiconductor
behaves as an insulator and no electron from V.B. can cross this energy gap to go to C.B.
At higher temperature some of the electrons gain energy due to thermal
energy and move from V.B. to C.B. As a result holes are created
in the V.B. Since the absence of negatively charged electron is
equivalent to presence of equivalent amount of positive charge              
therefore hole is considered as a positively charged particle having
charge equal to that of electron. A hole is considered active particle
in  V .B. and an electron in C.B. The motion of electrons in C.B.
and also the motion of holes in V.B. is responsible for electrical
conduction in semiconductors at room temperature.
 I = I
e
 + I
h
Doping : It is the process of adding impurities to pure semiconductor to modify its properties in controlled
manner. The impurity atoms are called dopants. The impurity added is 1 part per million. The dopant atoms take
the position in semiconductor at lattice points.
Methods of Doping :
(a) by adding the impurity atoms to the melted semiconductors.
(b) by heating the semiconductor in the atmosphere containing dopant atoms so that they diffuse into
semiconductor.
Extrinsic Semiconductors :  A doped semiconductor with suitable impurities atoms added to it is called
extrinsic semiconductor. They are of two types.
(i)   n-type (ii)    p-type
(i) n-type Semiconductor (Bond Model) : When a pure semiconductor of Si and  Ge which has four
valence electrons is doped with a controlled amount of pentavalent atoms (As, Sb, Bi) which have five valence
electrons then the impurity atoms will replace Si or Ge atom at its lattice point.
The four of the five valence electrons of the impurity atom will form four     
covalent bonds by sharing the electrons with adjoining four Si atoms while
the fifth electrons is left free. Thus each impurity atom added donates one
free electron to the crystal structure. There impurity atoms which donate
free electrons for conduction are called donor atoms. Since the conduction
is due to negatively charge electrons therefore this semiconductor is called
n-type. This semiconductor now conducts at low temperature. At room temperature, some of the covalent
bonds break producing free electrons and an equal no. of holes. Thus electrons are the majority carries and
holes are minority carries in n-type semiconductor.
Energy Bond theory of n-type : For a Si semiconductor with impurity
atoms of As or P, the energy level of the free electrons of impurity atoms             
are slightly less than the energy level of C.B. of Si atoms. This is called
donor energy level which lies at 0.01 eV below the bottom of C.B.
Page 4


On the basis of electrical conductivity ( ?) and resistivity ( ?), the solids are classified into three categories.
(1) Condutors : Those solids which have high conductivity and low resistivity even at low temperature.
(2) Insulators : Those solids which have very low conductivity and high resistivity even at very high
temperature.
(3) Semiconductors : Those solids which have low conductivity and high resistivity than conductors at
room temperature.
Energy Band Theory in Solids : According to Bohr's Model the electrons in an atom have definite descrete
energy levels. If large number of atoms are brought closer together to form a crystal they begin to influence
each other. Due to this interatomic interaction there is modification in the energy level of electrons in the outer
shells. Si confugration 1s
2
, 2s
2
, 2p
6
, 3s
2
, 3p
2
 / Ge 1s
2
, 2s
2
, 2p
6
, 3s
2
, 3p
6
, 4s
2
, 3d
10
 4p
2
. In Si and Ge four energy
levels as filled and four are empty.
(1) If the interatomic spacing of Si atom is [r = d] very large, there is no interatomic interaction. Each atom
has descrete energy levels.
(2) When the interatomic spacing (r = c) then interaction between outermost shell electrons [3s
2
 and 3p
2
]
with neighbouring silicon atoms begins. As a result the splitting of these energy levels starts and there is no
change in the energy levels of electrons of the inner shells.
(3) When interatomic spacing is (r = b) then the interaction becomes strong and spreading of energy levels
of 3s and 3p levels reduce the gap between them. So the energy levels change into energy band.
(4) When the mteratomic spacing becomes (r = a) then 4 N filled energy levels is separated from 4 N
unfilled energy levels by a gap called as energy gap (Eg). The lower completely filled band is called valence
band and the upper unfilled band is conduction band. The energy gap is small [< 3 eV].
Distinction between conductors, semiconductors and insulators on the basis of energy band theory.
Conductors :
(1) If C.B is empty then V.B should  be partially filled.
Eg
C.B
V .B
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(2) If V.B is filled then C.B should be partially filled.           
Eg
C.B
V .B
(3) The C.B and V.B overlap and there is no energy gap.
V .B
C.B
Insulators : V.B is completely filled and C.B is empty. The energy gap is large > 3eV. Due to large energy gap
electron are not able to go from V.B. to C.B. even if it is placed at high temperature. Hence electrical conduction
is these materials is impossible.
Semiconductors : Valence band is totally filled and conduction band is empty. But energy gap between C.B
and V .B. is small. The energy gap for Si is 1.1 eV and for Ge is 0.72 eV. At low temperature electrons are not
able to cross this small energy gap and C.B. remains completely empty. Thus a semiconductor behave as
insulator. However at room temperature some electrons in the V.B. acquire thermal energy greater than
energy gap and jump over to the C.B. where they are free to move under the influence of a small electric field.
As a result semiconductors acquire small conductivity.
Intrinsic Semiconductors : A pure semiconductor which is free of any impurity is called intrinsic semiconductor.
Ge and Si are the examples of intrinsic semiconductor.
Bond Model : Both these atoms have four valence electrons. The four valence electrons form four covalent
bonds by sharing the electrons with neighbouring four Ge atoms. Thus no free electron exist at low temperature
to conduct electricity.
At room temperature some of the electrons break away from covalent bond. The empty space or vacancy left
is called hole. When an external electric field is applied these free electrons and holes move in opposite
directions and constitute a current flow through intrinsic semiconductor.
Here ne = nh = ni
where ni is called intrinsic carrier concentration.
3 / 2015
Energy bond theory : The eneregy gap is 1eV in intrinsic semiconductor. At low temperature the semiconductor
behaves as an insulator and no electron from V.B. can cross this energy gap to go to C.B.
At higher temperature some of the electrons gain energy due to thermal
energy and move from V.B. to C.B. As a result holes are created
in the V.B. Since the absence of negatively charged electron is
equivalent to presence of equivalent amount of positive charge              
therefore hole is considered as a positively charged particle having
charge equal to that of electron. A hole is considered active particle
in  V .B. and an electron in C.B. The motion of electrons in C.B.
and also the motion of holes in V.B. is responsible for electrical
conduction in semiconductors at room temperature.
 I = I
e
 + I
h
Doping : It is the process of adding impurities to pure semiconductor to modify its properties in controlled
manner. The impurity atoms are called dopants. The impurity added is 1 part per million. The dopant atoms take
the position in semiconductor at lattice points.
Methods of Doping :
(a) by adding the impurity atoms to the melted semiconductors.
(b) by heating the semiconductor in the atmosphere containing dopant atoms so that they diffuse into
semiconductor.
Extrinsic Semiconductors :  A doped semiconductor with suitable impurities atoms added to it is called
extrinsic semiconductor. They are of two types.
(i)   n-type (ii)    p-type
(i) n-type Semiconductor (Bond Model) : When a pure semiconductor of Si and  Ge which has four
valence electrons is doped with a controlled amount of pentavalent atoms (As, Sb, Bi) which have five valence
electrons then the impurity atoms will replace Si or Ge atom at its lattice point.
The four of the five valence electrons of the impurity atom will form four     
covalent bonds by sharing the electrons with adjoining four Si atoms while
the fifth electrons is left free. Thus each impurity atom added donates one
free electron to the crystal structure. There impurity atoms which donate
free electrons for conduction are called donor atoms. Since the conduction
is due to negatively charge electrons therefore this semiconductor is called
n-type. This semiconductor now conducts at low temperature. At room temperature, some of the covalent
bonds break producing free electrons and an equal no. of holes. Thus electrons are the majority carries and
holes are minority carries in n-type semiconductor.
Energy Bond theory of n-type : For a Si semiconductor with impurity
atoms of As or P, the energy level of the free electrons of impurity atoms             
are slightly less than the energy level of C.B. of Si atoms. This is called
donor energy level which lies at 0.01 eV below the bottom of C.B.
4 / 2015
Thus a very small energy supplied can excite the electrons from donor level to C.B. and conductivity starts at
low temperature. At room temperature, more electrons jump to the C.B. so that there is majority of electrons
in C.B. and minority of holes in V.B. On applying electric field the conduction is
I = I
e
 + I
h
.
Energy Bond theory of p-type : For a Ge or Si semiconductor, the doping of impurity atoms of In or B will
have energy level slightly above the valence band of Si or Ge. These are called
acceptor levels. Which has a small diff. of 0.01 eV from V.B. The electrons
from V.B. move to the acceptor level at low temperature creating
holes in  V.B. Thus conductivity is possible at low temperature.
At room temperature the electrons from V.B. are transferred to C.B.             
due to thermal energy. This produces a large no. of holes in V.B.
When external electric field is applied then minority of
electrons in C.B. and majority of holes in the V.B. conduct electricity.
I = I
e
 + I
h
p-type Semiconductor (Bond Model) : When a pure semiconductor of Ge or Si in which each atom has
four valence electrons is doped with a controlled amount of trivalent atoms say (Ga, In, or B) which have three
valence electrons, the impurity atom will replace the Ge and Si atoms.
The three valence electrons of impurity atom form covalent bonds by
sharing the electrons with the adjoining Si atoms, while there will be
one incomplete covalent bond due to deficiency of electron with the             
neighbouring Si atom. This deficiency creates a hole. The electrons
move to fill up the holes and new holes are formed in opposite direction.
The conduction of electricity takes place due to holes at low temperature.
At room temperature some of covalent bonds break, releasing equal no. of holes and electrons. These holes are
the majority carriers and electrons are minority carries in p-type.
Intrinsic Extrinsic
(1) It is pure semiconductor . (1) It is impure semiconductor.
(2) Ex. Si and Ge (2) Ex. Si and Ge doped with pentavalent or
trivalent atoms.
(3) ne in C.B. = nh in V.B. (3) No. of electron and holes is never equal. There
is excess of electrons in n-type and excess of
holes in p-type.
(4) Does not conduct at low temperature (4) It conduct at low temperature.
(5) Conductivity is low. (5) Conductivity is high.
n-type p-type
(1) It is obtained by doping pentavalent (1) It is obtained by doping trivalent atoms to
atoms to Si or Ge crystal. Si or Ge crystal.
(2) The impurity atoms added provide (2) The impurity atoms added provide extra
extra electrons and are called holes and are called acceptor atoms.
donor atoms.
(3) The electron density is greater than (3) The hole density is greater than
hole density ne >>> nh. electron density nh >>> ne.
Page 5


On the basis of electrical conductivity ( ?) and resistivity ( ?), the solids are classified into three categories.
(1) Condutors : Those solids which have high conductivity and low resistivity even at low temperature.
(2) Insulators : Those solids which have very low conductivity and high resistivity even at very high
temperature.
(3) Semiconductors : Those solids which have low conductivity and high resistivity than conductors at
room temperature.
Energy Band Theory in Solids : According to Bohr's Model the electrons in an atom have definite descrete
energy levels. If large number of atoms are brought closer together to form a crystal they begin to influence
each other. Due to this interatomic interaction there is modification in the energy level of electrons in the outer
shells. Si confugration 1s
2
, 2s
2
, 2p
6
, 3s
2
, 3p
2
 / Ge 1s
2
, 2s
2
, 2p
6
, 3s
2
, 3p
6
, 4s
2
, 3d
10
 4p
2
. In Si and Ge four energy
levels as filled and four are empty.
(1) If the interatomic spacing of Si atom is [r = d] very large, there is no interatomic interaction. Each atom
has descrete energy levels.
(2) When the interatomic spacing (r = c) then interaction between outermost shell electrons [3s
2
 and 3p
2
]
with neighbouring silicon atoms begins. As a result the splitting of these energy levels starts and there is no
change in the energy levels of electrons of the inner shells.
(3) When interatomic spacing is (r = b) then the interaction becomes strong and spreading of energy levels
of 3s and 3p levels reduce the gap between them. So the energy levels change into energy band.
(4) When the mteratomic spacing becomes (r = a) then 4 N filled energy levels is separated from 4 N
unfilled energy levels by a gap called as energy gap (Eg). The lower completely filled band is called valence
band and the upper unfilled band is conduction band. The energy gap is small [< 3 eV].
Distinction between conductors, semiconductors and insulators on the basis of energy band theory.
Conductors :
(1) If C.B is empty then V.B should  be partially filled.
Eg
C.B
V .B
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(2) If V.B is filled then C.B should be partially filled.           
Eg
C.B
V .B
(3) The C.B and V.B overlap and there is no energy gap.
V .B
C.B
Insulators : V.B is completely filled and C.B is empty. The energy gap is large > 3eV. Due to large energy gap
electron are not able to go from V.B. to C.B. even if it is placed at high temperature. Hence electrical conduction
is these materials is impossible.
Semiconductors : Valence band is totally filled and conduction band is empty. But energy gap between C.B
and V .B. is small. The energy gap for Si is 1.1 eV and for Ge is 0.72 eV. At low temperature electrons are not
able to cross this small energy gap and C.B. remains completely empty. Thus a semiconductor behave as
insulator. However at room temperature some electrons in the V.B. acquire thermal energy greater than
energy gap and jump over to the C.B. where they are free to move under the influence of a small electric field.
As a result semiconductors acquire small conductivity.
Intrinsic Semiconductors : A pure semiconductor which is free of any impurity is called intrinsic semiconductor.
Ge and Si are the examples of intrinsic semiconductor.
Bond Model : Both these atoms have four valence electrons. The four valence electrons form four covalent
bonds by sharing the electrons with neighbouring four Ge atoms. Thus no free electron exist at low temperature
to conduct electricity.
At room temperature some of the electrons break away from covalent bond. The empty space or vacancy left
is called hole. When an external electric field is applied these free electrons and holes move in opposite
directions and constitute a current flow through intrinsic semiconductor.
Here ne = nh = ni
where ni is called intrinsic carrier concentration.
3 / 2015
Energy bond theory : The eneregy gap is 1eV in intrinsic semiconductor. At low temperature the semiconductor
behaves as an insulator and no electron from V.B. can cross this energy gap to go to C.B.
At higher temperature some of the electrons gain energy due to thermal
energy and move from V.B. to C.B. As a result holes are created
in the V.B. Since the absence of negatively charged electron is
equivalent to presence of equivalent amount of positive charge              
therefore hole is considered as a positively charged particle having
charge equal to that of electron. A hole is considered active particle
in  V .B. and an electron in C.B. The motion of electrons in C.B.
and also the motion of holes in V.B. is responsible for electrical
conduction in semiconductors at room temperature.
 I = I
e
 + I
h
Doping : It is the process of adding impurities to pure semiconductor to modify its properties in controlled
manner. The impurity atoms are called dopants. The impurity added is 1 part per million. The dopant atoms take
the position in semiconductor at lattice points.
Methods of Doping :
(a) by adding the impurity atoms to the melted semiconductors.
(b) by heating the semiconductor in the atmosphere containing dopant atoms so that they diffuse into
semiconductor.
Extrinsic Semiconductors :  A doped semiconductor with suitable impurities atoms added to it is called
extrinsic semiconductor. They are of two types.
(i)   n-type (ii)    p-type
(i) n-type Semiconductor (Bond Model) : When a pure semiconductor of Si and  Ge which has four
valence electrons is doped with a controlled amount of pentavalent atoms (As, Sb, Bi) which have five valence
electrons then the impurity atoms will replace Si or Ge atom at its lattice point.
The four of the five valence electrons of the impurity atom will form four     
covalent bonds by sharing the electrons with adjoining four Si atoms while
the fifth electrons is left free. Thus each impurity atom added donates one
free electron to the crystal structure. There impurity atoms which donate
free electrons for conduction are called donor atoms. Since the conduction
is due to negatively charge electrons therefore this semiconductor is called
n-type. This semiconductor now conducts at low temperature. At room temperature, some of the covalent
bonds break producing free electrons and an equal no. of holes. Thus electrons are the majority carries and
holes are minority carries in n-type semiconductor.
Energy Bond theory of n-type : For a Si semiconductor with impurity
atoms of As or P, the energy level of the free electrons of impurity atoms             
are slightly less than the energy level of C.B. of Si atoms. This is called
donor energy level which lies at 0.01 eV below the bottom of C.B.
4 / 2015
Thus a very small energy supplied can excite the electrons from donor level to C.B. and conductivity starts at
low temperature. At room temperature, more electrons jump to the C.B. so that there is majority of electrons
in C.B. and minority of holes in V.B. On applying electric field the conduction is
I = I
e
 + I
h
.
Energy Bond theory of p-type : For a Ge or Si semiconductor, the doping of impurity atoms of In or B will
have energy level slightly above the valence band of Si or Ge. These are called
acceptor levels. Which has a small diff. of 0.01 eV from V.B. The electrons
from V.B. move to the acceptor level at low temperature creating
holes in  V.B. Thus conductivity is possible at low temperature.
At room temperature the electrons from V.B. are transferred to C.B.             
due to thermal energy. This produces a large no. of holes in V.B.
When external electric field is applied then minority of
electrons in C.B. and majority of holes in the V.B. conduct electricity.
I = I
e
 + I
h
p-type Semiconductor (Bond Model) : When a pure semiconductor of Ge or Si in which each atom has
four valence electrons is doped with a controlled amount of trivalent atoms say (Ga, In, or B) which have three
valence electrons, the impurity atom will replace the Ge and Si atoms.
The three valence electrons of impurity atom form covalent bonds by
sharing the electrons with the adjoining Si atoms, while there will be
one incomplete covalent bond due to deficiency of electron with the             
neighbouring Si atom. This deficiency creates a hole. The electrons
move to fill up the holes and new holes are formed in opposite direction.
The conduction of electricity takes place due to holes at low temperature.
At room temperature some of covalent bonds break, releasing equal no. of holes and electrons. These holes are
the majority carriers and electrons are minority carries in p-type.
Intrinsic Extrinsic
(1) It is pure semiconductor . (1) It is impure semiconductor.
(2) Ex. Si and Ge (2) Ex. Si and Ge doped with pentavalent or
trivalent atoms.
(3) ne in C.B. = nh in V.B. (3) No. of electron and holes is never equal. There
is excess of electrons in n-type and excess of
holes in p-type.
(4) Does not conduct at low temperature (4) It conduct at low temperature.
(5) Conductivity is low. (5) Conductivity is high.
n-type p-type
(1) It is obtained by doping pentavalent (1) It is obtained by doping trivalent atoms to
atoms to Si or Ge crystal. Si or Ge crystal.
(2) The impurity atoms added provide (2) The impurity atoms added provide extra
extra electrons and are called holes and are called acceptor atoms.
donor atoms.
(3) The electron density is greater than (3) The hole density is greater than
hole density ne >>> nh. electron density nh >>> ne.
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(4) The donoar energy level is close (4) The acceptor energy level is close to V.B.
to C.B.
(5) Conductivity is more as drift speed (5) Conductivity is less as drift speed of holes
of electrons is more than holes. is less than that of electrons.
Electrical Conductivity : The potential difference applied to the semiconductor is V so that E = 
V
l
Due to electric field electrons and holes move in opposite directions with drift velocity ve and vh. So that
 I = I
e
 + I
h
I
e
= en
e
 Av
e
 + en
h
 Av
h
V
R
= eA [n
e
 v
e
 + n
h
 v
h
]
E
A
?
l
l
= eA [n
e
 v
e
 + n
h
 v
h
]
1
?
=
e e h h
[n v n v ]
eA
E
?
? = eA [n
e
 v
e
 + n
h
 v
h
]
Where v
e
 and v
h
 are the mobilities of electron and holes. Mobility is defined as drift velocity per unit electric
field.
P-N Junction : When a p-type semiconductor crystal is brought into close contact with n-type semiconductor
crystal then the resulting arrangement is called P-N junction diode.
Depletion Region and Potential Barrier :When a P-N diode is formed then due to difference in concentration
of charge carriers in the two regions, the electrons from N-region diffuse through the junction and fillup the
holes of P-region. Due to this diffusion electron leaves, behind a positively charged donor ion which is immobile
as bonded by surrounding atoms. Due to combination of electrons with hole there will be negatively charged
immobile ion in region and a small potential difference is devloped at the junction. As the process of diffusion
continue this potential difference increases and becomes maximum. This is called potential barriers V
B
 which
stops the diffusion of majority charge carriers. At 27°C, V
B
 is a bout 0.3 V for Ge and 0.7 V for Si. The small
region on both sides of P-N junction which has immobile ions and is free of any charge carriers is
called depletion region. The width of depletion layer is 10
–6
 m. Due to this potential barrier in a narrow width a
strong electric field is developed at the junction.
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6
V 0.7
E 7 10 V / m.
d 10
?
? ? ? ?