PPT: Solids & Semiconductor Devices

 Page 1


SOLIDS AND SEMICONDUCTOR DEVICES - I
1. Energy Bands in Solids
2. Energy Band Diagram
3. Metals, Semiconductors and Insulators
4. Intrinsic Semiconductor
5. Electrons and Holes
6. Doping of a Semiconductor
7. Extrinsic Semiconductor
8. N-type and P-type Semiconductor
9. Carrier Concentration in Semiconductors
10.Distinction between Intrinsic and Extrinsic Semiconductors
11.Distinction between Semiconductor and Metal
12.Conductivity of a Semiconductor
Page 2


SOLIDS AND SEMICONDUCTOR DEVICES - I
1. Energy Bands in Solids
2. Energy Band Diagram
3. Metals, Semiconductors and Insulators
4. Intrinsic Semiconductor
5. Electrons and Holes
6. Doping of a Semiconductor
7. Extrinsic Semiconductor
8. N-type and P-type Semiconductor
9. Carrier Concentration in Semiconductors
10.Distinction between Intrinsic and Extrinsic Semiconductors
11.Distinction between Semiconductor and Metal
12.Conductivity of a Semiconductor
Energy Bands in Solids:
? According to Quantum Mechanical Laws, the energies of electrons in a 
free  atom can not have arbitrary values but only some definite 
(quantized) values.
? However, if an atom belongs to a crystal, then the energy levels are 
modified.
? This modification is not appreciable in the case of energy levels of 
electrons in the inner shells (completely filled).
? But in the outermost shells, modification is appreciable because the 
electrons are shared by many neighbouring atoms.
? Due to influence of high electric field between the core of the atoms and 
the shared electrons, energy levels are split-up or spread out forming 
energy bands.
Consider a single crystal of silicon having N atoms.  Each atom can be 
associated with a lattice site.
Electronic configuration of Si is 1s
2
, 2s
2
, 2p
6
,3s
2
, 3p
2
.  (Atomic No. is 14)
Page 3


SOLIDS AND SEMICONDUCTOR DEVICES - I
1. Energy Bands in Solids
2. Energy Band Diagram
3. Metals, Semiconductors and Insulators
4. Intrinsic Semiconductor
5. Electrons and Holes
6. Doping of a Semiconductor
7. Extrinsic Semiconductor
8. N-type and P-type Semiconductor
9. Carrier Concentration in Semiconductors
10.Distinction between Intrinsic and Extrinsic Semiconductors
11.Distinction between Semiconductor and Metal
12.Conductivity of a Semiconductor
Energy Bands in Solids:
? According to Quantum Mechanical Laws, the energies of electrons in a 
free  atom can not have arbitrary values but only some definite 
(quantized) values.
? However, if an atom belongs to a crystal, then the energy levels are 
modified.
? This modification is not appreciable in the case of energy levels of 
electrons in the inner shells (completely filled).
? But in the outermost shells, modification is appreciable because the 
electrons are shared by many neighbouring atoms.
? Due to influence of high electric field between the core of the atoms and 
the shared electrons, energy levels are split-up or spread out forming 
energy bands.
Consider a single crystal of silicon having N atoms.  Each atom can be 
associated with a lattice site.
Electronic configuration of Si is 1s
2
, 2s
2
, 2p
6
,3s
2
, 3p
2
.  (Atomic No. is 14)
O
• •
• •
• • • • • •
• •
• •
1s
2
2s
2
2p
6
3p
2
3s
2
Inter atomic spacing   (r)
Energy
a b c
d
Conduction Band
Valence Band
Forbidden Energy Gap
Ion 
core 
state
Formation of Energy Bands in Solids:
Page 4


SOLIDS AND SEMICONDUCTOR DEVICES - I
1. Energy Bands in Solids
2. Energy Band Diagram
3. Metals, Semiconductors and Insulators
4. Intrinsic Semiconductor
5. Electrons and Holes
6. Doping of a Semiconductor
7. Extrinsic Semiconductor
8. N-type and P-type Semiconductor
9. Carrier Concentration in Semiconductors
10.Distinction between Intrinsic and Extrinsic Semiconductors
11.Distinction between Semiconductor and Metal
12.Conductivity of a Semiconductor
Energy Bands in Solids:
? According to Quantum Mechanical Laws, the energies of electrons in a 
free  atom can not have arbitrary values but only some definite 
(quantized) values.
? However, if an atom belongs to a crystal, then the energy levels are 
modified.
? This modification is not appreciable in the case of energy levels of 
electrons in the inner shells (completely filled).
? But in the outermost shells, modification is appreciable because the 
electrons are shared by many neighbouring atoms.
? Due to influence of high electric field between the core of the atoms and 
the shared electrons, energy levels are split-up or spread out forming 
energy bands.
Consider a single crystal of silicon having N atoms.  Each atom can be 
associated with a lattice site.
Electronic configuration of Si is 1s
2
, 2s
2
, 2p
6
,3s
2
, 3p
2
.  (Atomic No. is 14)
O
• •
• •
• • • • • •
• •
• •
1s
2
2s
2
2p
6
3p
2
3s
2
Inter atomic spacing   (r)
Energy
a b c
d
Conduction Band
Valence Band
Forbidden Energy Gap
Ion 
core 
state
Formation of Energy Bands in Solids:
Each of N atoms has its own energy levels.  The energy levels are identical, 
sharp, discrete and distinct.
The outer two sub-shells (3s and 3p of M shell or n = 3 shell) of silicon atom 
contain two s electrons and two p electrons.  So, there are 2N electrons 
completely filling 2N possible s levels, all of which are at the same energy.
Of the 6N possible p levels, only 2N are filled and all the filled p levels have 
the same energy.
(ii)  Oc < r < Od:
There is no visible splitting of energy levels but there develops a tendency 
for the splitting of energy levels.
(iii) r = Oc:
The interaction between the outermost shell electrons of neighbouring 
silicon atoms becomes appreciable and the splitting of the energy levels 
commences.
(i) r = Od (>> Oa):
(iv) Ob < r < Oc: 
The energy corresponding to the s and p levels of each atom gets slightly 
changed.  Corresponding to a single s level of an isolated atom, we get 2N 
levels.  Similarly, there are 6N levels for a single p level of an isolated atom.
Page 5


SOLIDS AND SEMICONDUCTOR DEVICES - I
1. Energy Bands in Solids
2. Energy Band Diagram
3. Metals, Semiconductors and Insulators
4. Intrinsic Semiconductor
5. Electrons and Holes
6. Doping of a Semiconductor
7. Extrinsic Semiconductor
8. N-type and P-type Semiconductor
9. Carrier Concentration in Semiconductors
10.Distinction between Intrinsic and Extrinsic Semiconductors
11.Distinction between Semiconductor and Metal
12.Conductivity of a Semiconductor
Energy Bands in Solids:
? According to Quantum Mechanical Laws, the energies of electrons in a 
free  atom can not have arbitrary values but only some definite 
(quantized) values.
? However, if an atom belongs to a crystal, then the energy levels are 
modified.
? This modification is not appreciable in the case of energy levels of 
electrons in the inner shells (completely filled).
? But in the outermost shells, modification is appreciable because the 
electrons are shared by many neighbouring atoms.
? Due to influence of high electric field between the core of the atoms and 
the shared electrons, energy levels are split-up or spread out forming 
energy bands.
Consider a single crystal of silicon having N atoms.  Each atom can be 
associated with a lattice site.
Electronic configuration of Si is 1s
2
, 2s
2
, 2p
6
,3s
2
, 3p
2
.  (Atomic No. is 14)
O
• •
• •
• • • • • •
• •
• •
1s
2
2s
2
2p
6
3p
2
3s
2
Inter atomic spacing   (r)
Energy
a b c
d
Conduction Band
Valence Band
Forbidden Energy Gap
Ion 
core 
state
Formation of Energy Bands in Solids:
Each of N atoms has its own energy levels.  The energy levels are identical, 
sharp, discrete and distinct.
The outer two sub-shells (3s and 3p of M shell or n = 3 shell) of silicon atom 
contain two s electrons and two p electrons.  So, there are 2N electrons 
completely filling 2N possible s levels, all of which are at the same energy.
Of the 6N possible p levels, only 2N are filled and all the filled p levels have 
the same energy.
(ii)  Oc < r < Od:
There is no visible splitting of energy levels but there develops a tendency 
for the splitting of energy levels.
(iii) r = Oc:
The interaction between the outermost shell electrons of neighbouring 
silicon atoms becomes appreciable and the splitting of the energy levels 
commences.
(i) r = Od (>> Oa):
(iv) Ob < r < Oc: 
The energy corresponding to the s and p levels of each atom gets slightly 
changed.  Corresponding to a single s level of an isolated atom, we get 2N 
levels.  Similarly, there are 6N levels for a single p level of an isolated atom.
Since N is a very large number (˜ 10
29 
atoms / m
3
) and the energy of each level 
is of a few eV, therefore, the levels due to the spreading are very closely 
spaced. The spacing is ˜ 10
-23
eV for a 1 cm
3
crystal.
The collection of very closely spaced energy levels is called an energy band.
(v) r = Ob:
The energy gap disappears completely.  8N levels are distributed 
continuously.  We can only say that 4N levels are filled and 4N levels are 
empty.
(v) r = Oa:
The band of 4N filled energy levels is separated from the band of 4N unfilled 
energy levels by an energy gap called forbidden gap or energy gap or   
band gap.
The lower completely filled band (with valence electrons) is called the 
valence band and the upper unfilled band is called the conduction band.
Note:
1. The exact energy band picture depends on the relative orientation of 
atoms in a crystal.
2. If the bands in a solid are completely filled, the electrons are not permitted 
to move about, because there are no vacant energy levels available.