RCPIT, SHIRPUR
DEPARTMENT OF APPLIED SCIENCE
Experiment No.1
Objective: Band gap in semiconductor material
a) To determine forbidden energy gap of given semiconductor
b) Compare analytical and the practical values.
Theory:
1.1 Materials and concept of energy bands:
Materials can be categorized into conductors, semiconductors or insulators by their
ability to conduct electricity. Free electrons are responsible for the conduction of electricity.
Free electrons are those outermost electrons, which are most weakly bound with atoms.
Hence these electrons get separated from their atoms and move freely inside the entire
substance from one atom to another atom. These free electrons act as the charge carrier.
Insulators do not conduct electricity because their valence electrons are not free to wander
throughout the material. Metals conduct electricity easily because the energy levels between
the conduction and valence band are closely spaced or there are more energy levels
available, than there are electrons to fill them so very little energy is required to find new
energies for electrons to occupy. The band theory of materials explains qualitatively the
difference between these types of materials. Electrons occupy energy levels from the lowest
energies to upwards. However, some energy levels are forbidden. The allowed energy levels
tend to form bands. In metals, there is no forbidden gap; the conduction band and the
valence band overlap, allowing free electrons to participate in the conduction process.
Insulators have an energy gap that is far greater than the thermal energy of the electron,
while semiconductor materials the energy gap is typically around 1eV.
1.2 Energy bands:
In case of single isolated atoms the electrons in any orbit possess discrete energy
level. An atom in a solid is greatly influenced by the closely packed neighboring atoms.
Hence the electron in any orbit of such an atom can have a range of energies rather than a
single energy level which is known as energy band.
As more atoms come together more valence electron orbitals come to form
molecular orbitals. When two atoms come close together one energy level splits into two
energy levels. When three atoms come close together, the original level split into three
energy levels. More generally ‘N’ interacting atom cause an energy level to split up into ‘N’
energy levels.
The transformation of single energy level into two or more energy level is called
energy level splitting. The range of energies possessed by an electron in solid is called as
energy band. The energy band is responsible for the electrical, magnetic, optical properties
of solid.
1.2.1 Valence band:
The valence band is the lower band of allowed states and a highest range of electron
energy at absolute zero temperature. Since electrons have a tendency to fill the lowest
available energy states, the valence band is always nearly completely filled with electrons.
Electrons in the valence band do not participate in the conduction process.
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DEPARTMENT OF APPLIED SCIENCE
1.2.2 Conduction band:
The conduction band is the upper band of allowed states and is generally empty. In
reference to conductivity in semiconductors, it is the band that accepts the electrons from
the valence band. It is the range of electron energy, higher than that of valence band
sufficient to make the electrons free to accelerate under the influence of an applied electric
field and thus constitute an electric current. Semiconductors may cross this conduction band
when they are excited.
1.2.3 Energy band gap:
The energy gap between the top of the valence band and bottom of the conduction
band is known as energy band gap. The region between the two energy bands is known as
forbidden band gap. i.e. Eg = Ec– Ev
Solids having different values of forbidden gap on the basis of which they can be
classified into conductor, insulator and semiconductor. The width of the band gap decides
the nature of the solid and no electron can exist in forbidden band ( gap ) in case of the
germanium, the width of band gap is 0.7 eV and that for silicon in 1.1 eV.
1.3 Conductor:
In case of conductor, there is no forbidden band overlap to each other, shown in following
figure. The electrons from valence band are freely enter in the conduction band. The most
important point in conductors is that due to absence of forbidden band, there is no structure
to establish holes. There for conductor have large number of free electrons is available for
electric conduction.. The total current in conductor is simply a flow of electrons.
Example:- Cu, Na, Al, W, etc.
1.4 Insulator:
Insulators are those substances in which valence electrons are tightly bound to their
parent atoms. Due to this they required very large electric field to get removes from the
valence band. Insulator have valence band completely filled and conduction band is empty.
The band gap (Eg) is so large (~5 eV). In case of insulator, at room temperature no thermal
excitation of electrons from valence band to conduction band is possible. At high
temperatures some of the electrons in valence band acquire sufficient energy to overcome
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RCPIT, SHIRPUR
DEPARTMENT OF APPLIED SCIENCE
forbidden band and enter the conduction band. These electrons are now, the free electrons
and therefore this is reason why insulator shows conductivity with increased temperature.
1.5 Semi-conductor:
In case of semi-conductor, electrical properties lie in between those of insulator and
conductor. In these substance conduction band is separated from the valence band by a
comparatively small forbidden band (~ 1eV).
At absolute zero temperature the valence band is totally filled and the
conduction band is empty. Electrons are not able to cross the small forbidden gap at very
low temperature and a pure semiconductor behaves like an insulator at this temperature. The
semiconductor becomes conducting, as the temperature is raises, width of forbidden band
decreases and some of the valence electron are excited into conduction band. Current in
semi-conductor is due to both electrons and holes.
Procedure:
1.
2.
3.
4.
5.
6.
7.
Switch ‘Off’ the power switch from trainer board.
Switch the toggle switch of +6 V and +15 V power supply towards off condition.
Set potentiometer of +16 V towards anticlockwise position.
Connect the mains cord to trainer.
Short terminal 2 to 3 and 6 to 7 by using patch cords.
Connect DC ammeter between terminals 8 and 9 (+)ve and (–)ve respectively.
Connect DC voltmeter across the terminals 1 and 10 (+)ve and (–)ve respectively.
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RCPIT, SHIRPUR
DEPARTMENT OF APPLIED SCIENCE
8.
9.
10.
11.
12.
13.
Switch ‘On’ the power supply.
Select the toggle switch of (+) 16 V power supply towards on condition.
Use potentiometer of (+) 6 V power supply to set voltage across diode to 2 V.
Select the toggle switch of (+) 15 V power supply towards on condition.
Note down the initial reading of current in micro ampere.
Wait until temperature reaches up to 65 degree Celsius after that switch ‘Off’ +15 V
power supply.
Note: Don’t increase temperature more than 65 degree Celsius.
4. Let temperature reaches up to 65 degree Celsius, note corresponding reading of current.
15. Further temperature decreases, as how temperature decreases up to 60 degree Celsius,
note corresponding readings of current.
16. Take several readings of current, at the interval of 5 degree Celsius decrement.
17. Tabulate all retrieved data in below table and calculate other factors of the table.
Measure the energy band gap of semiconductor by using s/w Procedure:
1.
2.
3.
4.
5.
6.
Install the s/w on your computer.
Convert USB cable between USB port of trainer and PC.
Make connections same as above (from step 1 to step 10).
Detect the device on PC by using s/w.
Select the toggle switch of +15V power supply towards on condition.
Wait until temperature reaches up to 65-67 degree Celsius after that switch ‘Off’ the
+15 V power supply.
7. Now click on the “Take Reading” command button. So that temperate and
corresponding reading of current note down on table automatically.
8. Wait again until temperature reaches up to 30-35 degree Celsius after that click on the
“Plot Graph” command button.
9. Values of the a, b, c, and d blocks show coordinates of two best fitted paints P and Q.
10. If graph is plotted then click on the “Energy Band Gap” command button.
Diagram:
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RCPIT, SHIRPUR
DEPARTMENT OF APPLIED SCIENCE
Observation Table:
Sr.
Temperature
no.
0
C
Temperature
0
(T) K
= (273 +0 C )
104/T
0)-1
(K
Current
IS
(in µA)
Conductivity
σ = I/V× 10-6
Log10 σ
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
Calculations:
From observation table, Take 104/T along the X-axis and along Log10 Y-axis; plot
a graph between Log10 and 104/T.
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RCPIT, SHIRPUR
DEPARTMENT OF APPLIED SCIENCE
Slope of line, S = AB / CD = A-B / C-D
S=
We know that,
Energy band gap of diode,
Eg = -3.97 X S
eV
Eg = -3.97 X
Eg =
Eg = ………………....eV
Result:
i)
Measured energy band gap = ………………….eV
Conclusion:
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