A very short note on Semiconductors

1. A SHORT NOTE ON
SEMICONDUCTORS

2. p-n junctions
 When p-type and n-type materials are placed in contact with each
other, the junction behaves very differently than either type of
material alone. Specifically, current will flow readily in one direction
(forward biased) but not in the other (reverse biased), creating the
basic diode. This non-reversing behavior arises from the nature of the
charge transport process in the two types of materials and the
formation of a depletion zone.
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Module 1: Semiconductors – p-n junction

3. Depletion Zone
 Some of the free electrons in the n-region diffuse across the junction
and combine with holes to form negative ions. In so doing they leave
behind positive ions at the donor impurity sites.
 This coulomb force creates a source of barrier as these ions oppose
further diffusion of electrons
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Module 1: Semiconductors – p-n junction

4. Forward and Reverse Bias
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Module 1: Semiconductors – p-n junction
 the p side is made more positive, so
an electron can move across the
junction and fill a vacancy or "hole"
near the junction. It can then move
from vacancy to vacancy leftward
toward the positive terminal, which
could be described as the hole
moving right.
• the p side is made more negative,
making it "uphill" for electrons
moving across the junction. The
conduction direction for electrons
in the diagram is right to left, and
the upward direction represents
increasing electron energy.

5. Solar Cells
 Solar cells (photovoltaic cells) convert solar energy in to electrical
energy. They primarily consist of wafer thin p-n junction diodes.
 Each solar cell is very small and series combination of many such
solar cells, which gives rise to a photovoltaic module, combination of
these modules make a solar cell array panel.
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Module 1: Semiconductors – Solar Cells

6.  Solar Cell is a p-n junction diode.
 Its construction is little bit different form conventional p-n
junction diode.
 A very thin layer of n-type semiconductor is grown on a relatively
thicker p-type semiconductor.
 Few finer electrodes are provided on the top of the n-type
semiconductor layer.
 These electrodes do not obstruct light to reach the thin n-type
layer.
 Just below the n-type layer there is a p-n junction. 6
Construction

7. 7
 A current collecting electrode is provided at the bottom of the
p-type layer.
 The entire assembly is enclosed by thin glass to protect the
solar cell from any mechanical shock.

8. Working
 When light reaches the p-n junction, the light photons can enter
easily into the junction, through very thin n-type layer.
 These light photons, carry sufficient energy to create a number of
electron-hole pairs.
 The incident light breaks the thermal equilibrium condition of the
junction.
 The free electrons in the depletion region can quickly come to the
n-type side of the junction.
 Similarly, the holes can quickly come to the p-type side of the
junction.
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9. 9
 These newly created free electrons and holes cannot further
cross the junction because of barrier potential of the junction.
 As the concentration of electrons becomes higher in n-type side
of the junction and concentration of holes becomes higher in p-
type side of the junction.
 Now the p-n junction will behave like a small battery cell. A
voltage is set up which is known as photo voltage.
 If we connect a small load across the junction, there will be a
meaningful current flowing through it.
 The amount of electric current produced depends on the amount
of light incident on it.

10. Characteristics
 Modern solar cells are only microns thick and have practical every day
efficiencies of about 10-20%
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11. Efficiency of Solar Cell
 The voltage – current graphs or the volt-ampere (V-I) curve of a solar
cell is determined by connecting a resistance box and a voltmeter
across a solar cell.
 The resistance values are varied step by step, and the corresponding
voltages across the resistance box are measured using a voltmeter.
From the acknowledged values of V and R, the value of I is determined
using the relation
I=
𝑉
𝑅
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Module 1: Semiconductors – Solar Cells

12. Max Power of Solar Cell
 The current obtained by short-circuit in the two terminal of the solar
cell is called short – circuit current (Isc).
 The voltage built up in open circuit is called as open-circuit voltage
(Voc).
 Max power, Pmax is obtained when the area of the rectangle formed is
the largest. The corresponding voltage is Vmax and current is Imax.
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Module 1: Semiconductors – Solar Cells

13. Efficiency of Solar Cell
 Efficiency of a solar cell can be defined as the ratio of total power
converted by the solar cell to the total power available for energy
conversion.
 Fill factor is defined as the ratio of maximum output power to ideal
output power
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Module 1: Semiconductors – Solar Cells
η =
𝑀𝑎𝑥𝑖𝑚𝑢𝑚 𝑜𝑢𝑡𝑝𝑢𝑡 𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑎𝑙 𝑝𝑜𝑤𝑒𝑟
𝐼𝑛𝑝𝑢𝑡 𝑜𝑝𝑡𝑖𝑐𝑎𝑙 𝑝𝑜𝑤𝑒𝑟 𝑜𝑟 𝑙𝑖𝑔ℎ𝑡 𝑖𝑛𝑡𝑒𝑛𝑠𝑖𝑡𝑦 𝑋 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑡ℎ𝑒 𝑠𝑜𝑙𝑎𝑟 𝑐𝑒𝑙𝑙
η =
𝑃𝑚𝑎𝑥
𝐼𝑛𝑝𝑢𝑡 𝑜𝑝𝑡𝑖𝑐𝑎𝑙 𝑝𝑜𝑤𝑒𝑟
×100
f =
𝑃𝑚𝑎𝑥
𝑉𝑜𝑐
∗𝐼𝑠𝑐

14. Multiple Choice Questions - Recollection
 Solar cells are primarily composed of
a) Diodes
b) Triodes
c) Transistors
d) Thermistors
 The efficiency of a solar cell is given by
a) η =
𝑷𝒎𝒂𝒙
𝑰𝒏𝒑𝒖𝒕 𝒐𝒑𝒕𝒊𝒄𝒂𝒍 𝒑𝒐𝒘𝒆𝒓
×100
b) η =
𝑃𝑚𝑎𝑥
𝑂𝑢𝑡𝑝𝑢𝑡 𝑜𝑝𝑡𝑖𝑐𝑎𝑙 𝑝𝑜𝑤𝑒𝑟
×100
c) η =
𝑃𝑚𝑎𝑥
𝑂𝑢𝑝𝑢𝑡 𝑜𝑝𝑡𝑖𝑐𝑎𝑙 𝑝𝑜𝑤𝑒𝑟
d) η =
𝑃𝑚𝑎𝑥
𝑉𝑜𝑐
∗𝐼𝑠𝑐
×100
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Module 1: Semiconductors – Questions

15. Multiple Choice Questions - Recollection
 Pure semi conductors are called
a) Extrinsic semiconductors
b) Doped semiconductors
c) Intrinsic semiconductors
d) n-type semiconductors
 In a semi conductor the energy bands in
increasing order of energies are
a) Valence, conduction and forbidden energy bands
b) Conduction, valence and forbidden energy bands
c) Valence, forbidden and conduction energy bands
d) Forbidden, conduction and valence energy bands
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Module 1: Semiconductors – Questions
 The forbidden energy gap in
semiconductors is
approximately
a) 10 eV
b) 5 eV
c) 1 eV
d) 0 eV
 Which of the following is a
semiconductor
a) Aluminium
b) Germanium
c) Boron
d) Lead

16. Multiple Choice Questions - Comprehension
 In n-type semiconductor Fermi level is closer to the conduction band,
it implies that
a) There is lower conductivity
b) There is less probability of finding electrons in conduction band
c) There is more probability of finding electrons in the valence band
d) There is higher conductivity
 We say that solar cell has efficiencies of about 20%, it implies that we
will have 100% efficiency if
a) There was less dust on the cells
b) There was continuous 24 hours sunlight
c) If product of Vmax and Imax was equal to input optical power
d) If product of Voc and Isc was equal to input optical power
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Module 1: Semiconductors – Questions

17. Multiple Choice Questions - Comprehension
 We read about reverse bias in p-n junction. What would happen if the
negative terminal connected to p-type was disconnected but the
positive to n-type was left connected
a) The depletion zone moves towards the p-type
b) The depletion zone moves towards the n-type
c) The depletion zone becomes larger
d) The depletion zone becomes smaller
 We say that solar cell has efficiencies of about 20%, it implies that we
will have 100% efficiency if
a) There was less dust on the cells
b) There was continuous 24 hours sunlight
c) If product of Vmax and Imax was equal to input optical power
d) If product of Voc and Isc was equal to input optical power
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Module 1: Semiconductors – Questions

18. Multiple Choice Questions - Comprehension
 In a solar cell, the electrons generated by photons moves along the
circuit towards the p-type semiconductor. Why do the electrons not
just cross the junction to the p-type
a) The pentavalent atoms in n-type prevent motion across the depletion zone
b) The trivalent atoms in p-type prevent motion across the depletion zone
c) The energy required for travelling through circuit is higher than to cross the
barrier potential
d) The energy required for travelling through circuit is lower than to cross the
barrier potential
 In an intrinsic semiconductor why don’t the electrons from lower levels
of valence band jump to the conduction band?
a) Energy levels are quantized and electrons are excited only if the energy available
matches energy level vacancy
b) Excitation occurs only in the order of upper to lower levels
c) Electrons in lower energy levels of valence band are bound tightly to the nucleus
d) Electrons in higher levels of valence band absorb energy from electrons in lower levels
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Module 1: Semiconductors – Questions