Lecture_1_Semiconductors_011100.pdf hjjj

school staff

1. Patrick J.C. Mzaza
Department of Physics & Electronics, School of Natural & Applied
Sciences,
University of Malawi (2022/2023)
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Materials are categorized into Conductors,
Semiconductors and Insulators depending on their
conducting properties.
Conductors: have low resistance which allows
electrical current flow (< 10
-4
·cm)
Semiconductors: can allow or suppress electrical
current flow
Insulators: have high resistance which suppresses
electrical current flow (10
10
· cm)
(
Si
Cu*10
11
· cm , 
Ge
Cu*10
7
· cm)

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Good conductors have low resistance so
electrons flow through them with ease.
Best element conductors include:
Copper, silver, gold, aluminum, & nickel
Alloys are also good conductors:
Brass & steel
Good conductors can also be liquid:
Salt water

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The atomic structure of good
conductors usually includes only one
electron in their outer shell.
It is called a valence electron.
It is easily striped from the atom, producing
current flow.

5. Semiconductors are materials that
essentially can be conditioned to act as
good conductors, or good insulators, or any
thing in between.
Common elements such as carbon, silicon,
and germaniumare semiconductors.
Silicon is the bestand most widely used
semiconductor.
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6. Insulators have a high resistance so current
does not flow in them.
Good insulators include: Glass, ceramic,
plastics, & wood
Most insulators are compounds of several
elements.
The atoms are tightly bound to one another
so electrons are difficult to strip away for
current flow.
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The highest
occupied energy
band is called the
valence band.
Most electrons
remain bound to
the atoms in this
band.

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The conduction
band is the band of
orbitals that are
high in energy and
are generally empty.
It is the band that
accepts the electrons
from the valence band.

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The “leap”required
for electrons from the
Valence Band to enter
the Conduction Band.

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As the temperature increases, some electrons
in the valence band become thermally
agitated and thus acquire enough energy to
move (jump) to the conduction band.
Once in conduction band these electrons can
move (drift) if an electric field (voltage) is
applied to the semiconductor (i.e. if the
semiconductor is biased).

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When an electron moves to the conduction
band, an empty state is left in the valence
band, this is called a hole.
A hole is essentially the absence of an
electron. When the electron moves in one
direction, the hole is perceived to move in the
opposite direction.

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In an intrinsic (pure) and electrically neutral
semiconductor, the number of electrons is always
equal to the number of holes.
The probability per unit time that an electron-
hole pair is thermally generated is given by
P(t) = CT
3/2
e
(-Eg/2KT)
Where; T = absolute temperature, Eg= band gap,
K = Boltzmann constant, C = proportionality
constant characteristic of the material.

22. To make the semiconductor conduct
electricity, other atoms called impurities
(dopants) must be added.
“Impurities” are different elements.
This process is called doping.
A doped semiconductor is called an
extrinsic semiconductor
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The semiconductor with donor atoms has a
large number of electrons and a small of
holes. Its conductivity will mainly be due to
the electrons and it is called an n-type
semiconductor.
Electrons in this case are called majority
carriers and the holes are the minority
carriers.

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In nearly all cases, the concentration of
impurities N
Dis large compared with the
concentration of electrons expected in the
conduction band for the intrinsic material.
Therefore the number of conduction
electrons becomes completely dominated by
the contribution from the donor impurities
and we can write n ≈ N
D.

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The added concentration of electrons in the
conduction band compared with the intrinsic
value increases the rate of recombination,
shifting the equilibrium between electrons
and the holes.
As a result the equilibrium concentration of
holes decreases by an amount such that the
equilibrium constant given by the product of
n and p is the same as for the intrinsic
material.
np = n
ip
i

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It takes only a very
small quantity of
the impurity to
create enough free
electrons to allow
an electric current
to flow through
the silicon.
N-type silicon is a
good conductor.

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If the concentration N
Aof the acceptor
impurities is made to be large compared with
the intrinsic concentration of holes p
i, then
the number of holes is completely dominated
by the concentration of the acceptors.
Thus p ≈ N
A.

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The increased availability of holes enhances the
recombination probability between conducting
electrons and holes and therefore decreases the
equilibrium number of conducting electrons.
The equilibrium constant also holds
np = n
ip
i.
In p-type material holes are the majority carriers
and dominate the electrical conductivity. The
electrons are thus minority carriers

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Holes can conduct
current. A hole
happily accepts
an electron from
a neighbor,
moving the hole
over a space.
P-type silicon is a
good conductor.

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