DO PHYSICS ONLINE
FROM IDEAS TO IMPLEMENTATION
9.4.3
ATOMS TO TRANSISTORS
SEMICONDUCTORS
ENERGY BANDS
Diamond is a very good insulator. The electronic configuration in the ground state is
1s2 2s2 2p2. It might appear that diamond is a conductor because it has only two
electrons in the 2p energy level and that the 2p band is only partly filled. However,
there are two distinct 2p energy bands separated from each other by an energy gap of
6 eV. The lower 2p band is completely filled. At room temperature, energy due to
thermal motion is only about 0.03 eV, much small than the energy gap, so virtually no
electrons will be found in the upper 2p energy band.
Silicon has a much smaller band gap and therefore less energy is required for electrons
to be free and take part in conduction, therefore, silicon is classified as a
semiconductor.
energy bands for diamond
insulator
C Z=6
energy bands for silicon
semiconductor
Si Z = 14
1s2 2s2 2p2
forbidden band exists
between two 3p bands
forbidden band exists
between two 2p bands
2p
conduction band
Egap
2p
Egap
valence band
3s
2p
2s
1s
3p
3p
valence band
1s2 2s2 2p6 3s2 3p2
Egap ~ 6 eV
2s
Egap ~ 1 eV
1s
Fig. 1. Energy band diagram for diamond (insulator) and silicon (semiconductor).
When an electric field is applied to a material, only if electrons can gain sufficient
energy to move into the conduction band, will they move freely to establish a current.
The greater the number of charged carriers in the conduction band, the better the
conduction.
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electron energy
insulator
semiconductor
conductor
Fig. 2. Schematic diagram of energy bands for different materials.
INTRINSIC SEMICONDUCTORS
An intrinsic semiconductor is a pure crystal, for example, silicon or germanium.
Germanium was the first semiconductor widely used because it was easier to purify
than other semiconductor materials. The first transistors were made from germanium.
However, there are thermal problems with using germanium in semiconductor devises.
The small energy band gap in germanium means that electrons can be easily excited
into the conduction band and so the conducting properties of germanium devices were
sensitive to fluctuations in temperature. Today, silicon is the most widely used
semiconductor material.
In metals, the charge carriers are the free electrons. However, in semiconductors, the
charge carriers are electrons and holes. When an electron is freed from being bonded
to a particular atom, it leaves behind a vacancy called a hole in the electronic structure
of the crystal. An electron requires little energy to move into the hole, but as it does so
it leaves a new hole in its previous location. So holes move like a positive charge
carrier with the mass of an electron. Therefore, in a semiconductor, holes drift in the
direction of the externally applied electric field and the free electrons move in the
opposite direction.
E
C C C C C C
C C C C C C
C C C C C C
C C C C C C
C C C C C C
C C C C C C
C C C C C C
C C C C C C
C C C C C C
hole (vacancy) propagates to the in the direction of applied electric field
electron propagates in a direction opposite to the
direction of the applied electric field as the electron
moves into the vacancy and creating a new vacancy
in its former position
Fig. 3. Movement of electrons and holes through an intrinsic semiconductor
under the action an applied electric field.
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electron energy
Fig.4. Semiconductor energy band structure showing the movement of the
positive holes and negative electrons as the charge carriers for an electric
current.
EXTRINSIC SEMICONDUCTORS
The conductivity of semiconductors is markedly affected by slight amounts of
impurities. When a pure (intrinsic) semiconductor has controlled amount of impurity
atoms embedded within its crystal structure, it is called an extrinsic semiconductor.
Suppose we add arsenic atoms to a silicon crystal. Silicon like carbon and germanium
has four electrons in its outer most shell, whereas an arsenic atom has five electrons in
its outer shell. Since only four of the five electrons of an arsenic atom can be shared
with four neighbouring silicon atoms in a covalent bond, the remaining electron needs
little energy to be detached and move about freely within the crystal. In terms of an
energy band explanation, the effect of the arsenic impurity is to create an energy
levels just below the empty conduction band in which electrons must be present in for
conduction to take place. These levels are called donor levels, and the material is
called an n-type semiconductor because electric current in it is carried by the motion
of electrons (negatively charged).
bonded electron pair
increasing energy
Si
conduction band
-
-
electrons need
little energy to
move from a donor
level into
conduction band
As
donor levels
valence band
semiconductor:
silicon
Eg = 1.1 eV
As
extra electrons can move freely
through the Si crystal
Fig. 5. n-type semiconductor (+5 valency impurity atoms).. Donor levels due to
presence of arsenic atoms in silicon crystal. Conduction is by means of excess
electrons.
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Suppose we add gallium atoms to a silicon crystal. A gallium atom has three electrons
in its outer shell, and their presence leaves vacancies called holes in the electronic
structure of the silicon crystal. An electron requires little energy to move into a hole,
but as it does so it leaves a new hole in its previous location. When an electric field is
applied to the crystal in which gallium is present as an impurity, electrons move
towards the positive electrode by successively filling holes. The current is best
described in terms of the motion of holes, which behave as though they are positive
charges since they move towards the negative terminal. A material of this kind is called
a p-type semiconductor. In the energy band diagram, the effect of gallium as an
impurity is to create energy levels called acceptor levels just above the valance band.
The electrons that enter these levels leave behind unoccupied levels in the formerly
filled band which makes possible the conduction of current.
bonded electron pair
Si
increasing energy
Ga
electrons need
conduction band little energy to
move from valance
band to an
acceptor level
-
-
acceptor levels
hole
hole
Ga
valence band
hole
semiconductor:
silicon
Eg = 1.1 eV
electrons can move freely
through the Si crystal by filing
vacancies creating a new hole
Fig. 6. p-type semiconductor (+3 valency impurity atoms). Acceptor levels due to
presence gallium atoms in silicon crystal. Conduction by means of holes (+) in the
valance band.
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valency 3:
p type
semiconductor
impurities
valance electrons:
# electrons in outer shell
valency 4:
semiconductors
3
4
valency 5:
5 type
semiconductor
impurities
5
Fig. 7. Section of periodic table showing the elements used for doping (adding
controlled amounts of impurities) to create p-type and n-type semiconductors.
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