Electronics for IT
Dr. Engs. Wesam Bachir, PhD.
Computer Engineering
Vistula University
b.wesam@vistula.edu.pl
ENGAGE
Lecture
Introduction to Semiconductors
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Semiconductors
In the 1940s, researchers developed a new technology
that lead to new components that were, literally, a
quantum-leap improvement over vacuum tubes. These
new components were called semiconductors.
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Lecture Outline
The basic structure of semiconductors and how they conduct current
The atomic structure of silicon and germanium
Covalent bonding in silicon
Explain how current occurs with electrons and holes in a
semiconductor
The properties of n-type and p-type semiconductors
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Why Semiconductors ?
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Silicon and Germanium Atoms
silicon (Si), a nonmetallic chemical
element in the carbon family (Group 14
[IVa] of the periodic table).
The germanium valence electrons are
at higher energy levels than those in
silicon and, therefore, require a smaller
amount of additional energy to escape
from the atom.
This property makes germanium more
unstable than silicon at high
temperatures, which is the main reason
silicon by far is the most widely used
semiconductive material.
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Atomic Bonding
When certain atoms combine to
form a solid material, they arrange
themselves in a fixed pattern called
a crystal. The atoms within the
silicon crystal structure are held
together by covalent bonds, which
are created by the sharing of the
valence electrons of each atom.
The center silicon atom shares an
electron with each of the four
surrounding silicon atoms, creating a
covalent bond with each.
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Bonding diagram. The red
negative signs represent the
shared valence electrons.
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Conduction Electrons and Holes
A pure silicon
crystal at room
temperature has
sufficient
heat
(thermal) energy
for some valence
electrons to jump
the gap from the
valence band into
the conduction
band, becoming
free electrons.
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Electron-hole pair
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Electron and Hole Current
The applied voltage across a
piece of silicon, will attract
the thermally generated free
electrons in the conduction
band, to the positive end.
This movement of free
electrons is called electron
current.
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The hole current
It occurs at the valence level.
Electrons remaining in the valence
band are still attached to their atoms
and are not free to move randomly in
the crystal structure.
A valence electron can move into a
nearby hole, with little change in its
energy level, thus leaving another
hole where it came from.
The hole has effectively, although not
physically, moved from one place to
another, it is called hole current.
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Hole electron currents
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Energy Gap
There exist three types of materials (insulators, semiconductors, and conductors).
Pure semiconductors are neither insulators nor good conductors because the
current in a material depends directly on the number of free electrons.
The energy gap for an insulator is so wide that hardly any electrons acquire enough
energy to jump into the conduction band.
The valence band and the conduction band in a conductor (such as Metals) overlap
as there are always many conduction electrons, even without the application of
external energy.
A semiconductor has an energy gap that is much narrower than that in an insulator.
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Energy Gap
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Types of Semiconductors
Because of the limited number of free electrons in the conduction
band and holes in the valence band in pure semiconductors, They do
not conduct current well.
To make them conductive they must be modified by increasing the
free electrons and holes.
This can be done by adding impurities to the semiconductive material.
Two types of (impure) semiconductive materials, n-type and p-type,
are the key building blocks for all types of electronic devices.
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The Doping Process
Adding impurities to the (pure) semiconductive material makes them
more conductive.
The conductivities of silicon can be increased and controlled by this
process.
This process s called doping.
The aim is to increase the number of current carriers (electrons or
holes).
The two categories of impurities are n-type and p-type.
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N-Type Semiconductor
The goal is to increase the number of conduction-band electrons in
pure semiconductors such as silicon material.
Doner atoms such as arsenic (As), phosphorus (P) can be used to
provide pentavalent impurity to silicon.
These doners are atoms, with five valence electrons, provide an extra
electron to the semiconductor’s crystal structure.
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N-Type Semiconductor
Each doner atom (Sb) forms
covalent bonds with four adjacent
silicon atoms.
Four of the Sb atom’s valence
electrons are used to form the
covalent bonds with silicon atoms
The extra electron from the Sb
atom becomes a free electron.
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P-Type Semiconductor
The aim is to increase the number of holes in pure silicon.
Trivalent impurity atoms can be added. These are atoms with three
valence electrons, such as aluminum (Al), boron (B), and gallium
(Ga).
These atoms are known as acceptor atoms because they leave a
hole in the semiconductor’s crystal structure.
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P-Type Semiconductor
Each trivalent atom (boron) forms
covalent bonds with four adjacent
silicon atoms.
All three of the boron atom’s valence
electrons are used in the covalent
bonds; and, since four electrons are
required, a hole is formed with each
trivalent atom.
The number of holes can be
controlled by the amount of trivalent
impurity added to the silicon.
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What is next?
To understand electronic devices a basic knowledge of the structure
of atoms is required.
Semiconductive materials are used in manufacturing diodes as well
as other semiconductive components such as transistors and
integrated circuits.
An important concept is that of the pn junction that is formed when
two different types of semiconductive material are joined.
The pn junction is fundamental to the operation of a diode.
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End of Lecture 7
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