Lec 5-semicondutors

SEMICONDUCTORS,CONDUCTORS
AND INSULATORS
 All materials are made up of atoms. These atoms contribute to the
electrical properties of a material, including its ability to conduct
current.

CONDUCTORS
• A conductor is a material that easily conducts electrical
current. The best conductors are single element materials
such as Copper, Silver, Gold, Aluminium & nickel
• Alloys are also good conductors:
– Brass & steel
• Good conductors can also be liquid:
– Salt water
 These elements are charachterized by atoms with only
one valence electron very loosely bound to the atom.
 These loosely bound valence electrons can easily break
away from their atoms and become free electrons.
 So a conductive material has many free electons that,when
moving in the same direction make up the current.

INSULATORS
 An insulator is a material that do not conduct electrical current under
normal conditions.
 Most good insulators are compounds rather than single element
materials.
 Valence electrons are very tightly bound to the atoms therefore,
there are very few free electrons in an insulator.

SEMICONDUCTORS
 A semiconductor is a material that is in between conductor
and insulator in its ability to conduct electric current.
 A semiconductor in its pure intrinsic state is neither a good
conductor nor a good insulator.
 The most common single element semiconductors are
• Silicon
• Germanium
• Carbon
 The single element semiconductors are charachterized by
atoms with four valence electrons.

Uses of semiconductors
Semiconductors are the foundation of modern electronics,
including radio, computers, and telephones.
Semiconductor-based electronic components include transistors, solar cells,
many kinds of diodes including the light-emitting diode (LED), the silicon controlled
rectifier, photo-diodes, and digital and analog integrated circuits.

ENERGY BANDS
 Recall that the valence shell of an atom represent the band
of energy levels and when an atom acquires additional energy
it can leave the valence shell, become a free electron, and exist
in what is known as conduction band.
 The differnce in energy between the valence band and the
conduction band is called an Energy Gap.
 This is the amount of energy that a valence electron must have
in order to jump from the valence band to the conduction band.
 The following figure shows energy diagrams for insulators,
semiconductors, and conductors.

 In part (a) of the fig the insulators shows a wide energy gap.
Valence electrons do not jump in to the conduction band except
under break down conditions.
 In part (b) semiconductors have a much narrower energy gap that
permits some electrons to jump in to the conduction band and
become free electrons.
 In part (c) the energy band in conductors overlap.Therefore in
conductive materials there is always a large number of free
electrons.

CONDUCTION IN SEMICONDUCTORS
CONDUCTION ELECTRONS AND HOLES :
 An intrinsic (pure) silicon crystal at room temp has sufficient energy
for some valence electrons to jump the gap from the valence band
in to the conduction band, becoming free electrons.
 Free electrons are also called Conduction Electrons.
 When an electron jumps to the conduction band a vacancy is left in
the valence band within the crystal.
 This vacancy is called a Hole.
 When an electron is raised to the conduction band there is one hole
left in the valence band creating what is known as Electron Hole pair

EXTRINSIC MATERIAL
• The characteristic of semiconductor can be altered by
adding impurity through doping process (extrinsic
material)
• Two type:
– N-type
– P-type

N-TYPE
• N-type is created by introducing
impurity elements that have five
valence electrons (pentavalent) –
antimony, arsenic, phosphorus
• Adding arsenic (doping) will
allow four of the arsenic
valence electrons to bond with
the neighboring silicon atoms.
• The one electron left over for
each arsenic atom becomes
available to conduct current
flow.
• Diffused impurities with five
valence electrons are called donor
atoms
Figure: Arsenic impurity
in n-type material

P-TYPE
• P-type is created by doping
with impurity atoms having
three valence electrons –
boron gallium, indium
• The 3 electrons in the outer
orbit do form covalent
bonds with its neighboring
semiconductor atoms.
• This hole is ready to accept
a free electron
• The diffused impurities with
three valence electrons are
called acceptor atoms.
Figure 1.11 Boron impurity
in p-type material

Semiconductor Diode
• Diode is formed by bringing these two material together p- and
n-type
• Electrons and holes at joined region will combine, resulting in a
lack of carriers in the region near the junction (depletion region)

Current Flow in N-type Semiconductors
• The DC voltage source has a
positive terminal that attracts the
free electrons in the
semiconductor and pulls them
away from their atoms leaving
the atoms charged positively.
• Electrons from the negative
terminal of the supply enter the
semiconductor material and are
attracted by the positive charge
of the atoms missing one of
their electrons.

Current Flow in P-type
Semiconductors
• Electrons from the negative
supply terminal are attracted
to the positive holes and fill
them.
• The positive terminal of the
supply pulls the electrons
from the holes leaving the
holes to attract more
electrons.

Figure 1-16 Reverse-biased p-n junction

Figure 1.18 Forward-biased p-n junction

Figure 1.19 Silicon semiconductor diode characteristics

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