Semiconductors:
Solids in nature are divided into three categories as far as electrical properties are
concerned, these are:
 Conductors,
they have very high conductivity (very low resistivity)
 Insulators,
they have very low conductivity (very high resistivity)
 Semiconductors, they have intermediate conductivity, which can be varied
over very wide range.
 Solids with “free electrons” – that is electrons not directly involved in the
inter-atomic bonding- are the familiar metals (Cu, Al, Fe, Au, etc).
 Solids with "no free electrons" are the familiar insulators (glass, quartz
crystals, ceramics, etc.)
 Silicon is an insulator, but at higher temperatures some of the bonding electrons
can get free and make it a little conducting – hence the term “semiconductor”
 Pure silicon is a poor conductor (and a poor insulator). It has 4 valence
electrons, all of which are needed to bond with nearest neighbors. No free
electrons.
Electronic devices are fabricated from semiconductor materials.
Good understanding of the semiconductor characteristics leads to good
comprehension of the behaviour of the electronic devices.
Silicon is the most common semiconductor material used for semiconductor
devices and integrated circuit.
Other semiconductors are used for specialized applications.
(a)
(b)
Atoms = central heavy nucleus, positively charged with protons and neutrons,
surrounded by a cloud of electrons.
No electrons = No protons → atoms are electrically neutral.
Electrons, are distributed in different orbits around the nucleus as shown in the
Figure for the atom of Si
14 protons, and 14 electrons Electrons are distributed among the three first orbits.
Each orbit is subdivide into shells that will contain the electrons, and it is known
that each shell can contain only a certain maximum number of electrons.
1st orbit has one shell,
2nd orbit has two shells,
3rd orbit has three shells,
1s
2s
3s
2 e2 e2 e-
2p
3p
6 e6 e-
3d
10 e-
- - - - etc….
Outer shell in atom = valence shell,
Electrons in Valence shell are valence electrons
Valence electrons govern the electrical and chemical properties of the atom.
Energy
3d
3p
3s
2p
2s
1s
1
2
4
N
Energy levels in isolated atoms are discrete,
Atoms in solid interact: the discrete energy levels split into very closely spaced
levels to form bands of energy.
These bands may overlap
The last band containing e- determines the electrical
properties of the solid.
INSULATOR
CONDUCTOR
Band partially filled
Band
empty
Eg
Eg
Band filled
Overlap between
Bands
Band filled
1- Last band completely filled with e- → an insulators or a semiconductors.
2- Last band partially filled with e- → it is a conductors. Example all Metals
Case 1:
Valence Band VB: last filled band;
Conduction Band CB: band just above
Energy Gap Eg: gap between the two bands;
The value of Eg determines the property of the material:
1- Eg very large > 5 eV,
Insulator,
2- Eg small,
Semiconductor,



Energy



Conduction
Band
Eg












Conductor
(a)
Valence
Band
Eg
Insulator
(b)
Semiconductor
(c)
CB
Eg
VB
In semiconductor due thermal energy, at room temperature → e- gain energy:
Some e- can cross Eg: from VB → CB
To make this transition e- need a minimum energy equal to Eg.
An equal number of vacant states, or holes h+, are left in the valence band.
e- in CB, are free their motion contribute to the electrical conduction of the
semiconductor.
h+ in CB, can move by attracting nearby e-, they are like positive charges,
their motion contribute to the electrical conduction of the
semiconductor.