4
SEMICONDUCTORS
and
Semiconductor devices
 Semi conductors are materials whose electrical conductivities are
between highly conducting metals and poorly conducting insulators.
 They have the property of high conductivity at high temperatures and
nearly zero conductivity at low temperatures.
 The resistivity of solid materials
property of semi conductor
Two classifications of semiconductor materials are;
a) Elemental semiconductor materials (group IV elements)
Silicon, Germanium and Gallium are the most famous.
 Silicon is the most dominant in the industry. Why?
 Simply, silicon is beach sand processed in high temperature so
it’s cheep and available.
 Easily oxidized to form electrical insulator.
 Has excellent mechanical properties, it’s strength exceeds highstrength steel.
b) Compound semiconductor materials (groups III-V, II-VI elem.)
AlP , Al As, GaP, GaAs, InP
 If one examines the electrical resistivity of elements, particularly elements in
group I, II, III, IV; it will be seen that they (resistivity) tend to increase with
increasing valence.
 Increasing valence means an increasing number of loosely bound electrons in the
outer shell.
 The conductivity/resistivity of elements is directly related with interatomic bonding.
 crystal with covalent bonding is a good insulator.
 In covalent bonding of group IV elements, electrons are localized in
the bonds.( It does not mean that these localised electrons are
immobile. Those of neighbouring bonds overlap slightly and
electrons in adjacent bonds change places with one another.)
 Group IV elements ( carbon, Silicon, Germanium, and Gallium)
usually form crystals by covalent bonding.

At 0oK, each electron is in its lowest energy state so each covalent
bond position is filled.
Thus, it is expected that these crystals are all good insulators
i.e. resistivity of Diamond ρ = 1012 ohm-m at 20oc
“
“ polymers ρ = 1015 ohm-m “ “
“
“ Silicon ρ = 5x104 ohm-m “ “
“
“ Germanium ρ = 2.2x10-1 ohm-m “ “
 Even though crystals of Si and Ge are formed by covalent
bonding, they have much lower resistivity. This is because
of the bond strength.
The bond strength of crystals of diamond, silicon,
germanium decreases as the atomic weight increases. Thus
it is easier to break the bonds in germanium than silicon;
while it is hardest of all to break the bonds of Diamond.
Conduction in Semi conductor
 When a voltage is applied to a crystal
containing these conduction band electrons,
the electrons move through the crystal
toward the applied voltage. This movement
of electrons in a semiconductor is referred
to as electron current flow
Continued….
Continued….
Types of Semiconductors
Intrinsic or pure
semiconductors
Extrinsic semi
conductor
Intrinsic semiconductors
A semiconductor in which holes and electrons are
created only by thermal excitation across the energy
gap is called an intrinsic semiconductor.
A pure crystal of silicon or germanium is an intrinsic
semiconductor.
In an intrinsic semiconductor the number of holes in
the valence band is equal to number of electrons in the
conduction band.
The Fermi level for an intrinsic semiconductor lies at
midway in the forbidden gap
Extrinsic semiconductors
Extrinsic Semiconductors are those which have been
doped, i.e. impurities are added purposely into their
crystal structure .
It is an impure semiconductor their band gap are
reducing up to 0.01 eV
Doping process
The pure semiconductor mentioned earlier is basically neutral. It contains
no free electrons in its conduction bands .
Even with the application of thermal energy, only a few covalent bonds
are broken, yielding a relatively small current flow.
A much more efficient method of increasing current flow in
semiconductors is by adding very small amounts of selected additives to
them, generally no more than a few parts per million.
These additives are called impurities or dopants and the process of
adding them to crystals is referred to as doping.
Contin..
The purpose of semiconductor doping is to increase the number of free
charges that can be moved by an external applied voltage.
When an impurity increases the number of free electrons, the doped
semiconductor is negative or n -type semiconductor and the impurity
that is added is known as an n-type impurity, or penta-valent.
However, an impurity that reduces the number of free electrons, causing
more holes, creates a positive or p-type semiconductor, and the impurity
that was added to it is known as a p-type impurity, or trivalent.
Semiconductors which are doped in this manner, either with n- or p-type
impurities, are referred to as extrinsic semiconductors.
N-Type Semi conductor
• The n-type impurity loses its extra valence electron
easily when added to a semiconductor material, and
in so doing, increases the conductivity of the material
by contributing a free electron. This type of impurity
has 5 valence electrons and is called a penta-valent
impurity. Arsenic, antimony, bismuth, and
phosphorous are penta-valent impurities. Because
these materials give or donate one electron to the
doped material, they are also called donor impurities.
P-Type Semi conductor
The second type of impurity, when added to a semiconductor
material, tends to compensate for its deficiency of 1 valence
electron by acquiring an electron from its neighbor. Impurities
of this type have only 3 valence electrons and are called
trivalent impurities. Aluminum, indium, gallium, and boron
are trivalent impurities. Because these materials accept 1
electron from the doped material, they are also called acceptor
impurities
Boron doped to Si
Conductivity of Extrinsic Semiconductors
Conductivity of Extrinsic Semiconductors
Location of fermi level in Energy Band
diagram for Intrinsic Semiconductors
In intrinsic semiconductor, the probability of finding an electron in the
conduction band is zero and the probability of finding a hole in the
valence band is zero, at absolute zero i.e. T = 0 °K.
Now let Ec be the lowest energy level in the conduction band while
Ev be the highest energy level in the valence band. As temperature
increases, equal number of electrons and holes get generated. Hence
probability of finding electron in conduction band and probability of
finding hole in valence band is same.
Continued…….
Thus in the energy band diagram, the fermi level for
the intrinsic semiconductor lies in the center of the
forbidden energy band. Hence the energy band diagram
for intrinsic semiconductor is shown figure below
Location of fermi level in Energy Band diagram
for Extrinsic Semiconductors
Continued…………
In n-type semiconductor, a donor impurity is added. Each donor atom
donates one free electron and there is large number of free electrons,
available in the conduction band. The donor energy level corresponding
to the donor impurity added is just below the conduction band. This
donor level is indicated as Ed and its distance is 0.01 eV below the
conduction band in germanium while it is 0.05 eV below the conduction
band in silicon. As this distance is very small, even at room temperature,
almost all the extra electrons from the donor impurity atoms jump into
the conduction band. Hence number of free electrons is very large in case
of n-type material. Due to abundant free electrons, the probability of
occupying the energy level by the electrons, towards the conduction band
is more. This probability is indicated by Fermi level Ef. So in n-type
material, the fermi level Ef gets shifted towards the conduction band. But
it is below the donor energy level. The overall energy band diagram for
n-type material is shown in the Figure 4.3(a).
Continued…………
In p-type material, acceptor impurity is added. Due to
this, large number of holes gets created in the valence
band. The acceptor energy level corresponding to
acceptor impurity gets introduced which is indicated
as EA and is very close to the valence band just above
it. At room temperature, the electrons from valence
band jump to acceptor energy level, leaving behind the
holes in valence band. This Shifts the Fermi level Ef
toward the valence band. It lies above the acceptor
energy level. The overall energy band diagram for ptype material is shown in the Figure 4.3(b).
N.B
In a band diagram, the position of the Fermi level
determines which carrier dominates. If the
semiconductor contains more electrons than holes,
n-type material, the Fermi level is positioned above
mid gap. If holes are more abundant than electrons,
p-type material, EF is positioned below mid gap.
When the electron and hole concentrations are
approximately equal, intrinsic material, EF is
positioned at mid gap. The Fermi function, or level,
also varies with temperature and carrier
concentration.
Carrier Concentrations
The concentrations of holes and free electrons are important
quantities in the behavior of semiconductors.
 Carrier concentration is given as the number of particles per unit
volume, or
- Carrier concentration = #/cm 3
 In an intrinsic semiconductor, the number of holes and free
electrons are the same because they are thermally generated.

 If an electron breaks its covalent bond we have one free electron
and one hole.
 In an intrinsic semiconductor, the concentration of holes and free
electrons are the same.
For any semiconductor in thermal equilibrium nopo=ni2, where
no = the concentration of free electrons.
po = the concentration of holes.
 For an n-type semiconductor with donor impurities, the
concentration of donor impurities is Nd with units #/cm3.
 If Nd >> ni, then the concentration of free electrons in the n-type
semiconductor is approximately no ≈ Nd.
 Since nopo=ni2 for any semiconductor in thermal equilibrium, and
 For an n-type semiconductor, no ≈ Nd
Where, po is the concentration of holes in the n-type
semiconductor
 For a p-type semiconductor with acceptor impurities, the
concentration of acceptor impurities is Na with units #/cm3.
 If Na >> ni, then the concentration of holes in the p-type
semiconductor is approximately po ≈ Na.
 Since nopo=ni2 for any semiconductor in thermal equilibrium, and
 For a p-type semiconductor, po ≈ Na
Where, no is the concentration of free electrons in the p-type
semiconductor.
Conduction in Semiconductors
• Under the action of applied electric field (Ex), the electron in CB and the holes
in VB immediately start gaining energy and accelerates with drift velocities Vde
and Vdh. Where,
Vde   e E x
Vdh   h E x
• Since both electrons and holes contribute to electrical conduction, we may write
the current density J, such that
J  enVde  ePVdh
• Where,
n – electron concentration in CB
P – hole concentration in VB
e e
e 
me
,
e h
h 
mh
Drift mobility of electrons and holes.
e, h - the mean free time between scattering events of electrons and holes
me, mh – mass of electrons and holes
Drift & Diffusion Current
• There are two current mechanisms which cause charges to move in
semiconductors. The two mechanisms are drift and diffusion.
Drift Current
• Drift is, by definition, charged particle motion in response to an applied electric
field.
• When an electric field is applied across a semiconductor, the carriers start
moving, producing a current.
• The positively charged holes move with the electric field, whereas the
negatively charged electrons move against the electric field.
Drift Current Equations
For undoped or intrinsic semiconductor ; n = p = ni
For electron
For hole
J n  nqE n
drift
current
density
for
electrons
number
of free
electrons
per unit
volume
mobility
of
electron
J p  pqE  p
drift
current
density
for holes
number
of free
holes per
unit
volume
mobility
of holes
Total current density
Ji  Je  J h
J i  nqE n  pqE  P
since
n  p  ni
J i  ni q ( n   p ) E
For a pure
intrinsic
semiconductor
J total  ?
for doped or extrinsic semiconductor
n-type semiconductor;
n  p  J T  nqn E  N D qn E
Where, ND is the shallow donor concentration
p-type semiconductor;
p  n  J T  pq p E  N Aq p E
Where, NA is the shallow acceptor concentration
Diffusion Current
• Diffusion is the process of particles distributing themselves from
regions of high concentration to regions of low concentration.
• It is possible for an electric current to flow in a semiconductor
even in the absence of the applied voltage provided a
concentration gradient exists in the material.
• A concentration gradient exists if the number of either electrons
or holes is greater in one region of a semiconductor as compared
to the rest of the Region.
• In a semiconductor material the charge carriers have the
tendency to move from the region of higher concentration to that
of lower concentration of the same type of charge carriers. Thus
the movement of charge carriers takes place resulting in a current
called diffusion current.
The Hall Effect
• When a current-carrying conductor is placed into a magnetic field, a voltage
will be generated perpendicular to both the current and the field. This
principle is known as the Hall effect.
Hall effect principle, no magnetic field
• It shows a thin sheet of semiconducting material (Hall element) through
which a current is passed. The output connections are perpendicular to the
direction of current. When no magnetic field is present (Figure 2-1), current
distribution is uniform and no potential difference is seen across the output.