Module 1
Study of Semiconductor
Devices
Presented By
Ridhi Bardoloi
Introduction
The word electronics has been derived from a Greek word ‘elektron’.
A field of science and Engineering which deals with the study ,design and use of devices
.
It depends on the conduction of electricity through a vacuum, gas or semiconductor.
Applications:
-Communication and entertainment applications
-Instrumentation and Control applications
-Defence applications
-Applications in medicine
Structure of Solids:
Operation of electronic devices depend upon the motion of charged
particles within them.
Therefore we must know the forces which control the motion of
charged particle.
But it is due to the physical structure of solids which exerts the
control.
Types of Crystalline Structure
In every cases, the crystal lattice contains a volume called a unit cell.
The unit cell is a representative of the entire lattice and is regularly repeated throughout the
crystal.
Types of crystalline Structure:
Contd…..
4.Diamond or tetrahedral
Semiconductors such as Silicon, Germanium and Gallium
Arsenide(GaAs) have this type of crystal structure.
The unit cell can be thought of as an FCC structure with an
extra atom located at a/4,+b/4,+c/4 from each of the FCC
atoms.
Structure of an atom
Atom is mainly composed of three fundamental particles i.e. electrons,
protons
and neutrons.
Atomic number: Atomic number of an atom is equal to the number of
protons (or
electrons) contained in an atom.
Electron orbits
The orbits through which electron revolves (called energy level) are
represented by K,L,M,N, etc.
The maximum number of electrons in any shell is given by the relation 2n2
where n is an integer and is called principal quantum number.
The electrons which exist in the outermost shell of an atom are called valence
electrons.
Energy Possessed by electron in orbit
When a body receives the electromagnetic radiation, the radiation absorbed are converted into
energy. As a result of this ,the energy of its electron is raised and electron is said to be excited.
According to Bohr theory, some energy is always emitted by an electron when it moves from a
higher orbit to lower orbit. Similarly, some energy is absorbed by an electron when it moves from
a lower orbit to higher orbit.
The amount of energy absorbed or emitted is given by
W2-W1=h.f (in Joules)
W1=Energy of the initial orbit h=Planck’s const. f= Frequency of radiation
W2=Energy of the final orbit
Energy level diagram
Bonding in Solids
Bond energy is used to classify the various types of bonds in solids:
1. Primary bonds
2. Secondary bonds
There are three types of primary bonds :
Ionic Bond:
Covalent Bond
Silicon Atom
Formation of covalent bond in silicon
crystal
Energy band
The valence band is completely filled with the electrons
The upper band contains no electrons. This empty band is called conduction
band.2
Insulators,Conductors and
Semiconductors
Semiconductor
The materials whose electrical conductivity lie between those of conductors and
insulators are known as semiconductors. This means conductivity is in the range of 10−3 to
10−8 S/cm.
Semiconductors are the foundation of modern electronics including radio, television,
computers etc.
Common semiconductors are crystalline solids for example: germanium(Ge),
silicon(Si),Gallium Arsenide(GaAs),cadmium sulfide(CdS) etc.
Types of semiconductors:
1. Intrinsic semiconductor 2. Extrinsic semiconductor
Intrinsic semiconductor
Semiconductor which is in its extremely pure form is known as an intrinsic semiconductor.
The silicon and germanium are the two most widely used intrinsic semiconductor.
FIG 1.1
Generation of electrons and holes
At very low temperature (at 0k) an intrinsic semiconductor will
have exactly the ideal structure as shown in FIG 1.1.
Let us assume that the temperature is increased to room
temperature(i.e 27∘ 𝐶 𝑜𝑟 300𝐾)
At this temperature some of the covalent bonds are broken. An
electron which was a part of covalent bond earlier is now shown
as dislodged electron. The electron is free to move anywhere
within the crystal. The energy required to break such a bond is
equal to band gap energy( 𝐸𝐺 ). The value of (𝐸𝐺 ) at room
temperature is about 0.72 eV for Ge and 1.1 eV for Si.
A hole can serve as carrier of electricity like the free electron.
Intrinsic Semiconductor
In intrinsic semiconductor, free electron concentration ‘n’ (i.e. no of
electrons/𝑐𝑚3 ) is equal to the concentration of holes ‘p’ (i.e no of
holes/𝑐𝑚3 ). Each of this intrinsic carrier concentration is commonly denoted
by letter 𝑛𝑖
Thus for an intrinsic semiconductor, concentration of free electrons or
concentration of holes
n=p= 𝑛𝑖
Energy band diagram of Intrinsic semiconductor
The horizontal line marked 𝐸𝐹 indicates Fermi Level.
Fermi level is the maximum energy level which is occupied
by an electron at absolute zero temperature.
Mechanism of holes contributing in
conductivity
Generation and recombination of carriers
• When electron jumps from valence band to conduction band it leaves a vacancy in the
valence band .This vacancy is called a hole. For every electron raised to the conduction
band there is one hole left in the valence band. Thus thermal energy creates an electron
hole pair.
• When that free electron lose energy it falls back into a hole in the valence band. This
process is known as recombination of electron hole pair.
• Due to thermal energy new electron hole pairs are generated and after a few nanoseconds
or several µ seconds, depending on the crystal structure they recombine.
• The average time its creating and disappearing of an electron hole pair is known as lifetime.
Extrinsic semiconductor
In order to increase the conductivity of intrinsic semiconductor, it is necessary
to change their characteristics by adding certain amount of desired impurity
atoms. The resulting semiconductors are called extrinsic semiconductor.
The process of adding impurity atoms is called doping.
Generally, the impurities are added at the rate of only one atom of impurity
per 106 to 1010 semiconductor atoms.
Depending upon the type of impurity atoms added to the semiconductor, the
extrinsic semiconductors are classified into two types:
1.N-type semiconductors
2.P-type semiconductors.
N-type semiconductor
The semiconductors which are obtained by introducing pentavalent impurity
atoms are known as N-type semiconductor.
The element from Group V of the periodic table contain 5 valence electrons.
These elements donate excess (negative) electron carriers. Therefore, such
elements known as donor or N-type impurities.
Pentavalent Impurities
Sl. No. Element Symbol
1 Phosphorus P
2 Antimony Sb
3 Arsenic As
4 Bismuth Bi
N –Type Semiconductor
Fig shows the structure of a Silicon crystal lattice
containing an Antimony atom at the central position.
The four electrons out of 5 valence electrons form
covalent bond by sharing one electron each with the
electrons of neighbouring atom. The 5th electron is
loosely bound with the Antimony atom. This extra
electron detached from the Antimony atom and will be
available as a carrier of current. The energy required to detach the electron is of
the order of .05 eV for silicon and 0.01eV for germanium.
P type semiconductor
The semiconductors which are obtained by introducing trivalent impurity
atom are known as P-type semiconductor.
The element from Group III of the periodic table contain 3 valence electrons.
These elements make available positive charge carriers because they create
holes, which can accept electrons. Therefore, such elements known as
acceptor or P-type impurities.
Sl no Element Symbol
1 Gallium Ga
2 Indium In
3 Aluminium Al
4 Boron B
Contd…..
Fig shows the structure of a Silicon crystal lattice
containing an Indium atom at the central position.
The 3 valence electrons of Indium atom forms 3
covalent bond by sharing one electron with the
electrons of neighboring atoms.
However the fourth covalent bond is incomplete. A vacancy which exist in
the incomplete covalent bond constitutes a hole. Now the indium atom
acquire the 4th electron from its surrounding atom to complete the bond.
Thus an electron which is in favorable position is captured by Indium atom
and becomes immobile.
