Atomic theory and energy band
Insulator and semiconductor materials
Formation of PN junction
Distribution of holes and electrons
Forward-biased and reversed-biased PN
IV characteristics of forward-biased
and reversed-biased PN junction
1.1 Atomic Theory
The atom has 3 basic particles:
Same magnitude but different pole with
Same magnitude but different pole with hole
Protons and neutrons form the nucleus
Electrons appear in fixed orbits around the nucleus.
• For each atom;
No. of proton in nucleus = no. of electron
ATOM IS NEUTRAL
• If an atom losses 1 valence electron - +ve
• If an atom gains 1 valence electron - -ve
1.1.1 Bohr Model
• The orbital paths or shells
are identified using
K through M.
• The innermost shell- K shell.
• The outermost atom- valence shell.
• Valence shell – determines the
The orbital shells for an atom
conductivity of atom.
• The conductivity of atom depends on
the number of electron in valence shell
Element in periodic table
are arranged according to
The atomic number of an
element = the number of
protons (which also equals the
number of electrons) in the
nucleus of a neutral atom.
The Atomic Structure
Atomic number, often represented by the symbol Z.
Shells are divided into sub shells :
i. s – max 2 electrons
ii. p – max 6 electrons
iii. d – max 10 electrons
iv. f – max 14 electrons
The structure for nickel atom
1.2 Energy Band
• Electron energy level in valence shell is
changing depend on the atomic force.
• Electron energy level always stated as energy
• In any material, there are 2 energy band;
i. Valence band – the outermost shell that
ii. Energy band – the band outside the
• The 2 bands are separated by one energy gap
called – forbidden gap.
• The valence band
contains with electrons.
• The electrons can move
to the conduction band if
it have enough energy
( eg: light or heat)
• When the electron absorbs
Energy band in Silicon Atom
enough energy to jump from
valence band to the conduction band,
the electron is said to be in excited state.
The energy band gap for conductor, insulator and semiconductor
1.3 Insulator, Semiconductor and
The concept of
energy bands is
Energy Diagram for Three Types of Material
• Insulator - very wide energy gap. The wider
this gap, the greater the amount of energy
required to move the electron from the
valence band to the conduction band.
• Therefore, an insulator requires a large
amount of energy to obtain a small amount of
• The insulator "insulates" because of the wide
forbidden band or energy gap.
• Semiconductor - has a smaller forbidden band
and requires less energy to move an electron
from the valence band to the conduction band.
• Therefore, for a certain amount of applied
voltage, more current will flow in the
semiconductor than in the insulator.
• Conductor - no forbidden band or energy gap and
the valence and conduction bands overlap.
• With no energy gap, it takes a small amount of
energy to move electrons into the conduction
band; consequently, conductors pass electrons
• The valence shell determines the ability of material to
• The number of valence electron in valence shell:
1 e – perfect conductor ( < 4e)
(Easy to drift or move to other atom)
8 e – insulator
4 e – semiconductor
Note: conductivity decreases with an increase in
the number of valence electrons
Most of the conductors used in electronics
are metals like copper, aluminum and steel.
Conductors are materials that obey Ohm's
law and have very low resistance.
They can also carry electric currents from
place to place without dissipating a lot of
• i.e: glass, most polymers (plastics), rubber
• Materials which will refuse to carry an
• Useful for jobs like coating electric wires to
prevent them from 'shorting together' or
giving a shock.
• Silk and cotton are also good insulators
(when they're dry!!)
• Modern insulators like PVC
(Polyvinylchloride) are much better and
• Insulators are also very useful to fill the 'gap'
in between the metal plates of a capacitor.
Special class of elements having a conductivity
between that of a good conductor (like cooper) and
that of an insulator (like plastic).
Most of the transistors, diodes, integrated circuits,
etc. used in modern electronics are built using a
range of semiconductors.
The basic property of a semiconductor is given away
by its name - it 'conducts a little bit'.
A semiconductor will carry electric current, but not
as easily as a normal conductor.
The semiconductor atoms complete their valence
shells by sharing valence electrons with other atoms
– covalent bonding.
For low temperature, semiconductor material will act
as an insulator.
• In room temperature, the stability of atom is
threatened. Some of the electrons free from its
bonding and jump to forbidden gap.
• When the temperature increases, more valence
electrons (free electron) jump to conduction band
and increase the conductivity.
• When the covalent bonding break, the hole is
created by free electrons in valence bands.
• The thermal energy (heat) causes the constant
creation of electron – hole pairs.
• Recombination occurs when the free electrons loss
their energy and fall down to valence band (fill the
1.4 Types of Semiconductor
• Semiconductors are mainly classified into two
Intrinsic - chemically very pure and possesses
- It has equal numbers of negative
carriers (electrons) and positive
- Impurities do not affect its electrical
Extrinsic - improved intrinsic semiconductor with
a small amount of impurities
a process, known as
which alters the
electrical properties of
semiconductor and improves its
- Introducing impurities into the
semiconductor materials (doping
process) can control their conductivity.
1.4.1 Intrinsic Semiconductor
o The pure semiconductor material without
o Example: Silicon and Germanium.
The Silicon bonding
1.4.2 Extrinsic Semiconductor
• Adding impurities atom into intrinsic
semiconductor = extrinsic semiconductor.
• The process of adding specific types of
atoms to a semiconductor to favorably alter
electric characteristics - Doping
• 2 types of extrinsic (impure)
• When an impurity increases the number of
free electrons, the doped semiconductor is
NEGATIVE or N-TYPE.
• An impurity that reduces the number of
free electrons, causing more holes,
creates a POSITIVE or P-TYPE
N– type material
- Diffused impurities with
5 valence electrons are
called donor atoms.
Antimony (Sb) impurity in n-type material
-The diffused impurities
with 3 valence electrons
are called acceptor
Boron (B) impurity in p-type material
1.5 PN Junction Formation
• A PN junction is fabricated from a single slice of
• One side doped with acceptor impurity atoms – p region
• One side doped with donor impurity atoms –
• The interface separating the n and p regions is referred
as the metallurgical junction.
The PN junction
Majority and minority carriers
a) n-type material b) p-type material
In trying to neutralize charges;
- free electrons in n-type diffuse across
junction to p-type
- free holes in p-type diffuse to n-type
- electrons & holes close to junction
The movement of holes and electrons in diffusion process.
E-field force on holes
E-field force on electrons
A depletion region formation due to electrons and holes
movement in diffusion process and electric field.
Forward biased narrows the depletion region and produces a voltage
drop across the PN junction equal to the barrier potential.