EEE201 Lecture 1 www.fida.com.bd

1. Lecture 1
Introduction to Semiconductor

2. Semiconductor
Semiconductors are materials with conductivity
that can be controlled through methods such as
doping or changing the temperature.
Conductivity can be increased through doping,
creating either p-type semiconductors or n-type
semiconductors.

3. Atomic Theory
• Atom is smallest piece of an element that keeps its
chemical properties
• Atom contains 3 basic particles
– Protons
– Neutrons - Form the nucleus
– Electrons – orbit around nucleus

4. Bohr models
• The major advantage of the Bohr model was that it worked. It
explained several things:
• Atomic spectra - discussed above
• Periodic behavior of elements - elements with similar properties
had similar atomic spectra.
• Each electron orbit of the same size or energy (shell) could only
hold so many electrons.
• First shell = two electrons
• Second shell = eight electrons
• Third shell and higher = eight electrons
• When one shell was filled, electrons were found at higher levels.
• Chemical properties were based on the number of electrons in
the outermost shell.
• Elements with full outer shells do not react.
• Other elements take or give up electrons to get a full outer shell.

5. Valence shell
• Outermost shell for a given atom
• Determines the conductivity of the atom
• Contains up to 8 electron
1 electron in valence shell – nearly perfect conductor
8 electron in valence shell – complete insulator
4 electron in valence shell - semiconductor

6. Covalent Bonding
• A method by which atoms complete their valence shells by
sharing valence electron with other atoms
• Covalent bond will result in a stronger bond between the
valence electrons and their parent atom (insulator)
• However, valence electrons still possible to absorb
sufficient kinetic energy from natural causes to break the
covalent bond and assume free state
• Refer figure 1.7

7. Energy Level
• There are discrete energy levels associated with each
orbiting electron
• The more distance the electron from the nucleus, the
higher energy state

8. Insulators
• Electrons tightly bound to host ion
– need large amounts of energy to break free
– very low numbers of free electrons low conductivity
– electric currents do not pass easily
e.g. paper, rubber, PVC
Conductors
• Electrons very loosely bound to host ions
– very easy to break free from ions
– free to "wander" around crystal large numbers of free electrons
– about one per atom high conductivity
– movement of electrons produces current in opposite direction
e.g. metals - Cu, Ag, Al etc
Semiconductors
• Electrons have moderate binding energies
– at absolute zero, all electrons are tightly bound insulator
– at very high temps, material can conduct conductor
– usually moderate numbers of free electrons about one per million atoms
e.g. Si, Ge, GaAs

9. Conduction in metals
• Free electrons in metal have a wide range of energies &
velocities
– behave as a "cloud" of electrons
– individual electrons wander through crystal & collide with ion
cores
– individual electrons may travel in many different directions
• No net flow of current - flow in one direction balanced by
flow in another
• Electron cloud can be accelerated by applied external
electric-field
– p.d. across the ends
– cloud moves in opposite direction to field with drift velocity vd
– constitutes an electric current in direction of field
• Can show that
V = IR (OHM's LAW)

10. Electron cloud Electric Field
Drift of electron cloud
vd
- +
I
Current flow
l Metal bar of
Cross-sectional
area A
Individual electrons
may travel in
random directions