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