2. Review of
Atoms are comprised of
electrons, neutrons, and
Electrons are found orbiting
the nucleus of an at atom at
specific intervals, based
upon their energy levels.
The outermost orbit is the
3. Energy Levels
Valence band electrons
are the furthest from the
nucleus and have
higher energy levels
than electrons in lower
The region beyond the
valence band is called
the conduction band.
Electrons in the
conduction band are
easily made to be free
4. Intrinsic Semiconductors
Silicon, germanium, and gallium arsenide are
the primary materials used in semiconductor
Silicon and germanium are elements and are
In pure form, silicon and germanium do not
exhibit the characteristics needed for practical
5. Semiconductor Crystals
Tetravalent atoms such as silicon, gallium
arsenide, and germanium bond together to
form a crystal or crystal lattice.
Because of the crystalline structure of
semiconductor materials, valence electrons
are shared between atoms.
This sharing of valence electrons is called
covalent bonding. Covalent bonding makes it
more difficult for materials to move their
electrons into the conduction band.
6. Electron Distribution
As more energy is applied to a
semiconductor, more electrons will move into
the conduction band and current will flow
more easily through the material.
Therefore, the resistance of intrinsic
semiconductor materials decreases with
This is a negative temperature coefficient.
7. Semiconductor Doping
Impurities are added to intrinsic
semiconductor materials to improve the
electrical properties of the material.
This process is referred to as doping and the
resulting material is called extrinsic
There are two major classifications of doping
Trivalent - aluminum, gallium, boron
Pentavalent - antimony, arsenic, phosphorous
9. Forward Biased Junction
An external source can either oppose or aid the barrier
If the positive side of the voltage is connected to the p-
type material, and the negative side to the n-type material,
then the junction is said to be forward biased.
10. Forward Biased Junction
In a forward biased junction, the following
Forward bias overcomes barrier potential.
Forward bias narrows the depletion region.
There is maximum current flow with forward
11. Reverse Biased Junction
Reverse bias occurs
when the negative
source is connected
to the p-type
material and the
positive source is
connected to the n-
Reverse bias widens
the depletion region.
Current flow is
12. Reverse Biased Junction
A reversed biased junction has zero current
Reverse current is temperature dependent.
If reverse biased is increased enough, the
reverse current increases dramatically.
This breakdown is called junction breakdown.
The voltage required to reach this point is the
reverse breakdown voltage.
As the breakdown occurs, avalanche may
occur and destroy the device if uncontrolled.
diode = “biased p-n junction”, i.e. p-n junction
with voltage applied across it
“forward biased”: p-side more positive than n-
“reverse biased”: n-side more positive than p-
14. forward biased diode:
the direction of the electric field is from p-side
⇒ p-type charge carriers (positive holes) in p-
side are pushed towards and across the p-n
n-type carriers (negative electrons) in n-side are
pushed towards and across n-p boundary
⇒ current flows
across p-n boundary
16. The Diode: a tiny P-N Junction
A diode comprises a section of N-type material
bonded to a section of P-type material, with
electrodes on each end. This arrangement
conducts electricity in only one direction.
17. When no voltage is applied, to the diode,
electrons from the N-type material fill
holes from the P-type material along the
junction between the layers, forming a
depletion zone. In a depletion zone, the
semiconductor material is returned to its
original insulating state -- all of the holes are
filled, so there are no free electrons or empty
spaces for electrons, and charge can't flow.