This presentation is on Semiconductor

1. Semi-conductor Material
Presenter : Damion Lawrence

2. Review of
Basic Atomic
Model
 Atoms are comprised of
electrons, neutrons, and
protons.
 Electrons are found orbiting
the nucleus of an at atom at
specific intervals, based
upon their energy levels.
 The outermost orbit is the
valence orbit.

3. Energy Levels
 Valence band electrons
are the furthest from the
nucleus and have
higher energy levels
than electrons in lower
orbits.
 The region beyond the
valence band is called
the conduction band.
 Electrons in the
conduction band are
easily made to be free
electrons.

4. Intrinsic Semiconductors
 Silicon, germanium, and gallium arsenide are
the primary materials used in semiconductor
devices.
 Silicon and germanium are elements and are
intrinsic semiconductors.
 In pure form, silicon and germanium do not
exhibit the characteristics needed for practical
solid-state devices.

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
increasing temperature.
 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
semiconductor.
 There are two major classifications of doping
materials.
 Trivalent - aluminum, gallium, boron
 Pentavalent - antimony, arsenic, phosphorous

8. Atomic Outer Electron Shells
Silicon
Tetravalent
Boron
Trivalent
“Acceptor”
Phosphorus
Pentavalent
“Donor”

9. Forward Biased Junction
 An external source can either oppose or aid the barrier
potential.
 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
conditions exist:
 Forward bias overcomes barrier potential.
 Forward bias narrows the depletion region.
 There is maximum current flow with forward
bias.

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-
type material.
 Reverse bias
strengthens the
barrier potential.
 Reverse bias widens
the depletion region.
 Current flow is
minimum.

12. Reverse Biased Junction
 A reversed biased junction has zero current
flow (ideally).
 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.

13. DIODE
 diode = “biased p-n junction”, i.e. p-n junction
with voltage applied across it
 “forward biased”: p-side more positive than n-
side;
 “reverse biased”: n-side more positive than p-
side;

14.  forward biased diode:
 the direction of the electric field is from p-side
towards n-side
 ⇒ p-type charge carriers (positive holes) in p-
side are pushed towards and across the p-n
boundary,
 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.