2. What is Semiconductor?
- Is a material which has an electrical
conductance which is between that of an
insulator and a conductor.
3. A semiconductor behaves as an
insulator at very low
temperature, and has an
appreciable electric conductance
at room temperature.
4. It can be distinguished from a
conductor by the fact that, at
absolute zero, the uppermost
filled electron energy band is fully
filled In a semiconductor, but only
partially filled in a conductor.
5. A semiconductor has a band gap which is
small enough such that its conduction band is
appreciably thermally populated with
electrons at room temperature
An insulator has a band gap which is too
wide for there to appreciable thermal
electrons in its conduction band at room
temperature.
6. Theory of Semiconductors
The operation of semiconductors is best
understood using band theory. When a large
number of atoms combine to form a
solid, the electrons e − in the solid are
distributed into energy bands among all the
atoms in the solid. Each band has a different
energy, and the electrons fill these bands
from the lowest energy to the highest, similar
to the way electrons occupy the orbitals in a
single atom.
7. The variation in properties between electrical
insulators, conductors ( metals ), and
semiconductors stems from differences in the
band structures of these materials.
8. Valence Band
- the highest energy band that contains
electrons
Conduction Band
- the lowest energy empty band
Band gap
- the difference in energy between the
valence and conduction bands
9. • In a metal, the valence band is only
partially filled with electrons (Figure 1a&b).
This means that the electrons can access
empty areas within the valence band, and
move freely across all atoms that make up
the solid. A current can therefore be
generated when a voltage is applied.
• In general, for electrons to flow in a
solid, they must be in a partially filled band or
have access to a nearby empty band.
10. •In an electrical insulator, there is no
possibility for electron flow (Figure
1d), because the valence band is completely
filled with electrons, and the conduction band
is too far away in energy to be accessed by
these electrons (the band gap is too large).
11. A semiconductor (Figure 1c) is a special case in
which the band gap is small enough that
electrons in the valence band can jump into the
conduction band using thermal energy. That
is, heat in the material (even at room
temperature) gives some of the electrons
enough energy to travel across the band gap.
Thus, an important property of semiconductors
is that their conductivity increases as they are
heated up and more electrons fill the conduction
band.
12. Fig 1. Schematic of the electronic band structures of different types of solids.
(Electrons are represented in red)
13. DOPING OF SEMICONDUCTORS
intentionallly introduces impurities into an
extremely pure (intrinsic) semiconductor fro
the purpose of modulating its electrical
properties.
14. One of the main reasons that
semiconductors are useful in electronics
is that their electronics properties can be
greatly altered in a controllable way by
adding small amounts of impurities.
15. Intrinsic & Extrinsic
Semiconductors
Intrinsic Semiconductor
- Is one that is pure enough that impurities do
not appreciably affect its electrical behavior.
Extrinsic Semiconductor
- Is one that has been doped with impurities to
modify the number and type of free charge
carriers.
16. TYPE OF DOPING
N – Type Doping
- Is to produce an abundance of mobile or
“carrier” electrons in the material.
P – Type Doping
- It is to create an abundance of holes.
17. P-n Junctions
It is maybe created by doping adjacent of the
semiconductor with p-type and n-type
dopants.
18. Fig. 2. Schematic diagram of the band structures of (a) p-type
semiconductors, (b) n-type semiconductors, & (c) a p-n junction
19.
20.
21. SEMICONDUCTORS IN ELECTRONICS
Semiconductors used in electronics perform a
variety of tasks from enabling
communication to speeding up processing.
22. • Semiconductors are used extensively in solid-
state electronic devices and computers.
•An important property of p-n junctions is that
they allow electron flow only from the n side to
the p side. Such one-way devices are called
diodes.(Figure 2c )
23. •If a positive voltage (also called a forward bias)
is applied that lowers the energy barrier
between n and p, then the electrons in the
conduction band on the n side can flow across
the junction (and holes can flow from p to n ).
•A reverse bias, however, raises the height of the
barrier and increases the charge separation at
the junction, impeding any flow of electrons
from p to n.
24. Diodes have several important applications
in electronics. The power supplied by most
electrical utilities is typically alternating current
(AC); that is, the direction of current flow switches
back and forth with a frequency of sixty cycles per
second. However, many electronic devices require
a steady flow of current in one direction (direct
current or DC).
25. Since a diode only allows current to flow
through it in one direction, it can be combined
with a capacitor to convert AC input to DC output.
For half the AC cycle, the diode passes current and
the capacitor is charged up. During the other half
of the cycle, the diode blocks any current from the
line, but current is provided to the circuit by the
capacitor. Diodes applied in this way are referred
to as rectifiers.
26.
27. REPORTERS:
Rotchil A. Casurra & Ma. Diana R. Coñado
