2. PROPERTIES OF SEMICONDUCTORS
Conductors are materials
which allow current to flow
through them easily.
Reason : conductors have
free electrons which can
drift between their atoms.
Hence, conductors have low
resistance.
Semiconductors are a group
of materials that can conduct
better than insulators but
not as good as metal
conductors
Semiconductors can be pure
element such as silicon,
germanium, boron, tellurium.
At 0 Kelvin it behaves as an
insulator. When the temperature
increases, the conductivity of
the electricity will increase
because its resistance will be
lowered.
3. IN TERMS OF RESISTANCE
Good conductors of
electricity because
they have free
electrons that can
move easily between
atoms
The resistance of
metals is generally
very low.
Poor conductors of
electricity because
they have too few
free electrons to
move about.
The resistance of
insulators is very high
4. TWO TYPE OF CHARGE CARRIERS
TYPE OF CHARGE CARRIERS
Hole
Electron
which is
negatively
charge
which is
positively
charge
5. CHARACTERISTICS OF A SILICON ATOM
Figure on the top shows
the outer electrons in a
silicon crystal which all
are involved in perfect
covalent bonds, leaving
no free electrons to
conduct electricity.
There are four electrons in
the outermost shell of a
silicon atom and they are
shared between four other
neighbouring atoms to form
four covalent bonds.
Each of the covalent bonds
has a pair of electrons. Every
atoms shares one electron
with each of its neighbours.
6. These holes
are said to
be carriers
of positive
charge
At very low temperature, pure silicon
crystal is an insulator and has a high
resistance to current flow.
As the temperature of pure silicon
crystal increases, the energy of the
vibrating atoms in the silicon crystal
causes some electrons to break free.
For every electron that is broken
free, there is a hole in the bonding
structure between the atoms of the
crystal. (atom X)
7. One outer electron from
the neighbouring atom (Y)
will fill the hole and at the
same time will produce a
hole at Y.
When the valence/outer
electron moves to the left,
the hole ‘move’ to the right
This is the physical origin of
the increase in the
electrical conductivity of
semiconductors with
temperature
8. Doping is a process of adding a small amount of impurities into
the crystalline lattice of semiconductors to increase their
conductivity.
Atoms of the impurities added should have almost the same size
as the atoms of the semiconductors.
2 types of semiconductors can be obtained :
p-type semiconductor
n-type semiconductor
DOPING
Similarities
Undergoes the
‘doping’ process
Made from
pure silicon or
germanium
Has electrons
and holes as
charge carriers
9. Boron, indium, gallium,
aluminium
Doping substance Antimony, arsenic,
phosphorus
Acceptor atom/trivalent Type/ valency of atom Donor atom/ pentavalent
Hole Majority charge carriers Electron
Electron Minority charge carriers Hole
Current flow
p-type semiconductor n-type semiconductor
10. A semiconductor diode
is also called a p-n
junction diode.
It consists of a p-
type semiconductor in
contact with an n-type
semiconductor.
The regions of p-type
and n-type materials
are called anode and
cathode respectively.
SEMICONDUCTOR DIODES
11. WHAT IS THE P-N JUNCTION?
• A p-n
junction
(depletion
layer) is
formed when a
n-type and p-
type
semiconductor
s are joined
together.
• The boundary
between the p-type
and n-type regions
is called the
junction. Holes from
the p-side similarly
move into the n-
side, where they
recombine with
electrons.
At the p-n
junction,
electrons from
the n-side
move to the p-
side and
recombine
with the holes.
• This result in a
potential difference
which is called
junction voltage that
acting from the n-
type to the p-type
material across the
junction. It is to
prevent charge
carriers from drifting
across the junction
• As a result of
this flow, the
n-side has a
net positive
charge, and
the p-side has
a net negative
charge.
12. The region around the
junction is left with neither
holes nor free electrons.
• This neutral region
which has no charge
carriers is called
the depletion layer.
• This layer which
has no charge
carrier is a poor
conductor of
electricity.
Figure shows the depletion layer and
junction voltage of a diode.
• In order for electric current to flow through the
diode, the voltage applied across the diode must
exceed the junction voltage.
• The junction voltages for germanium and silicon are
approximately 0.1V and 0.6V respectively.
13. WHAT IS FORWARD-BIASED ?
The p-type of the diode is
connected to the positive
terminal and the n-type is
connected to the negative
terminal of a battery.
The diode conducts current
because the holes from the p-
type material and electrons
from the n-type material are
able to cross over the
junction.
A light bulb will light up.
The depletion layer is narrow,
and the resistance of the
diode decreases.
14. WHAT IS REVERSE-BIASED ?
The n-type is connected to the
positive terminal and the p-type is
connected to the negative
terminal of the battery.
The reversed polarity causes a
very small current to flow as both
electrons and holes are pulled
away from the junction.
When the potential difference
due to the widen depletion region
equals the voltage of the battery,
the current will cease. Therefore
the bulb does not light up.
15. When a p-n junction diode is in a
forward-biased arrangement, it only
allows the current to flow from the
anode to the cathode. It is said to act
as a valve.
A diode can convert alternating
current into direct current. This is
known as rectification. So, a diode
can act as a rectifier.
An alternating current is a current
which changes its direction with a
certain frequency. This is due to the
alternate change of the polarity of
the power supply
DIODE AS A RECTIFIER
16. A complete cycle of alternating current consists
of 2 half cycles;
a positive half-cycle
negative half-cycle
There are 2 ways to convert an alternating
current into a direct current:
Half-wave rectification
Full-wave rectification
17. The current can only flow in the
forward direction through the
diode.
In the first half-cycle, the diode is
forward-biased. The current can
flow through the diode
In the second half-cycle, the
diode is reverse-biased. The
diode blocks the current.
The process of rectification using
a diode which allows current to
flow in the half-cycle is known as
half-wave rectification
Half-wave rectification
18. FULL-WAVE RECTIFICATION
The arrangement of
diodes in Figure 4.38 is
called a bridge
rectifier, because it
reverses the negative
half of each alternating
current cycle instead
of blocking the flow of
the current.
19. During the forward
half of each cycle,
diodes X are forward-
biased but diodes Y
are reverse-biased.
Therefore, diodes X
conduct current (solid
arrows) but diodes Y
block current.
During the reverse
half, diodes Y are
forward-biased but
diodes X are reverse-
biased. Therefore,
diodes Y conduct
current (broken arrow)
but diodes X block
current.
As a result, the
current always flows
in the same direction
through the load
regardless of which
way it leaves the
power supply.
The process of
rectification using 4
diodes to allow
current to flow in a
complete cycle and in
the same direction is
called full-wave
rectification.
20. The output from a rectifier
circuit can be smoothed by
connecting a capacitor across
the load, as shown in Figure
4.39
During the forward peaks
(positive half-cycles), the
capacitor is charged up.
Energy is stored in the
capacitor.
In between the forward
peaks(negative half-cycles),
the capacitor releases its
charge (discharges). It
discharges partly through the
load. The energy stored in the
capacitor acts as a reservoir
and maintains the potential
difference across the load.
SMOOTHING
21. Figure 4.40 shows the
potential difference
for half-wave
rectification with
smoothing.
A capacitor with
great capacitance
produces a smoother
current. This is
because the capacitor
can store more charge.
