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Understanding Semiconductor Diodes
Properties of Semiconductor
Semiconductor : Material whose conductivity is between conductor and
Semiconductor behaves as an insulator at very low temperature with
appreciable electrical conductivity at room temperature but good
conductor at high temperature
Pure Element Semiconductors: Silicon, germanium, boron,
tellurium and selenium
Compound Semiconductors: Lead sulphide, gallium arsenide,
cadmium selenide and indium arsenide
A silicon atom
Typical semiconductor materials are silicon and germanium. Each
has 4 electrons in its outer orbital (outermost shell)
Each electron in the outer orbital can form a covalent bond with one
electron from outer orbital of another atom (Form four covalent
bonds with four neighbouring atoms)
When at high temperature, the thermal energy of the vibrating atoms
causes some electrons to break free
For every electron that is broken free, there is a hole in the
bonding structure. The atom is neutral, it has a complete set of
protons and electrons, the hole behaves as if it has a positive
Two types of charge carriers in a semiconductor: the electron and hole
When electron is removed from a covalent bond, it leaves a vacancy
(hole). An electron from a neighbouring atom can fill this vacancy,
leaving neighbouring atom now with a vacancy. Therefore the
vacancy (hole) can travel through the lattice and act as a charge carrier.
Doping of Semiconductor
Doping: Process of adding a certain amount of specific impurities
(dopants) to a crystalline lattice of semiconductor, to increase the
Type of semiconductors produced by doping: p-type semiconductor
and n-type semiconductor
p-type doping (Create an abundance of holes in the material)
By doping trivalent atoms such as boron, gallium or aluminium to
replace some of the silicon atoms.
One electron is missing from one of the four covalent bond
The trivalent atoms are called acceptor atoms, because they accept
any free electrons to fill the holes. When accepting an electron
from its neighbouring atom, a hole is created due to deficiency of
If more trivalent atoms are added, there will be more holes than
free electrons, hence holes are now majority carriers and free
electrons are the minority carriers
n-type semiconductor (Produce abundance of electrons)
By doping pentavalent atoms such as phosphorus, arsenic or antimony
to replace some of the silicon atoms.
Each of the pentavalent atoms will have four covalent bonds and one
extra electron. A free electron is available as a charge carrier
The pentavalent atom donates an extra electron it is therefore called the
The free electrons are the majority charge carriers and the holes are the
minority charge carriers.
Semiconductor diode is an electronic device made from a p-n
The region of p-type and n-type materials are called anode and
A diode is a device that allows current to flow in one direction only
The depletion layer is formed when there is a decrease in the free
electrons and holes in the region around the p-n junction.
The electrons from the n-type material drift across the junction to fill
in the holes in the p-type material. The holes from the p-type material
drift in the opposite direction
As a result the p-type region becomes more negative and the n-type
becomes more positive.
This result in a potential difference acting from the n-type to the p-
type region across the junction
The potential difference is called the junction voltage (it prevents
the charge carriers from drifting across the junction)
In order for electric current to flow through the diode, the voltage
applied across the diode must exceed the junction voltage
The junction voltage for germanium and silicon are approximately
0.1V and 0.6 V respectively
Ways of connection of diodes
Forward bias connection
The p-type (anode) is connected to the positive terminal and the n-
type (cathode) is connected to the negative terminal
When the potential difference supplied by the cell exceeds the
junctions power, the holes from p-type material and the electrons from
n-type material drift across the p-n junction
The depletion layer becomes narrow and the resistance of the diode
decreases, the bulbs lights up.
The n-type is connected to the positive terminal and the p-type is
connected to the negative terminal
The reversed polarity causes a very small current to flow as both
electrons and holes are pulled away from the junction
The depletion layer becomes wider. The resistance of the diode
When the potential difference due to the widen depletion region
equals the voltage of the battery, the current cease flowing
completely, therefore the bulb does not light up
Diode as a rectifier
A diode in forward bias only allows current to flow in one direction
(from anode to cathode-act as a valve)
A rectifier is an electrical device that converts alternating current to
A complete cycle of an a.c. consists of two half cycle: a positive half
cycle and a negative half cycle
Two type of rectification:
a) Half-wave rectification
b) Full-wave rectification
The current will only flow in the first half-cycle when the diode is
forward bias and is blocked in the second half, when the diode is in
Current flowing is half-wave rectified
When 4 diodes in the form of bridge rectifier (each pair allows
current flow on alternate half-cycles)
First half current flows through D1 and D3
Second half current flows through D2 and D4
Full-wave rectification is a process where both halves of every cycle
of an alternating current is made to flow in the same direction
Both half-wave and full-wave rectification do not provide a steady
direct current like battery.
A capacitor is used to smoothen the rectified output
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 acts as an reservoir and maintain
the p.d. across the load
Identify which bulb will light up in each circuit when the switches
The circuit in the figure below is a full-wave rectifier
a). Draw the symbol of diodes in the figure so that the current flow in
the direction indicated by the arrow
b). Draw the graph to show the effect of these diodes on the alternating
c). If a capacitor is connected across R, what is the effect on the
d). Is it possible to turn alternating current below into a constant direct
Current using a capacitor as shown below? Explain how this can be
c). The alternating current will be “smoothed”.
d). Yes. By using a capacitor with higher capacitance.