41 Electronics.pptx These are form four teaching notes

Specific Objectives
By the end of this topic, the learner should be able to:
a) State the difference between conductor and insulators.
b) Define intrinsic and extrinsic conductors.
c) Explain doping in semi-conductor.
d) Explain the working of a pin junction diode.
e) Sketch current voltage characteristics for a diode.
f) Explain the application of diode rectification.

Contents
 Conductors, semiconductors insulators
 Intrinsic and extrinsic semi-conductor
 Doping
 P-n junction diode
 Application of diodes: half wave rectification and full wave rectification

Introduction
Definition – study of motion of free electrons in electrical circuits.
Applications – pocket calculators, clocks, musical instruments, radios, TVs,
computers, robots etc.
Classes of Material
Conductors – have free electrons – not tightly bound to the nucleus of the
atom e.g. copper, aluminium.
Insulators – have immobile (fixed) electrons
Semi-conductors – with conducting properties between conductors
insulators e.g. silicon, germanium.

The Energy Band Theory
When two or more atoms are brought closer to each other, the energy levels
split into smaller energy levels called bands. This is due to the interaction of
both electric and magnetic fields of electrons.
Types of bands
Conduction band – electrons are free to move under the influence of an
electric current.
Valence band – here electrons are not free to move.
Forbidden band/energy band – represents the energy level that cannot be
represented by electrons. The width of the band determines the conductivity
of the material.

Conductors, insulators and semi-conductors in terms of energy band theory
Conductors:-
 Conduction band – free electrons.
 Valence band – un filled, few electrons.
 Forbidden band/energy gap–no forbidden
band, conduction and valence band
overlap.
 Resistance increases with rise in
temperature. A rise in temperature
increases the vibrations of the atoms and
this interferes with the electron flow.
Hence the resistance of a conductor
increases with temperature.

Insulators:-
 Conduction band – has no electrons,
empty.
 Valence band – filled with electrons.
 Forbidden band – has very wide gap
 Temperature increase has no effect on
their conductivity.

.
Semi-conductors:-
Conduction band – empty at O.K. Partially
filled at room temperature.
Valence band – filled at O.K; full of electrons
at very low temperatures
Forbidden band – have very narrow gap.
Resistance reduces with rise in temperature.
Increase in temperature increases the chance
of electrons moving from the valence band to
conduction band. Electrical resistance
therefore reduces because the total current
flow is due to the flow of electrons and holes.
Have negative temperature coefficient of
resistance.

.
Note: semi–conductors
 At room temperature:- Has holes in the valence band & free electrons in
the conduction band. At OK it behaves like an insulator.
 Holes: Holes are created when an electron moves from valence band to
conduction band.
 Holes are very important for conduction of electric current in semi-
conductors.

Types of Semi-Conductors
Intrinsic semi-conductors
 They are pure semi-conductors, electrical properties of a pure substance.
 Has equal number of electrons and holes.
 Conductivity is very low, insulator at low temperatures.
 Usually not used in a pure state e.g. silicon

Extrinsic semi-conductors
 With added impurities to improve its electrical properties.
 All semi-conductors in practical use has added impurities.
Doping:- A process of adding a very small quantity of impurities to a pure
semi-conductor to obtain a desired property.
 Process of introducing an impurity atom into the lattice of a pure semi-
conductor.

Extrinsic Semi-Conductors
 Made by adding a controlled amount of different element to an intrinsic
semi-conductor.
Two types of extrinsic semi-conductors:-
 N–Type semi-conductor – formed by doping a group 4 element with a
Group 5 element.
 P–Type semi-conductor – formed by doping a group 4 element with a
group 3 element.
 Group 4 elements – Tetravalent – Silicon, germanium, etc.
 Group 5 elements – Pentavalent– doping element, donor impurity –
phosphorous, antimony.
 Group 3 elements – Trivalent – boron, aluminium and indium

N-Type Semi–Conductor
 Formed by adding a Pentavalent atom (Phosphorus) to a group 4 semi-
conductor (Silicon) and an extra electron is left unpaired and is available for
conduction.
 Majority charge carriers are electrons; minority charge carriers are positive
holes.
 Phosphorous is called a donor atom. Silicon has now more electrons

P-Type Semi–Conductor
 Formed by adding a trivalent atom (Boron) to a group 4 atom (Silicon), a
fourth electron will be unpaired and a gap will be left called a positive hole.
 Pure semi-conductor is doped with impurity of group 3 element;
combination creates a positive hole which accepts an electron.
 The doping material creates a positive hole, which can accept an electron –
called an Acceptor.

.
P-N Junction Diodes (Junction Diodes)
Definition
 An electronic device with two electrodes, which allows current to flow in
one direction only.
 It is an electrical one way valve. It is a solid device.
Formation of P-N Junction Diode
 It consists of such a p-n junction with the p-side connected to the Anode
and the n-side to the cathode.
 Formed by doping a crystal of pure silicon so that a junction is formed
between the p-type and n-type regions.

.
Depletion Layer
 The region between the p-type and n-type semiconductor which conducts
very poorly.
 At the junction electrons diffuse from both sides and neutralize each other.
Junction
 The place (boundary) between two different types of semi-conductors.
 An arrow depletion layer if formed on either side of the junction free from
charge carriers & high resistance.

.
Diagram of unbiased Junction Diode

.
Biasing
i) Forward Bias
 A diode is forward biased when the cathode is connected to n-side and
anode to the p-side in a circuit.
 In forward bias, the depletion layer is narrowed and resistance is reduced.
 It allows holes to flow to n-side and electrons to p-side.
 The majority charge carriers cross the junction. It conducts current and the
bulb lights

.
Biasing
ii) Reverse Bias
 A diode is reverse biased when the cathode
 Is connected top-side and anode to the n-side in a circuit.
 The current through the diode is virtually zero. It hardly conducts, the bulb
does not light. Electrons and holes are pulled away from the depletion
layer, making it wider.
 The electrons and holes are attracted to opposite ends of the diode away
from the junction. The wider the depletion layer, the higher the resistance
of the junction.

.
The characteristic graph of current, I against reverse bias voltage is obtained
as shown below. The curve is non-ohmic. it is non-linear. The current
increases exponentially with voltage up to a point where a sharp increase in
current is noticed. This voltage is called threshold/cut-in/break point voltage.
At this voltage potential the barrier is overcome by bias and charges easily
flow across the junction.

.
Reverse Biasing
In reverse biasing, resistance is very high, however, the flow of leakage
current results from flow of minority charge carriers. At breakdown voltage
or Zener break down covalent bonds rapture liberating electrons. Those
electrons collide with some atoms causing ionisation this is called avalanche
breakdown. The two processes produce excess electrons for heavy
conduction. Beyond breakdown voltage a diode is damaged.

.
The Zener Diode
Definition
 A zener Diode is a silicon p-n semi-conductor, which is designed to work in
reverse biased connection.
Principle of operation
When the reverse-bias of the diode is increased, a large sudden increase in
current is obtained at one particular reverse voltage.
At the reverse voltage, the p-n junction diode breaks down into a
conductor, by breaking down the barrier layer.
The break down of the p-n junction diode is known as zener break down
or zener effect.
The characteristic is almost a vertical line, i.e. the zener current, which
occur as a result of the zener voltage.

.
Application of zener Diodes
 Used in industry as voltage regulators or stabilizers, by providing a constant
voltage to a load.
 Voltage remains constant as current increases.
Application of p-n Junction Diodes
a) To protect equipment, circuits or devices by a reverse power supply.
b) To rectify ac to dc
c) Enable the Audio Frequency energy carrier by modulated radio waves to
be detected.

.
Rectification and Smoothing
A) Definition
 Rectification is the process of converting a.c current to d.c current.
 A Rectifier is a device that changes a.c to d.c.
b)Reasons for rectification
 The conversion of a.c. to d.c.is often necessary for all electric equipment,
such as radios, T.V. sets, computers, musical instruments etc., which use
steady d.c.
Types of rectification
There are two types of rectification, namely:-
 Half-wave rectification
 Full-wave rectification.

.
Half-wave rectification and smoothing
 One diode is used which removes the negative half-wave cycle of the
applied a.c.
 It gives a varying but one-way direct current across the load R. R is a piece
of electronic equipment requiring a d.c. supply.
 If the Y-input terminals of a CRO are connected first across the input, the
wave form on the left will be displayed on the screen.
 When a CRO is connected across R, the output wave form is seen to be
positive half-wave of the a.c.

.
 Smoothing is done using a capacitor connected across R, to give a much
steadier varying d.c. supply.
 The smoothing capacitor provides extra charge so that current flows
continuously even as the phase current changes and the current go to zero.
 The larger the capacitor, the better the smoothing.
 On the positive half-cycle of the a.c. input the diode conducts, current
passes through R and also into the capacitor C to charge it up.
 On the negative half- cycle, the diode is reversing biased and cannot
conduct, but C partly discharges through R.

.
 The charge-storing action of the capacitor, C thus maintains current in R
and a steadier p.d across it when the diode is not conducting.
Note:-A single diode only allow half of the a.c. to flow through the load R, so
far half of the power supply is cutoff.

.
Full-wave Rectification and smoothing rectification and smoothing
 There are two methods for obtaining a full-wave rectification namely:-
 Using two diodes – Full-wave centre-tap transformer.
 Using four diodes – Full-wave bridge rectifier
Using Centre-Tap Transformer
 In a full-wave rectifier, both halves of the a.c. cycles are transmitted but in
the direction, i.e. same side.

.
or
 During the first half-cycle, when A is positive, D1 conducts through the load
R. at the same time B is negative with respect to T, so no current flows in
the diode D2.
 In the next half-cycle when B is positive, D2 conducts through the load R in
the same direction as before. A is positive with respect to T so no current
flows in D1.

.
Using the bridge Rectifier – four diodes
 In the 1st
half-cycle, diode D2 andD4 conducts.
 In the 2nd
half-cycle, diode D3 and D1conducts.
 During both cycles, current passes through R in the same direction, giving a
p.d. that varies as shown by the CRO.
 When a large capacitor is connected across R, the output d.c. is smoothed
as shown.
D1
D2
a.c. RL
A
D3
D4
B
C
D
R

.
 During the first half cycle, point A is positive with respect to C, diode D1
and D3 are forward biased while diodeD2 and D4 are reverse biased.
Current therefore flows through ABDCA. During the second half cycle, point
A becomes negative with respect to point C. diodes D2 andD4 become
forward biased while D1 and D3 are reverse biased. Conventional current
therefore flows through CBDAC.
 If a capacitor is connected across the resistor, the rectified output is
smoothened.
D1
D2
a.c. RL
A
D3
D4
B
C
D
R C

.
Advantages of bridge rectifier
 A smaller transformer can be used because there is no need for centre-
tapping.
 It is used for high voltage regulation.

.
 .

Questions
1. Draw the structure of a crystal lattice to show the arrangement of
electrons in following:
• Pure silicon.
• P-type semiconductors
• N-type semiconductors
2. Explain how temperatures rise affects the electrical conductivity or pure
semiconductors.
(a) Draw the symbol of a p-n diagram junction diode.
(b) Use a circuit diagram to distinguish between forward and reserve bias
of p-n junction diode.
3. (a) Use a labelled diagram to explain how a full valve rectification may be
achieved by using a resistor and: (i) Two diodes. (ii) Four diodes.

Questions
4. With the aid of a diagram explain how a capacitor can be used to
smoothen a full wave which has been rectified. Show using a sketch how
the smoothened wave will appear on the screen of C.R.O.
5. What is meant by the following terms: semiconductor, intrinsic
conduction, extrinsic conduction, doping, donor atoms, acceptor atoms,
n-type semiconductor, p-type semiconductor, depletion layer, forward
bias, hole, reverse bias and Zener effect?
6. Explain how doping produces a p-type and an n-type semiconductor.
7. Distinguish between electronics and electricity.
8. a) What is rectification?
(b)With diagrams, describe how half-wave and full-wave rectification can
be achieved.
9. Explain why a diode conducts easily on forward bias and not in reverse
bias.

Reference
Oliver Minish etal, Secondary Physics, Students Book Four, 4th
edition,
KLB, Nairobi, 2017 Page 166 - 186

41 Electronics.pptx These are form four teaching notes

More Related Content

Similar to 41 Electronics.pptx These are form four teaching notes (20)

More from biezcome (7)

Recently uploaded (20)

41 Electronics.pptx These are form four teaching notes