Electronics  
Is a branch of physics that deals with the emission, behavior and  
effects of electrons in materials  
OR  
Is the study of how to control the flow of electrons  
NB:  
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The various electronic components connected to each other to form systems or  
circuits called Electronic systems (Electronic circuits)  
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An electronic circuit is used to perform a wide variety of tasks. The main uses of  
electronic circuits are:  
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Conversion (ac to dc) and distribution of electric power.  
Controlling and processing of data  
Electronic Component  
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Is any basic discrete device or physical entity in an electronic system used to  
affect electrons or their associated fields  
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For example power sources  
integrated circuits etc.  
,
resistors  
,
capacitors  
,
diodes  
,
transistors, and  
Types of Electronic Components  
(a) Passive Electronic Components  
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Are electronic components that consume energy but do not produce energy  
These include power sources (battery or generator), resistors, capacitors and inductors.  
(b) Active Electronic Components  
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Are electronic components that consume energy in the form of voltage or  
current and supply energy in the form of voltage or current.  
They include semiconductor devices such as diodes transistors and integrated circuits  
,
Insulators, Conductors and Semiconductors  
An Insulator  
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I
s a material which resists the flow of electrical charges through it.  
Insulator has infinite resistance and zero conductance.  
For example glass  
,
mica  
,
paraffin  
,
hard rubber and also many plastics  
Why resists the flow of electrical charges?  
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It has no free electrons which are responsible to pass through electrical charges  
The atoms have tightly bound electrons  
Conductor  
:
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Is a material which allows the flow of electrical charges through it  
For example all metals and some non-metals such as graphite (carbon)  
A
semiconductor  
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Is a material in which its electrical conductivity intermediate between that of  
conductor and insulator.  
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For example silicon  
A semiconductor behaves as an insulator at very low temperature  
Has a significant electrical conductance at room temperature, however, much  
lower than that of a conductor  
,
germanium  
,
cadmium sulphide and gallium arsenide  
Band theory  
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Is the theory which explains about energy levels in a solid in terms of energy band  
Energy Band  
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Is a collection of closely spaced energy levels  
OR  
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Is the series of “allowed” and “forbidden” energy bands that it contains  
Diagram  
:
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We have about three bands in which a band electrons possesses energy called  
energy level  
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Conduction band  
Band gap (forbidden band)  
Valence band  
Conduction Band  
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Is the upper most part of semiconductor in which there is few or no electrons  
It is sufficient to make the electrons free to accelerate under the influence of an  
applied electric field and thus constitute an electric current  
N.B  
In conductors some electrons occupy the conduction band  
Band Gap (Forbidden energy gap)  
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Is the energy gap between the valence band and conduction band which cannot  
be occupied by electrons  
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(OR Is the energy required to shift an electron from valence band to conduction band)  
Valence Band  
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Is the lower part of semiconductor in which there is completely filled with electrons  
The valence band is the highest range of electron energies where electrons are  
normally present at the absolute zero temperature  
Forbidden energy gap (Fermi energy level).  
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Is the energy gap between the valence band and conduction band which cannot  
be occupied by electrons  
The band obtained by separating conduction band and valence band is called  
forbidden energy band or forbidden gap  
.
The figure below shows the conductor, semiconductor and insulator in terms of  
their energy level (band gap)  
For conductor  
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There is no forbidden gap available, the valence and conduction band overlap  
each other (figure a)  
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The electrons from valence band freely enter into conduction band  
Due to the overlapping of the valence and conduction bands, a very low  
potential difference can cause the continuous flow of current  
For semiconductor  
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The forbidden gap is very small (fig b)  
There are no electrons in the conduction band. The valence band is completely filled at 0 K  
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With a small amount of energy that is supplied, the electrons can easily jump  
from the valence band to the conduction band  
For example, if the temperature is raised, the forbidden gap is decreased and  
some electrons are liberated into the conduction band  
Germanium and Silicon are the best examples of semiconductors with  
forbidden energy gap of 0.7 eV and 1.1 eV respectively  
For insulator  
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The forbidden energy gap is very large (fig c). It is more than 3 eV and almost no  
electrons are available for conduction  
Therefore, a very large amount of energy must be supplied to a valence  
electron to enable it to move to the conduction band  
If the electron is supplied with high energy, it can jump across the forbidden  
gap. When the temperature is increased, some electrons will move to the  
conduction band. This is the reason, why certain materials, which are  
insulators at room temperature become conductors at high temperature  
Difference between conductor, insulator and semiconductor  
Conductor  
Insulator  
Semiconductor  
The conductivity of  
conductor is very high.  
It has very low resistivity.  
The conductivity of  
insulator is very low.  
It has very high resistivity.  
The conductivity of  
semiconductor is moderate.  
It has moderate resistivity.  
It has no forbidden gap.  
It has large forbidden gap.  
It has small forbidden gap.  
Has positive temperature  
coefficient of resistance.  
Has negative temperature  
coefficient of resistance.  
Has negative temperature  
coefficient of resistance  
Both the effect of resistance  
and temperature are  
increasing  
The effect of resistance  
decreases with the increase  
of temperature  
The effect of resistance  
decreases with the of  
temperature increases.  
There is large number of  
electrons available for  
conduction.  
There is small number of  
electrons available for  
conduction.  
There is moderate number of  
electrons available for  
conduction.  
Examples  
:
are Metals  
Paper, Wood, Mica glass.  
Silicon, Germanium.  
(aluminium, copper.etc)  
How Semiconductor Conducts Electricity  
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As the temperature is increased, some of the electrons in the valence band  
acquire thermal energy that is greater than the forbidden gap energy and move  
to the conduction band. Therefore, the material becomes a conductor. When  
an electron moves out of a valence band it leaves behind a small space called  
a
hole. Electrons and holes in the conduction and valence bands, respectively,  
are referred to as free charge carriers  
.
Effect of temperature on metal conductivity  
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Increase in temperature tends to increase the random motion of electrons. It  
reduces the electrical conductivity of metals  
Types of Semiconductors  
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Intrinsic semiconductors  
Extrinsic semiconductors  
Intrinsic Semiconductors  
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These are pure semiconductors in which there is no addition of impurities.  
Examples are silicon and germanium  
Conductivity in intrinsic semiconductors is limited hence ,they do not conduct electricity  
Extrinsic Semiconductors  
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These are impure semiconductors materials which contains added impurities  
Examples are N – Silicon , N – Germanium P – Silicon and P – Germanium  
Difference between intrinsic from extrinsic semi-conductor  
intrinsic  
extrinsic  
Is the pure form of semi-conductor  
Is an impure form of semiconductor  
It has equal number of holes and  
electrons in conduction and valence  
band respectively  
It has unequal number of holes and  
electron  
Its electrical conductivity depends on both  
temperature and amount of doping  
Its electrical conductivity depends on  
temperature only  
It has low conductivity  
It has high conductivity  
It is of no practical use  
It is used in electronic devices  
Doping  
Is the process of adding impurities to intrinsic semiconductors to alter their  
properties  
OR  
OR  
Is process of adding impurity atoms to intrinsic crystal to produce an  
extrinsic semiconductor.  
Is the process of adding impurities in a pure semiconductor in order to  
increase electrical conductivity  
Terms used In Doping  
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Hosts are atoms which can accept or donate an electron. Example All group IV  
elements (Tetravalent) ie Silicon and Germanium  
Acceptor atoms are atoms which receive electrons from other atoms.  
Example all group III elements (Trivalent)  
Donor atoms are atoms which supply electrons to other atoms. Example all  
group V elements (Pentavalent)  
Dopant is the element/impurity which added to modify the conductivity of an atom  
NB:  
o
Heavily doping a semiconductor increases its conductivity. That is why heavily  
doped silicon is often used as a replacement for metals  
o
Silicon and Germanium are the best semiconductors as they are used to  
make the most common electronic devices/components such as transistors  
and diodes (This is because the energy required to break their covalent bonds  
is very small ie 0.7 eV for Ge and 1.1 eV for Si)  
Types of Doped Semiconductor (Extrinsic semiconductor)  
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N-type semiconductor  
P-type semiconductor  
N-type Semiconductor  
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Is the type of semiconductor in which the majority carriers are electrons  
Is formed when pure semiconductors are doped with pentavalent elements  
The purpose of n-type doping is to produce an abundance of mobile or carrier  
electrons in the material  
Mechanism of Doping  
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Consider the silicon with four valences (with four electrons in their outer  
most shell) combine with dopant of more than four electrons they will share  
the four valences results the extra electrons from dopant (group V) remaining  
as extra (free electrons). This extra electron is only weakly bound to the atom and  
can easily be excited into the conduction band, since the silicon atoms with five  
valence atoms have an extra electron to “donate”, they are called donor atoms  
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Diagram of silicon after doping (n-doping with Antimony, Sb)  
[
P-type Semiconductor  
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Is the type of semiconductor in which the majority carriers are holes  
Is formed when pure semiconductors are doped with trivalent elements  
The purpose of p-type doping is to produce an abundance of holes in the  
valence band.  
Mechanism of Doping  
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Consider the silicon with four valences (with four electrons in their outer  
most shell) combine with Dopant (group III) of less than four electrons in  
their outer most shell they will share the three electrons results the semiconductor  
with less electrons (holes) to attain stability, since the silicon atoms with three  
valence atoms have a less electron to “acceptor”, they are called acceptor atoms  
•
Diagram of silicon after doping (P-doping with boron)  
COMPARISON BETWEEN N – TYPE AND P – TYPE SEMICONDUCTOR  
N – TYPE  
P – TYPE  
Produced by adding pentavalent  
impurities to a pure semiconductor.  
Produced by adding trivalent impurities to  
a pure semiconductor.  
The number of free electrons exceed the The number of holes exceeds the number  
number of holes.  
of free electrons.  
The majority charges are negative charges.  
The majority charges are positive charges.  
The donor energy level is just below the  
bottom of the conduction band.  
The acceptor energy level is just above  
the valence band.  
JUNCTION DIODE  
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This is the p–n junction semiconductor material which is connected to supply  
voltage.  
P–N Junction  
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This is the junction made up by two semiconductor material of n – type and p –  
type melted together to form a junction.  
The boundary (junction formed) between the p – side and n – side is referred to as a  
p – n junction  
Terms used in P-N Junction  
1. Diffusion of charge is the spreading out of charges (holes and electrons) which  
can result repelling and attraction of charge  
2. Potential barrier is the maximum voltage at the junction when there is no further  
diffusion of charge  
3. Depletion layer  
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Is a region in a P–N junction diode where no mobile charge carriers are present  
It acts like a barrier that opposes the flow of electrons from n – side and  
holes from p – side  
Biasing of the P – N Junction  
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A
p–n junction is said to be biased when a potential difference is applied across it  
When a P- N junction is connected to a power supply it is said to be biased  
A P- N junction allows current to flow only in one direction when the p – side is  
connected to the positive terminal of the power source and n – side to the  
negative terminal of the power source  
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There are two modes of action of P-N junction, these are  
(a) Forward – bias  
(b) Reverse- bias  
(a) Forward- Bias of P-N Junction  
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A p–n junction is said to be forward biased when the p–type region is  
connected to the positive terminal while the n–type region is connected to the  
negative terminal of an external cell or battery  
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In forward bias, the positive charge applied to the p–region repels the holes while  
the negative charge applied to the n–type repels the electrons .As the electrons  
and holes are pushed toward the junction, the distance between them decreases  
This reduces the size of depletion layer and lowers the potential barrier  
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Therefore the charge carriers interact easily and makes the flow of an electric  
current possible  
The graph of voltage against current for forward is given below  
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When the voltage of the battery is greater than potential barrier majority charge  
carries (holes and electrons) are pulled towards and large electric current flowing  
(b) Reverse - Bias in P-N Junction  
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A p–n junction is said to be reverse biased when the p–region is connected to the  
negative terminal of the cell or battery while the n–region is connected to the  
positive terminal of the battery  
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When the diode is connected in this manner, the holes in the p-type are attracted  
away from the junction by the external negative potential. Also electrons are  
attracted away from the junction by the external positive potential. This increases  
the thickness of the depletion layer .Thus the potential barrier and hence the  
resistance of the junction is increased .A very small current (leakage current) may  
flow in the circuit due to the flow of minority charge carries.  
The graph of voltage against current for reverse bias is shown from the fig below  
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When the voltage of the battery is greater than barrier potential majority charge carries  
(holes and electrons)are pushed away and very small or no electric current is flowing  
N.B  
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Potential barrier is the potential required to overcome the barrier at the PN junction  
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Zener  
/Break down voltage Is the reverse Voltage at which p-n junction breaks  
down with the sudden rise in reverse current.  
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Knee voltage Is the forward biased voltage at which the current through the junction  
starts to increase rapidly.  
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Reverse (leakage) current Is the current in a semiconductor device when the  
device is reverse biased  
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Saturation current (Scale current  
current in a semiconductor diode caused by diffusion of minority carriers from  
the neutral regions to the depletion layer  
)
Is that part of reverse current of the reverse  
Diodes  
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A diode is an electrical device that allows current to flow through it in one direction.  
NB:  
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When the junction is reverse-biased, the diode blocks the voltage  
When the junction is forward-biased, the diode conducts  
The magnitude of the current through the diode depends on the current in the  
external circuit  
Types of Diode  
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Semiconductor diode  
Metal semiconductor diode  
Light-emitting diode  
Zener Diode  
Semiconductor Diode  
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Most semiconductor diodes are made up of silicon or germanium.  
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Semiconductor diodes are most used for rectification  
Metal Semiconductor Diode  
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These types of diodes are formed by the deposition of a metal on the surface of a  
metal conductor.  
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The metal-semiconductor diode is used for very fast switching and microwave  
applications.  
Light-Emitting Diode (LED)  
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Is a semiconductor diode that emits light when an electrical current is applied in  
the forward direction of the diode  
NB:  
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LEDs are made from a variety of semiconductor materials depending on the  
wavelength of the light required  
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The most commonly used materials for visible LEDs are gallium phosphide and  
gallium arsenic phosphide  
LEDs have a wide range of applications, from simple indicator lamps and huge  
display screens to optical fiber communication links  
Zener Diode  
➢
Zener diodes are specifically manufactured and designed to be operated in the  
reverse breakdown voltage.  
➢
Every Zener diode is manufactured for a specific reverse breakdown voltage  
called the Zener voltage  
.
Its symbol:  
NB:  
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Zener diodes are used as voltage regulator devices.  
It allows required voltage to pass through  
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Advantage of semiconductor junction diode over vacuum tube diodes  
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They are less expensive to make  
They consume less power  
They are reliable in circuits  
They are much easier to produce  
They occupy less space due to their small size  
APPLICATIONS OF JUNCTION DIODES IN RECTIFICATION  
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A rectifier Is an electrical device used to convert an alternating current into a  
direct current by allowing a current to flow through it in one direction only  
OR  
Is a device that is used to convert alternating voltage into a direct  
(unidirectional) voltage  
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Rectification Is the process of converting alternating current to direct current  
OR  
Is the process of conversion of alternating voltage to direct voltage  
Diodes are used in rectification because they offer high resistance when reverse  
biased and low resistance when forward biased  
There are two types of rectification  
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Half-wave rectifiers  
Full -wave rectifiers  
Half-Wave Rectification  
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The half wave rectification is achieved by connecting a single diode in series  
with the load across which a unidirectional voltage is required  
Mechanism  
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During the first half-cycle of the AC sine wave, A is positive and B is negative.  
The diode is forward-biased and current flows around the circuit formed by the  
diode, the transformer winding and the load (R)  
✓
During the second half-cycle, A is negative and, B is positive. The diode is  
reverse-biased therefore no current flows in the circuit  
NB:  
o
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The output signal can be displayed on a CRO screen which outlines the above trace  
The output voltage of half wave rectification flows in pulse (half rectified) because  
the diode allows current to flow during the first half of the cycle when it is forward  
biased and stops the current during the second half when it is reversed biased  
o
o
o
o
The diode conducts on every half- cycle  
The rectified voltage is d.c and is always positive in value  
If the diode is reversed, then the output voltage is negative  
The voltage is not steady and needs to be smoothed (by putting a large  
capacitor, C in parallel with the load) for it to be useful (see fig below)  
o
The capacitor is charged during the positive half-cycle of the a.c. and discharges  
through the load in the negative half-cycle  
Advantages of half wave rectification  
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Low cost of construction, since it includes few components  
Easy to constructs