Electronics

1. INTRODUCTION TO ELECTRONICS AND SEMI
CONDUCTOR THEORY
Enock Mwape Walobele
*

2. *
What is Electronics?
It is the study of flow and control of electrons and
the study of their behaviour and effects
Can also be defined as the study of active devices
and their uses.

3. *
What are active devices?
*An active device is any type of circuit component with
the ability to electrically control electric charge flow.
*Passive devices can only receive energy which it can
either dissipate, absorb or store it

4. *
Examples of active devices ;
Diodes
Transistors
Integrated circuits
Amplifiers
Transmitters

5. *
Examples of Passive devices :
oResistors
oInductors
oCapacitors
oTransformers

6. *
Rectification: The conversion of AC into DC is called rectification
Amplification; The process of raising the strength of a weak
signal is known as amplification
Control: Electronic devices find wide applications in automatic
control systems
Generation: Electronic devices can convert DC power into AC
power of any frequency. When performing this function, they are
known as oscillators

7. *
Conversion of Electricity into Light : This
valuable property is utilized in television
and radar
Conversion of light into Electricity: This
conversion of light into electricity is known
as Photo-electricity

8. *
*DIGITAL ELECTRONICS
*ANALOGUE ELECTRONICS

9. *
*Digital electronic devices produces an output of only limited
states, usually two states. This could be 1 representing high
voltage or 0 representing low voltage
*An analogue device can produce an output with infinity
number of states. Example an analogue voltage can vary
between 0 and 10.The actual value may be 1.5, 2.8 or 3.763
V.

10. *
Analogue signals are continuous eg sine wave.
Digital signals are discrete ( discontinuous)
Analogue information is translated into electric pulses by
varying the amplitude
In Digital technology, translation of information is into binary
format ( 0 and 1)

11. *
*Analogue signal is denoted by sine wave while
digital signal by square wave
*Human voice in air is an example of analogue
while CD, DVD are examples of digital devices
*Analogue signals can deteriorate and are affected
by noise while digital signals are immune to noise
*Analogue hardware is not flexiblen while digital
hardware is flexible in implementation

12. *
*Analogue signals are best used in audio and video
transmission while digital ones are best suited for
computing and digital electronics
*Analogue signal processing consumes more
bandwidth than digital processing
*Analogue signal equipment usually give
considerable observational errors ,digital meters
have no observational errors

13. *
*Produces fewer errors- it is easy to detect and correct errors in
digital transmission
*Permits higher maximum transmission rates e.g. optical fiber
cable which is designed for digital transmission
*More efficient- It is easier to send more digital data in a given
circuit
*More secure –Easy to encrypt

14. *
*All matter is composed of atoms. Atoms consist of
electrons, neutrons and protons.
*Protons are positively charged, electrons are
negatively charged, while neutrons are neutral
*Nucleus is the central part of an atom and
contains protons and neutrons

15. *
*outermost shell is known as the valence shell and electrons in
this shell are called valence electrons.
*The valence electrons which are very loosely attached to the
nucleus and move randomly within the material are known as
free electrons
*The electrons in an atom revolve around the nucleus in different
orbits or paths. The number and arrangement of electrons in any
orbit is determined by the following rules

16. *
*The number of electrons in any orbit is given by
2n2 where n is the number of the orbit. For
example
*First orbit contains 2 × 12 = 2 electrons
*Second orbit contains 2 × 22 = 8 electrons
*Third orbit contains 2 × 32 = 18 electrons
*The last orbit cannot have more than 8 electrons.
*The last but one orbit cannot have more than 18
electrons

17. *
*When electrons are further away from the nucleus the force of
attraction between them and the positive charge of the nucleus
is small. This makes it possible for them to be detached from the
atom.
*In metals electrons move randomly within the metal thus
contributing to the high conductivity of metals. Each electron
experiences a small force of attraction towards neighboring cells
since atoms are closely packed together. So electrons will be free
to move about. Such electrons are called conduction electrons

18. *
*Silicon is a semiconductor and copper is a conductor
*Bohr diagrams of the silicon atom and the copper atom are
shown in Figure 1.0. Notice that the core of the silicon atom has
a net charge of +4 (14 protons - 10 electrons) and the core of the
copper atom has a net charge of +1 (29 protons - 28 electrons).

19. *
*The valence electron in the copper atom “feels” an attractive
force of +1 compared to a valence electron in the silicon atom
which “feels” an attractive force of +4. Therefore, there is more
force trying to hold a valence electron to the atom in silicon than
in copper.
*This is the reason why a large number of valence electrons in
copper already have sufficient energy to be free electrons at
normal room temperature

20. *

21. *
*Valence band: The range of energies (i.e. band)
possessed by valence electrons is known as
valence band.
*The electrons in the outermost orbit of an atom
are known as valence electrons. In a normal atom,
valence band has the electrons of highest energy.
This band may be completely or partially filled.
For instance, in case of inert gases, the valence
band is full whereas for other materials, it is only
partially filled. The partially filled band can
accommodate more electrons

22. *
*Conduction band: In certain materials (e.g.
metals), the valence electrons are loosely
attached to the nucleus. Even at ordinary
temperature, some of the valence electrons may
get detached to become free electrons. In fact, it
is these free electrons which are responsible for
the conduction of current in a conductor. For this
reason, they are called conduction electrons.
*The range of energies (i.e. band) possessed by
conduction band electrons is known as conduction
band.

23. *

24. *
*All electrons in the conduction band are free electrons. If a substance
has empty conduction band, it means current conduction is not possible
in that substance. Generally, insulators have empty conduction band.
On the other hand, it is partially filled for conductors.
*Forbidden energy gap: The separation between conduction band and
valence band on the energy level diagram is known as forbidden
energy gap.
*No electron of a solid can stay in a forbidden energy gap as there is no
allowed energy state in this region. The width of the forbidden energy
gap is a measure of the bondage of valence electrons to the atom. The
greater the energy gap, the more tightly the valence electrons are
bound to the nucleus.

25. *
*In order to push an electron from valence band to
the conduction band (i.e. to make the valence
electron free), external energy equal to the
forbidden energy gap must be supplied.

26. *
*Solids can be classified into three groups according their electrical
conductivity. These are as mentioned below.
*An insulator - is a material that offers a very low level of conductivity
under pressure from an applied voltage source.
*A Conductor- is any material that will support a generous flow of
charge when a voltage source of limited magnitude is applied across its
terminals.
*A semiconductor- is a material that has a conductivity level somewhere
between the extremes of an insulator and a conductor

27. *
*As a rough rule, one can determine the electrical behaviour of a
material from the number of valence electrons as under:
*When the number of valence electrons of an atom is less than 4 (i.e.
half of the maximum eight electrons), the material is usually a metal
and a conductor. Examples are sodium, magnesium and aluminium
which have 1, 2 and 3 valence electrons respectively.
*When the number of valence electrons of an atom is more than 4, the
material is usually a non-metal and an insulator. Examples are
nitrogen, sulphur and neon which have 5, 6 and 8 valence electrons
respectively.
*.

28. *
*When the number of valence electrons of an atom is 4 (i.e.
exactly one-half of the maximum 8 electrons), the material has
both metal and non-metal properties and is usually a
semiconductor. Examples are carbon, silicon and germanium
*Let us look at each of these types of materials in more details.
i.e. insulators , conductors and semiconductors

29. *

30. *
*Insulators are those materials in which valence electrons are bound very
tightly to their parents atoms, thus requiring very large electric field to
remove them from the attraction of their nuclei. In other words,
insulators have no free charge carriers available with them under normal
conditions.
*In terms of energy bands, it means that insulators:
*have a full valence band.
*have an empty conduction band.
*have a large energy gap (15eV) between the valence and conduction
bands.
*at ordinary temperatures, the probability of electrons from full valence
band gaining sufficient energy so as to surmount energy gap

31. *
*and thus become available for conduction in the conduction band is
low.
*This is shown in Figure 1.2a. For conduction to take place, electrons
must be given sufficient energy to jump from the valence band to the
conduction band.
* Increase in temperature enables some electrons to go to the
conduction band which fact accounts for the negative resistance-
temperature coefficient of insulators. Compare their large energy gap
with that in the semiconductors

32. *
*(b) Conductors
*Put in a simple way, conducting materials are those in which plenty
of free electrons are available for electric conduction.
*In terms of energy bands, it means that electrical conductors are
those which have overlapping valence and conduction bands as
shown in Figure 1.2b.

33. *
*In fact, there is no physical distinction between the two
bands. Hence, the availability of a large number of
conduction electrons. Another point worth noting is that
in the absence of forbidden energy gap in good
conductors, there is no structure to establish holes. The
total current in such conductors is simply a flow of
electrons.

34. *

35. *
*(c) Semiconductors
*Materials such as Silicon (Si), Germanium (Ge) and Gallium
Arsenide (GaAs), have electrical properties somewhere between
those of a “conductor” and an “insulator”.
*They are neither good conductors nor are they good insulators
(hence their name semiconductors). They have very few “free
electrons” in their valence shell because their atoms are closely
grouped together in a tight crystalline pattern called a “crystal
lattice

36. *
*In terms of energy bands, semiconductors can be defined as those
materials which have:
*almost full valence band
*almost empty conduction band
*small energy gap (1 eV) between valence and conduction bands.
*Figure 1.2c shows the energy bands in a semiconductor material.

37. *

38. *
*An illustration of energy gap levels for various materials is shown
in figure 1.3.

39. *

40. *
*Energy is required to remove an electron from the valence band to a
higher unoccupied band, a conduction band.
*Since the valence and conduction bands overlap in metals, little
energy removes an electron. Metals are excellent conductors.
*The large gap between the valence and conduction bands of an
insulator requires high energy to remove an electron. Thus,
insulators do not conduct.
*Semiconductors have a small non-overlapping gap between the
valence and conduction bands. Pure semiconductors are neither
good insulators nor conductors

41. *
*Valence electrons: Electrons in the outermost shell of an atom.
*Free electrons: Are valence electrons which are very loosely attached
to the nucleus and move randomly within the material.
*Nucleus: is the central part of an atom which contains protons and
neutrons.
*Semiconductor: A material like germanium or silicon whose electrical
properties are between those of an insulator and a conductor.
*Insulator: is a material that offers a very low level of conductivity under
pressure from an applied voltage source.
*Conductor: is any material that will support a generous flow of charge
when a voltage source of limited magnitude is applied across its
terminals.

42. *
*What is the nucleus of an atom composed of? Define each component
*What is a valence electron?
*What is a free electron?
*What is the basic difference between conductors and insulators?
*How do semiconductors differ from conductors and insulators?
*How many valence electrons does a conductor such as copper have?
*How many valence electrons does a semiconductor have?
*Name three of the best conductive materials.

43. *
*What is electronics? Mention some important applications of
electronics.
*Describe briefly the structure of atom
*What are active electronic devices?
*What are the characteristics of a digital signal?
*What is the difference between a digital and analogue
electronics?