Basic Electronics

Introduction

We know that earlier electronic devices were made up of vacuum tubes or valves

Vacuum tubes or Valves:

The evolution of vacuum tubes started with diode and proceeded to triode, tetrode and pentode
Valves control the flow of electrons; In diodes, there were two electrodes – cathode and anode
Similarly, triode had three electrodes – cathode, grid and anode; tetrode had four electrodes – anode, two grids and cathode; pentode had five electrodes –anode, three grids and cathode
Generally in vacuum tubes, the electrons are produced by heating the cathode using low tension battery; The vacuum helps in electron not losing its energy by collision with air molecules in the way
However, the vacuum tube devices had some disadvantages and they are
- Bulky
- Operate at high voltages
- Consume more power
- Have limited life
- Low reliability

Diodes and transistors

Next came the discovery of semiconductor junction, namely, junction diode and transistors
These replaced the vacuum tubes or valves

The advantages of semiconductor devices are
- Small in size
- Operate at low voltages
- Consume small power
- Long life
- High reliability
Though the above advantages are present, the circuits consisting of transistors were still bulky, less shock proof
Hence, it led to the discovery of integrated circuits which is a major revolution in the electronic industry

Example – The earlier generation television and computer monitors were very bulky as they were based on the principle of vacuum tubes; now days, we have LCD (Liquid Crystal Display) monitors which support solid state electronics

Classification of metals, semiconductors and insulators

On the basis of conductivity

The conductivity of a material indicates whether they can be classified as metals, semiconductors or insulators

The electrical conductivity is expressed as σ whereas the reciprocal of conductivity is resistivity ρ = 1/σ

Metals – Metals have high conductivity and low resistivity ; ρ is of order10-2 to 10-8 Ω m ; σ is of order 102 to 108 S/m

Semiconductors – They have conductivity and resistivity in between metals and insulators ; ρ is of order 10-5 to 10-6 Ω m ; σ is of order 105 to 106 S/m

Insulators – They have high resistivity and hence low conductivity ; ρ is of order 10-11 to 10-19 Ω m ; σ is of order 102 to 1019 S/m

Classification of semiconductors

The semiconductors can be classified into Elementary type semiconductor and compound type semiconductor

Elementary type semiconductor – These type of semiconductors are available in natural form like Silicon (Si) and germanium (Ge)
Compound type semiconductor – When semiconductors are made by compounding the metals, we get compound type semiconductor. They can be further classified into

Inorganic semiconductors like CdS, GaAs etc
Organic semiconductors like Antracene, doped pthalocyanines
Organic polymers – Polypyrole, polyaniline, polythiophene

Band theory of solids

In a substance, as many atoms are close to each other, the energy levels of the atom form a continuous band, where in the electrons move. This is called band theory of solids.

We know that in an atom, the protons and the neutrons constitute the central part called the nucleus
The electrons revolve around the nucleus in defined orbits
The orbits are named as 1s, 2s, 2p, 3s, 3p, 3d etc. each of which has a discrete energy level
All electrons in the same orbit have the same energy
The electrons in the innermost orbits which are completely filled constitute the valence electrons whereas the electrons in the outermost orbit which do not completely fill that shell are called conduction electrons
As seen in the diagram below, both Si and Ge have 4 electrons in the outermost shell

When in the crystal, the atoms are close to each other and hence they may be flow of electrons from one atom to another in the conduction band
Let us discuss in detail by considering interatomic distance in the X-axis and energy in the Y-axis:
As seen in the diagram below, the graph is divided into 4 regions – Region A, B, C and D
In the region A, the interatomic distance is large between atoms and in region D, the interatomic distance is small

Region A

Consider that the Si or Ge crystal contains N atoms. Electrons of each atom will have discrete energies in different orbits
If the atoms are isolated, that is, separated from each other by a large distance, the electron energy will be the same
However, in a crystal, the atoms are close to each other separated by a distance of 2-3 Ao. Hence, electrons interact with each other and also with the neighbouring atoms
The overlap or the interaction will be felt more by the electrons in the outermost orbit while the inner electron energies will remain unaffected
Hence, in the case of Si and Ge crystals, we need to consider the changes in energies of electron in the outer most orbit only
For Si, the outermost orbit is the third orbit (n = 3) while for Ge, the outermost orbit is fourth orbit (n = 4)
The number of electrons in both cases is 4 – namely 2s and 2p. Hence, the outer electrons in the crystal is 4
The maximum possible number of outer electrons in the orbit is 8 (2s + 6p electrons)
This is the case of well separated or isolated atoms as shown in region A

Region B

Suppose the atoms start coming nearer to each other to form a solid.
The energies of the electrons in the outermost orbit may increase or decrease, due to the interaction between electrons of different atoms
The 6N states for l=1, which originally had identical energies in the isolated atoms, spread out and form an energy band as shown in the region B
Similarly, the 2N states for l = 0 split into a second band separated from the first one

Region C

At still smaller spacing, however, there comes a region in which the bands merge with each other
The lowest energy state that is a split from the upper atomic level appears to drop below the upper state that has come from the lower atomic level
In this region, no energy gap exists where the upper and the lower energy states gets mixed

Region D

If the distance between the atom further decreases, the energy bands again split apart and are separated by an energy gap Eg
The total number of available energy states 8N has been re-apportioned between the two bands (4N states each in the lower and upper energy bands)
Here there are exactly as many states in the lower band (4N) as there are available valence electrons from the atoms (4N)
This lower band called the valence band is completely filled while the upper band is completely empty. The upper band is called the conduction band

Classification on the basis of energy bands

Depending upon the relative position of the valence band and the conduction band, the solids can be classified into conductors, insulators and semiconductors

Conductors

The conduction band and the valence band partly overlop each other and there is no forbidden energy band gap in between
The electrons from the valence band can easily move into the conduction band
Hence, large number of electrons are available for conduction
The resistance of such materials is low and conductivity is high

Insulators

In case of insulators, a large energy gap exists between the valence band and the conduction band
The energy gap is so high that the electrons from the valence band cannot move to the conduction band by thermal excitation
As there is no electrons in the conduction band, electrical conduction is not possible

Semiconductors

A finite but a small energy gap exists between the valence band and the conduction band
At room temperature, some of the electrons from the valence band acquire energy and move into the conduction band
Hence, at high temperature, semiconductors have conductivity and resistance is also not as high as insulators

Types of semiconductors

There are two types – Intrinsic semiconductor and Extrinsic semiconductor

Intrinsic semiconductor

A pure semiconductor, free from impurities is called intrinsic semiconductor
- The electrical conductivity of pure semiconductor is called intrinsic conductivity
- Structure – Consider pure Germanium and Silicon. Both have 4 valence electrons

Crystalline structure

At temperature 0K –

In the crystal structure, the four valence electron of the Ge atom forms four covalent bond by sharing of electron with the neighbouring atoms
Each covalent bond is made of two atoms, each one from each atom
By forming covalent bond, each Ge atom in the crystal behaves as if the outermost orbit of each atom is complete with 8 electrons, having no free electrons in the crystal

At room temperature

The conduction is possible if the electrons break away from the covalent bonds and are free by the thermal energy
When electron breaks away from the covalent bond, the empty space or vacancy left in the bond is called a hole
An electron from the neighbouring atom can break away and can be attracted by the hole, creating hole in the other place
In the crystal structure, thus, we can see, electrons break the covalent bond and keep moving. Similarly, due to attraction of hole and electron, hole also keeps moving in a crystal
Thus, breakage of a covalent bond produces one free electron and one hole in the crystal
In an intrinsic semiconductor, the number of holes = number of electrons. Thus ne = nh = ni

Energy band theory

There is an energy gap of about 1 eV between the valence and the conduction band

At temperature 0K –

In terms of energy band theory, the valence band is full and the conduction band is totally empty
As no electrons are available for conduction, the Ge crystal behaves like a electrical insulator

At room temperature -

The thermal vibrations of the atoms provide energy to the electrons in the valence band to cross the energy gap and move into the conduction band as free electrons
This results in electrical conductivity of the semiconductor
As electrons move from the valence band to the conduction band, a vacancy is created in the valence band. This vacancy is called a hole
As electrons move in the conduction band, the holes move in the valence band and electrical conduction in semiconductors is possible

At a higher temperature, when electric field is applied, the holes move towards the negative potential, giving rise to hole current and electrons move towards the positive potential giving rise to electron current. Thus I = Ie + Ih

Extrinsic semiconductor

A doped semiconductor or a semiconductor with suitable impurity added to it is called extrinsic semiconductor
There are two types – n-type and p-type
In n-type semiconductor, the electrons are the majority carriers while the holes are the minority carriers
In p-type semiconductor, the holes are the majority carriers while the electrons are the minority carriers
A detailed study of n-type and p-type semiconductors, help us to understand better about the extrinsic semiconductor

n-type semiconductor

When pure Si or Ge which has four valency electrons is doped with controlled amount of pentavalent atoms, like Arsenic, Phosphorus, Antimony or Bismuth, we get a n-type semiconductor
The four valence electron from the impure atom will combine with four electrons of the Si or Ge atom to form 4 covalent bonds
The fifth electron of the impure atom is free to move. Thus, each atom of the impure substance, donates a free electron for conduction. Hence, it is called as donor atom
Giving the free electron for conduction, the impure atom becomes positively charged, giving rise to a hole
Thus in n-type semiconductors, electrons are the majority carriers and holes are minority carriers

Energy band theory

Comparing Si or Ge doped with impurities like Arsenic with a pure Si or Ge, the lowest energy level of the conduction band is less
The electrons occupy discrete energy levels called the donor energy level between the valence band and the conduction band
This donor energy level is below the bottom of the conduction band
Thus, very small energy supplied can excite the electron from the donor level to the conduction band, hence, conductivity of semiconductor becomes remarkably improved

p-type semiconductor

When pure Si or Ge which has four valency electrons is doped with controlled amount of trivalent atoms, like Gallium, Indium, Boron or Aluminium, we get a p-type semiconductor
The three valence electron from the impure atom will combine with three electrons of the Si or Ge atom to form 3 covalent bonds
There will be one unbounded electron in the Ge atom which would try to form a covalent bond with the neighbouring Ge atom
This Ge-Ge covalent bond creates a deficiency of electron in Ge atom. Thus, creating a hole
This hole is compensated by the breakage of Ge-Ge covalent bond in the neighbourhood. Hence, electron moves towards the hole, resulting in hole formation at some other place
The trivalent atoms are called acceptor atoms and conduction of electricity is due to the motion of holes
Thus in p-type semiconductors, holes are the majority carriers and electrons are minority carriers

Energy band theory

Si or Ge doped with impurities like Aluminium, produces energy level which is situated in the energy gap slightly above the valence band
This is called as acceptor energy level
At room temperature, the electrons in the valence band can easily be transferred to the acceptor level. This produces a large number of holes in the valence band.
The valence band becomes the hole conducting band

p-n junction formation

A p-n junction is the basic building block of many semiconductor devices like diodes, transistor etc.

Consider a thin p-type silicon semiconductor wafer. Convert a part of the p-type semiconductor into n-type silicon semiconductor by adding a small quantity of pentavalent impurity
The holes are the majority carriers in the p-type semiconductor and electrons are the majority carriers in the n-type semiconductor

In n-type semiconductor, the concentration of electrons is more compared to the concentration of holes. Similarly, in p-type semiconductor, the concentration of holes is more compared to the concentration of electrons
The first process that occurs in the p-n semiconductor is diffusion
In the formation of the p-n junction, due to the concentration gradient across the p and the n sides, the electrons diffuse from n region to p region and the holes diffuse from p region to n region

Diffusion current –
- The motion of charge carriers due to the difference in concentration in two regions of the p-n junction, across the junction gives rise to diffusion current
As the diffusion continues, it leaves behind a positive charge on the n-side close to the junction. This positive charge, also called as ionised donor is immobile due to bonding with the surrounding atoms
Similarly, in the p-region, close to the junction, there is a negative charge or acceptor ions which are immobile

Depletion region formation –
- The space charge region on both sides of the p-n junction has immobile ions and is also devoid of any charge carriers
- This results in the formation of depletion region near the junction
Field setup –
- The depletion region formation results in setting up a field at the junction
- The field set up along the junction acts like a fictitious battery connected across the junction with positive terminal connected to the n-region

Barrier creation –
- The electric field at the junction sets a barrier which opposes further diffusion of majority charge carriers through the junction
- Thus, the barrier gets created at the junction prevents further diffusion
- Width of the barrier – The physical distance from one side of the barrier to the other is called the width of the barrier
- Height of the barrier –The difference in potential from one side of the barrier to the other side is known as the height of the barrier
Drift current
- Due to the electric field developed at the junction, the electrons from the p-region move to the n-region. Similarly, the hole from the n-region move to the p-region. This results in drift current
- The motion of charge carriers across the junction due to the electric field is called drift. This results in drift current
Drift current Vs Diffusion current
- The drift current is in a direction opposite to that of the diffusion current
- At a particular stage, the drift current becomes equal to the diffusion current
- This stage is set to be equilibrium state when no current flows across the p-n junction
- Potential barrier becomes maximum and is equal to VB
Thus, a p-n junction is formed. Thus, in a p-n junction under equilibrium there is no net current
The diagram below shows the p-n junction at equilibrium. . The n-material has lost some electrons and the p-material has acquired the electrons

Thus the n material is positive with respect to p material. The potential also prevents the movement of electron from the n-region to the p-region. This potential is called the barrier potential and is indicated as

When doping concentration is small, the electrons or holes move a large distance before collision with another electron or hole
Hence, the width of the p-n junction becomes large
As width of p-n junction increases, the electric field becomes small

p-n junction diode

A semiconductor diode is a p-n junction with metallic contacts provided at the ends for the application of an external voltage
Thus p-n junction diode is a two terminal device represented as

The equilibrium potential barrier can be altered by applying an external voltage V across the diode

There are two methods of biasing a p-n unction – Forward bias and reverse bias

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