B.tech 1st year Electronics Notes | Engineering

Useful notes on electronics for B.tech 1st year students.

The science of electronics is primarily concerned with the theory and application of devices which control current. Among these vacuum tubes and semiconductor devices are included. The diode introduced by Fleming in 1904, consisted of two metal electrodes, anode and cathode, may be taken as the first electronic device.

B.tech 1st year Electronics Notes

Notes on the Concept of Electrons:

It was Dalton (1802) who first gave an idea that all matters could be broken down into elements, the tiniest particles of which were called atoms. According to Bohr’s theory, an atom has a central nucleus of positive charge around which tiny negatively charged particles called electrons revolve in stationary orbits, just as the planets revolve round the sun. The nucleus of the atom is, in fact, made up of two fundamental particles, known as proton and neutron.

Out of the various elementary particles, electron is the most fundamental one. Its values of charge, mass and radius are given below:

Proton has a positive charge, magnitude equal to Q_e and mass 1837m_e. Neutron has no charge but mass equal to 1837m_e. There exists many other charged particles such as positron, etc. But due to high ratio of the charge to mass of an electron the motion of electron has become so useful in the field of electronic devices.

Electrons dislodged from the outer shell of an atom are called free electrons. Free electrons carry the current in all types of electron tubes and also in ordinary conductors. The movement of free electrons in antenna gives rise to electromagnetic radiations. Free electrons are thus responsible in most electrical and electronic phenomena.

Electron Emission:

Electrons perform a certain amount of work to escape from the surface of emitter. To do this work energy is imparted to them from some external sources such as heat energy, kinetic energy of electric charges, energy stored in electric or magnetic fields and the light energy.

Accordingly, the emission of electrons from a metallic surface is classified as follows:

i. Thermionic (Primary) Emission:

If electrons are emitted from a metal by supplying thermal energy, the process is called thermionic emission. The number of electrons released depends on temperature. At a given temperature the thermionic emission current density is obtained by an equation known as Richardson’s equation.

ii. Secondary Emission:

When electrons at a high speed suddenly strike a metallic surface, some will collide directly with free electrons on the metal surface and project them outward. The electrons freed in this manner are called secondary emission.

iii. Field Emission:

The presence of strong electric field set up by a high positive voltage outside the emitter surface may cause electron emission from it. The stronger the field applied, the greater will be the field emission from the cold emitter surface.

iv. Photoelectric Emission:

In this process light energy called ‘quanta’ falling upon the emitter is transferred to the free electrons within the metal and speeds them up sufficiently to eject from the surface. The number of electrons emission will depend on the intensity of the beam falling upon the metallic surface.

Out of the four methods of emission discussed above, the thermionic emission is the most vital one and most commonly used in electron tubes.

Thermionic Emitters:

Thermionic emitters are of two different types:

(i) Directly heated emitter or filamentary emitter, and

(ii) Indirectly heated emitter or oxide-coated emitter.

In the direct heating, the electric current is applied directly to a wire called filament which itself serves as electron emitter while in indirect heating, electric current is applied to a separate heater located inside a cylindrical cathode that serves as emitter. The arrangement of these two different types of heating is shown in Fig. 1.1.

Both alternating and direct current can be used in either methods of heating. Two important directly heated emitter’s are- (a) tungsten emitter and (b) thoriated tungsten emitter. Indirectly heated cathodes always use oxide-coated emitters. Receiving tubes are, in general, indirectly heated types.

Space Charge:

For simplicity, let us consider a vacuum diode consisting of an emitter called cathode and a collector of electrons called anode. At low temperatures, there is no emission of electrons from the cathode surface and hence anode current is zero. As the temperature is raised to emit electrons from the cathode and the anode is disconnected from a source of potential, electrons from the emitter form a cloud in the space.

This atmosphere of thermoelectrons that is formed in the interelectrode space between cathode and plate-is known as the space charge. Since it is made up of electrons, this atmosphere constitutes a negative charge that has a repelling effect on the electrons being emitted from the cathode. So the effect of the negative space charge alone is to force the emitted electrons back into the cathode by a considerable amount and prevent others from reaching the plate.

However, the space charge does not act alone. It is counteracted by the positive plate which attracts electrons through the space charge. When the plate voltage is low, only those electrons which are nearest to the plate are attracted to it and constitute a small plate current.

With the increase of plate voltage, a greater number of electrons is attracted towards the plate and eventually a point is reached when all the electrons emitted from the cathode are attracted to the plate overcoming the effect of space charge completely. With further increase of plate voltage there will be no further increase of current. The current is then temperature controlled. The total number of electrons emitted by an emitter is always same at a given operating temperature and is determined by using Richardson’s equation.

Atomic Structure:

The matter consists of atoms arranged in a particular array called lattice. Bohr’s theory of atom supposes that an atom consisting of a positively charged nucleus is surrounded by a group of negatively charged moving electrons in some definite states, orbits or energy levels.

The mass of the nucleus is several thousand times that of an electron. The number of negative electrons is equal to the atomic number of an element. For a normal atom the negative charge of these electrons is balanced by a positive charge in the nucleus due to the presence of protons.

An isolated silicon atom has 14 protons in its nucleus, 2 electrons travel in the first orbit, 8 in the second and 4 in the outer or valence orbit. This is shown in Fig. 1.2(a). Germanium atom resembles silicon in this respect. It has a valence orbit with 4 electrons as shown in Fig. 1.2(b).

Orbital Radius:

The electrons of an atom can travel with right velocity to balance the inward pull of the nucleus and the outward push of the centrifugal force. If an electron is in a large orbit, the nuclear attraction becomes low. Consequently, the velocity required to balance is also low.

On the other hand, for a smaller orbit the nuclear attraction is greater so that the electrons must travel faster for maintaining the balance. It is thus seen that an electron can travel in an orbit of any radius provided its velocity has the right value. According to modern theory the radii must have discrete values. In a hydrogen atom the smallest orbit has a radius of-

It may be noted here that silicon has 14 protons, so the radius r₁ of silicon is lower than that of hydrogen. However, its radius formula is more complicated than the equation (1.1) above.

Energy Levels:

The magnified orbits of silicon atom are shown in Fig. 1.3(a). Each orbit may be identified with an energy level as shown in Fig. 1.3(b). When an electron is moved from the first to the second orbit some energy is taken by it since the electron is pulled away from the nucleus. At the time of movement of electron to an orbit of larger radius it acquires potential energy with respect to smaller orbits. But if the electron falls back to its initial orbit, it will give up its potential energy.

The total energy of an electron which is the sum of its potential energy, kinetic energy and a few other forms of energy can be identified with the radius of its orbit. That means each radius in Fig. 1.3(b) is equivalent to an energy level. The larger is the radius, the greater is the energy of an electron.

The importance of the energy level is that if some external energies like light, heat or other radiation bombard an atom, then an electron can be lifted to a larger orbit with higher energy level. The atom is then said to be as an excited state which does not last long. It has been found that an electron remains in the larger orbit for about 10^-8 second before falling back to its smaller orbit.

The Crystals:

When instead of an isolated atom they are combined, a new picture appears. Thus, when silicon atoms are combined to form a solid, they arrange in a regular pattern called a crystal. Forces holding the atoms together are the covalent bonds.

The covalent bond can be explained by considering Fig. 1.4(a). Silicon atoms combine so as to have eight electrons in the valence orbit. For this each atom arranges itself with four other atoms so that each neighbour shares an electron with the central atom. In this fashion the central atom picks up four electrons resulting a total of eight electrons in the valence orbit. In this way they remain bounded together.

The bonding diagram is shown in Fig. 1.4(b). It symbolizes the mutual sharing and pulling on electrons. Each line in the figure represents a shared electron which establishes a bond between the central atom and a neighbour.

If by some external energy an electron of valence orbit is lifted to a higher energy level, a vacancy is created in the outer orbit called a hole. This is shown in Fig. 1.5(a). The hole is nothing but a broken covalent bond as shown in Fig. 1.5(b). In presence of a hole, the covalent force is cut in half.

The bonding diagram of many silicon atoms in a crystal. Since germanium atom has also four outer orbit electrons, the bonding diagram of its crystal will also be identical.

Energy Bands:

Inside the crystal each electron has a different position. Due to this the energy of each electron is different. Since there are billions of first-orbit electrons with slightly different energy levels. Similarly, the billions of second-orbit electrons form the second energy band and all third-orbit electrons form the third band.

The shade indicates saturated or filled band while the unshaded part of the band means that some orbits are empty, i.e., some energy levels are vacant.

Current Carriers:

In vacuum tubes, negatively charged electrons are considered as the current ‘carriers’. In order to explain current flow in semiconductor diodes and transistors this concept must be modified by the addition of positive charge carriers known as holes, which have mass, mobility and velocity. Current flow in semiconductors is thus carried on by the flow of both negative charges (free electrons) and positive charges (holes).

Holes are usually attracted by free electrons and when they combine, the free electron ‘fills’ the hole neutralizing its charge. When this happens the holes and free electron both are lost as current carriers and form new current carrier at other points in the semiconductor.

Impurity like arsenic or antimony (pentavalent atoms) increases the conductivity of silicon, which has four valence electrons in its outer shell, by increasing the number of negative charge carriers. Silicon which has been doped here with arsenic or antimony is designated as ‘N-type’.

Similarly, impurity like indium or gallium (trivalent atoms) increases the conductivity of silicon by increasing the number of positive charge carriers. For this reason, silicon which has been doped with indium or gallium is called ‘P-type’. Current flow in N-type and P-type silicon is carried on by ‘free electrons’ and ‘holes’ respectively, and these are known as majority carriers.

Some holes, however, exist in N-type silicon and some free electrons in P-type. But these are in the minority and are, therefore, called minority carriers.

By applying an external battery voltage V_AA across the semiconductor as shown in Fig. 1.8, one can easily control the movement of current carriers. In the P-type silicon holes are repelled by the positive terminal of the battery and move towards the negative terminal. While free electrons from the negative terminal move towards the hole. Thus a combination of the two takes place continuously and the flow of current is maintained in the external circuit.

Conduction in Crystals:

A bar of silicon with metal surfaces is shown in Fig. 1.9(a). Ah external voltage sets up an electric field between the ends of the crystal.

At absolute zero temperature electrons are unable to move through the crystal. We know that all electrons are tightly held by the silicon atoms. The outer-orbit electrons are part of the covalent bonding and cannot break away without getting any outside energy while the inner orbit electrons are buried deep within atoms. Thus at absolute zero temperature a silicon crystal behalves like an insulator.

In Fig. 1.9(b) the energy band diagram is shown. It is seen from the figure that the first three bands are filled and electrons are unable to move easily in these bands. But beyond the valence band a conduction band is there which represents that next large group of radii that satisfy the wave-particle nature of an electron. If an electron is lifted into the conduction band, it becomes virtually free to move from one atom to the next. That is why electrons in the conduction band are often termed as free electrons.

If now the temperature is raised above absolute zero, the incoming heat energy breaks some covalent bonds. That means it knocks valence electrons into the conduction band and thus we get a limited number of conduction-band electrons. These we have symbolized by the negative signs in Fig. 1.10(a). By the influence of the electric field, these free electrons move to the left and set up a current.

The energy bands above absolute zero is shown in Fig. 1.10(b). Here some electrons has lifted into the conduction band by thermal energy where they move in orbits of larger radii than before. Electrons in these orbits are loosely held by the atoms and can go easily from one atom to the next. In Fig. 1.10(b) each time an electron jumps up to the conduction band and a hole is created in the valence band which represents an available orbit of rotation.

With the increase of temperature a greater number of electrons is kicked up to the conduction band and thus increases the current flow. It has been found that at room temperature of about 25°C, the flow of current is too small for silicon and then it behaves neither a good insulator nor a good conductor; it is then called a semiconductor.

It may be noted that a germanium crystal also behaves as a semiconductor at room temperature like silicon. But silicon crystal at the room temperature has fewer free electrons than that of germanium. This is the main reason why silicon is preferably used as a semiconductor now-a- days.

Hole Current:

Holes in a semiconductor can also move to produce a current. Thus we find that in a semiconductor there are two different types of current flow. These are conduction- band current and holes current.

With a very small change of energy the valence electron at A can move into the hole and so the original hole disappears to produce a new one at position A. The new hole at A can attract and capture the valence electron at B. At the time the valence electron moves from B to A, the holes moves from A to B. The valence electrons can continue to move along the path shown by the arrows. Since holes are present in the valence orbits, there is a second path for electrons to move through a crystal.

Thermal energy jumps an electron from the valence band to the conduction band which leaves a hole in the valence band. With minor changes of energy the valence electrons can go along the path shown by arrows. Since the holes moves in the opposite direction, so through valence band it will go along ABCDEF path.

Electron-Hole Pairs:

As soon as an external voltage is applied across a crystal, it forces the electrons to move. The movement of both conduction-band electrons and valence electrons is shown in Fig. 1.13(a). The movement of the valence electrons to the right indicates that the holes are moving to the left. Fig. 1.13(b) we have replaced the valence electrons by holes showing their movement to the left. Since holes behaves like positive charges they have been shown by plus signs.

Each minus sign is a conduction-band electron in a large orbit while each plus sign is a hole in a smaller orbit. A conduction-band orbit of one atom may overlap the holes orbit of another. The merging of a conduction-band electron and hole is known as recombination. When recombination occurs, the hole does not move anywhere, rather it disappears. In a semiconductor such recombination takes place continuously.

The average time between the creation and disappearance of an electron-hole pair is known as the lifetime which varies from a few nano-seconds to several micro-seconds. The lifetime depends on the crystal structure and also on a few other factors.