Specific objectives.

  1. State the difference between conductors, insulators and semi- conductors.
  2. Define intrinsic and extrinsic semi- conductors.
  3. Explain doping in semi- conductors.
  4. Explain the working of a P.N junction diode.
  5. Sketch the current voltage characteristics for a diode.
  6. Explain the application of diodes in rectification.

Conductors, insulators and semi- conductors.

  • Insulators are materials which do not allow electric charges to pass through or heat to flow through them e.g. plastic, rubber, wood.
  • Conductors are materials that contain free electrons that allow heat to pass through them.
  • Semi- conductors are materials that allow the flow of electrical energy through them under certain conditions, temperature e.g. silicon, germanium, sulphide, gallium. These are elements in group 4.

                                                Band theory.

  • Assumes that for electrons to conduct electricity, they must have certain ranges of energy.
  • The electrons revolve round the nucleus in energy levels or paths.
  • When two or more atoms are brought together, the energy levels split into small energy levels called bands.
  • The last or outermost energy level splits into two bands called valence and conduction bands.
  • The splitting of energy levels into bands and interbands distance dictates the electrical optical and magnetic property of a given material.
  • Conduction band is the highest occupied energy band in which electrons conduct electric current.
  • Valence band is below conduction band and it is where electrons form chemical bonds.
  • Forbidden band is the gap between the conduction band and valence band. Electrons are not allowed to be in this gap.

                                                Conductors.

  • These are mainly metals.
  • They do not have forbidden bands between valence and conduction band.
  • The conductivity of these materials depend on the spacing of these energy bands and the extent to which these bands are occupied by electrons.
  • Conductors have either partly filled conduction band- copper or empty conduction band overlapping a filled valence band- magnesium.
  • They have free mobile electrons in the outer valence band which are responsible for the conduction of electricity.
  • They need little energy to enter into the conduction band. When the temperature increases. The vibration of the atom increases, more electrons are promoted in to the conduction band and this interfere with the electron flow hence increase in resistance.
  • Resistance is directly proportional to time.

                                                Insulators.

  • They have neither partly filled conduction band nor overlapping bands.
  • The valence band is completely filled.
  • There is a wide forbidden band between the valence band and conduction band of approximate 3 electron volts.
  • A lot of energy I required to promote electrons into conduction band. It cannot be provided by any source of p.d and if supplied, the crystal will break down hence insulators are poor conductors of electricity.

                                                Semi- conductors.

  • These are metals whose conductivity is between that of a good conductor and a good insulator e.g. silicon, germanium, selenium.
  • The forbidden band between the conduction band and valence band is very small.
  • An increase in temperature makes the electrons to cross the forbidden band and enter the conduction band and conduct electricity.
  • Resistance is inversely proportional to temperature.
  • These promoted electrons are called thermal electrons.
  • There are two types of semi- conductors determined by their conductivity which is determined by the electrons.
  1. Intrinsic semi- conductors.
  • Are pure semi- conductors whose charge carriers (electrons) are within the material.
  • They conduct electric current using electrons from within.
  1. Extrinsic semi- conductors.
  • Are impure semi- conductors whose charge carriers come from a foreign atom or impurity atom- an atom from outside the matter.
  • They are made by introducing a foreign atom or impurity atom into a pure semi- conductor from group 3 or group 5.
  • The process of introducing an impurity atom into the crystal of a pure semi- conductor is called doping.
  • The impurity atom must have a similar size to that of the semi- conductor atom so that it can occupy a position in the crystal lattice without distorting it.
  • There are two types of extrinsic semi- conductors.
  1. P- type.
  • It is made by introducing an atom from group 3 or trivalent atom into the crystal of a pure semi- conductor e.g. boron, indium and gallium.
  • The impurity atom has 3 electrons in the outermost energy level. When introduced into a crystal of silicon or germanium, there will be one electron less to complete the bonding.
  • This vacant place or bond due to missing electron is called a hole or positive. These are now the charge carriers in the semi- conductors.
  • An electron from silicon atom will move to this hole and make the silicon atom a positive ion.
  • The silicon crystal becomes an extrinsic semi- conductor with majority charge carriers as positive hence called P- type or positively charged extrinsic semi- conductors.
  • When connected to an electric field, the material conduct electric current.
  1. N- type.
  • It is made by introducing or doping a pure semi- conductor with an atom from group 5 or pentavalent atom e.g. phosphorous, arsenic, antimony.
  • When introduced, an extra electron will be set free and phosphorous is said to donate an electron which is a charge carrier.
  • When this free electron move to silicon atom, it makes it to be negatively charged hence the crystal becomes negatively charged or pentile.
  • The majority charge carriers are electrons. When connected to a p.d source, it conducts electricity.

                                    P- N junction diode.

  • When a crystal of silicon is doped simultaneously by trivalent on one side and pentavalent on the other side. It forms a reaction known as P- N junction.
  • The electrons and holes (positive) near the junction diffuses across the junction such that the electrons enter the P- zone as the holes enter the N- zone.
  • This forms a region of uncharged/neutral section which set up a potential barrier which in turn set up a field that stop further diffusion of mobile electrons and this region is called division layer or potential barrier. For silicon is 0.7 V and germanium is 0.3 V.

                                                Biasing.

  • Is the way an electric device is connected in an electric circuit.
  • A diode can be connected in two ways.
  1. Forward biasing/ connection.
  • In this connection, the P- type is connected to the positive terminal of the cell and the N- type to the negative terminal.
  • The current flows when the applied voltage is greater than the potential barrier.
  • It opposes the internal potential barriers by repelling the poles from P- type and electrons from N- type.
  • The forward resistance is then reduced and a large current flows through the circuit.
  1. Reverse biasing.
  • In this connection, the N- type is connected to the positive terminal of the cell and the P- type to the negative terminal.
  • The potential voltage is in the same direction as the potential barrier.
  • The holes (positive) and electrons in their respective regions are attracted away from the junction by the external voltage.
  • The potential barrier increases hence increasing the resistance of the P- N junction.

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