CHAPTER 14 SEMICONDUCTOR ELECTRONICS: MATERIALS, DEVICES AND
- Semiconductors are the basic materials used in the present solid state electronic devices like diode, transistor, ICs, etc.
- Lattice structure and the atomic structure of constituent elements decide whether a particular material will be insulator, metal or semiconductor.
- Metals have low resistivity. Insulators have a very high resistivity while semiconductors have intermediate values of resistivity.
- Pure semiconductors are called ‘intrinsic semiconductors’. The presence of charge carriers (electrons and holes) is an ‘intrinsic property’ of the material and these are obtained as a result of thermal excitation. The number of electrons (ne) is equal to the number of holes (nh) in the intrinsic conductors. Holes are essentially electron valencies with an effective positive charge.
- The number of charge carriers can be changed by ‘doping’ of a suitable impurity in pure semiconductor. Such semiconductors are called extrinsic semiconductors. These are of two types:
- n-type: ne >> nh
- p-type: nh >> ne
- n-type semiconducting Si or Ge is obtained by doping with pentavalent atoms (donors) like As, Sb, P, etc.
- p-type semiconducting Si or Ge can be obtained by doping with trivalent atom (acceptors) like B, Al, In, etc.
- There are two distinct band of energies (called valence band and conduction band) in which the electrons in a material lie. Valence band energies are low as compared to conduction band energies. All energy levels in the valence band are filled while energy levels in the conduction band may be fully empty or partially filled. The electrons in the conduction band are free to move in a solid and are responsible for the conductivity. The extent of the conductivity depends upon the energy gap (Eg) between the top of the valence band (EV) and the bottom of the conduction band (EC).
- The electrons from valence band can be excited by heat, light or electrical energy to the conduction band and thus produce a change in the current flowing in the semiconductor.
- p-n junction is the ‘key’ to all the semiconductor devices. When such a junction is made, a depletion layer is formed consisting of immobile ion-cores devoid of their electrons or holes. This is responsible for a junction potential barrier.
- Diodes can be used for rectifying an ac voltage (restricting the ac voltage to one direction). With the help of a capacitor or a suitable filter, a dc voltage can be obtained.
- Transistor is an n-p-n or p-n-p junction device. The central block (thin and lightly doped) is called the ‘Base’ while the other electrodes are ‘Emitter’ and ‘Collectors’. The emitter-base junction is forward biased while collector-base junction is reverse biased.
- When a transistor is used in the cutoff or saturation state, it acts as a switch.
- In the modern day circuit, many logical gates or circuits are integrated in one single ‘Chip’. These are known as Integrated circuits (ICs).