Semiconductors – Points to remember

  • Semiconductors are the basic materials used in the present solid state electronic devices like diode, transistor, ICs, etc.
  • Semiconductors are elemental (Si, Ge) as well as compound (GaAs, CdS, etc.).
  • 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 intrinsic conductors. Holes are essentially electron vacancies with an effective positive charge.
  • The number of charge carriers can be changed by ‘doping’ of a suitable impurity in pure semiconductors. Such semiconductors are known as extrinsic semiconductors. These are of two types (n-type and p-type).
  • In n-type semiconductors, ne >> nh while in p-type semiconductors nh >> ne.
  • n-type semiconducting Si or Ge is obtained by doping with pentavalent atoms (donors) like As, Sb, P, etc., while p-type Si or Ge can be obtained by doping with trivalent atom (acceptors) like B, Al, In etc.
  • p-n junction is the ‘key’ to all 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.
  • By changing the external applied voltage, junction barriers can be changed. In forward bias (n-side is connected to negative terminal of the battery and p-side is connected to the positive), the barrier is decreased while the barrier increases in reverse bias. Hence, forward bias current is more (mA) while it is very small (?A) in a p-n junction diode.
  • 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.
  • Zener diode is one such special purpose diode. In reverse bias, after a certain voltage, the current suddenly increases (breakdown voltage) in a Zener diode. This property has been used to obtain voltage regulation.
  • Transistor is an n-p-n or p-n-p junction device. The central block(thin and lightly doped) is called ‘Base’ while the other electrodes are‘Emitter’ and ‘Collectors’. The emitter-base junction is forward biasedwhile collector-base junction is reverse biased.
  • In transistors, the base region is both narrow and lightly doped,otherwise the electrons or holes coming from the input side (say, emitter in CE-configuration) will not be able to reach the collector.
  • In modern day circuit, many logical gates or circuits are integrated in one single ‘Chip’. These are known as Intgrated circuits (IC).
    Now Answer this

    1. In an n-type silicon, which of the following statement is true:

    (a) Electrons are majority carriers and trivalent atoms are the dopants.

    (b) Electrons are minority carriers and pentavalent atoms are the dopants.

    (c) Holes are minority carriers and pentavalent atoms are the dopants.

    (d) Holes are majority carriers and trivalent atoms are the dopants.

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