Semiconductor: A material that is neither a good conductor of electricity nor a good insulator, but has properties of electrical conductivity somewhere between the two. Silicon and germanium are good semiconductor materials.

There are two types of semiconductors on the basis of impurity added:

  • Intrinsic Semiconductor: The pure semiconductors in which the electrical conductivity is totally governed by the electrons excited from the valence band to the conduction band and in which no impurity atoms are added to increase their conductivity are called intrinsic semiconductors and their conductivity is called intrinsic conductivity. Electrical conduction in pure semiconductors occurs by means of electron-hole pairs. In an intrinsic semiconductor-
    ne = nh = ni
    where ne = the free electron density in conduction band, nh = the hole density in valence band, and ni = the intrinsic carrier concentration.
  • Extrinsic Semiconductors: A Semiconductor doped with suitable impurity atoms so as to increase its conductivity is called an extrinsic semiconductor.
  • Types of Extrinsic Semiconductors: Extrinsic semiconductors are of two types:
  1. n-type semiconductors
  2. p-type semiconductors
  • n-type semiconductors: The pentavalent impurity atoms are called donors because they donate electrons to the host crystal and the semiconductor doped with donors is called n-type semiconductor. In n-type semiconductors, electrons are the majority charge carriers and holes are the minority charge carriers. 

no. of electrons>>no. of holes

  • p-type semiconductors: The trivalent impurity atoms are called acceptors because they create holes which can accept electrons from the nearby bonds. A semiconductor doped with acceptor type impurities is called a p-type semiconductor. In p-type semiconductor, holes are the majority carriers and electrons are the minority charge carriers.

no. of electrons<<no. of holes

 

 biasing

  • Forward Biasing of a pn-junction: If the positive terminal of a battery is connected to the p-side and the negative terminal to the n-side, then the pn-junction is said to be forward biased. Both electrons and holes move towards the junction. A current, called forward current, flows across the junction. Thus a pn-junction offers a low resistance when it is forward biased.
  • Reverse Biasing of a pn-junction: If the positive terminal of a battery is connected to the n-side and negative terminal to the p-side, then pn-junction is said to be reverse biased. The majority charge carriers move away from the junction. The potential barrier offers high resistance during the reverse bias. However, due to the minority charge carriers a small current, called reverse or leakage current flows in the opposite direction. Thus junction diode has almost a unidirectional flow of current.

Transistor

transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. 

  • Three Configurations of a Transistor: A transistor can be used in one of the following three configurations:-
  1. Common-base (CB) circuit
  2. Common-emitter (CE) circuit
  3. Common-collector (CC) circuit

Zener diode

A zener diode is a special type of device designed to operate in the zener breakdown region. Zener diodes acts like normal p-n junction diodes under forward biased condition. When forward biased voltage is applied to the zener diode it allows  large amount of electric current and blocks only a small amount of electric current.

Zener diode is heavily doped than the normal p-n junction diode. Hence, it has very thin depletion region . Therefore, zener diodes allow more electric current than the normal p-n junction diodes.

Zener diode allows electric current in forward direction like a normal diode but also allows electric current in the reverse direction if the applied reverse voltage is greater than the zener voltage. Zener diode is always connected in reverse direction because it is specifically designed to work in reverse direction.

Zener breakdown or avalanche breakdown

The avalanche breakdown occurs in both normal diodes and zener diodes at high reverse voltage. When high reverse voltage is applied to the p-n junction diode, the free electrons (minority charge carriers) gains large amount of energy and accelerated to greater velocities. 

The zener breakdown occurs in heavily doped p-n junction diodes because of their narrow depletion region. When reverse biased voltage applied to the diode is increased, the narrow depletion region generates strong electric field.

Merits of zener diode

  • Power dissipation capacity is very high
  • High accuracy
  • Small size
  • Low cost

Uses of zener diode

  • used as voltage reference
  •  used in voltage stabilizers or shunt regulators.
  • used in various protection circuits, clipping and clamping circuits.

 

  • Zener breakdown occurs at low reverse voltage whereas avalanche breakdown occurs at high reverse voltage.
  • Zener breakdown occurs in zener diodes because they have very thin depletion region. 
  • Breakdown region is the normal operating region for a zener diode.
  • Zener breakdown occurs in zener diodes with zener voltage less than 6V.

Logic Gate: A logic gate is a digital circuit that has one or more inputs but only one output.

    • OR Gate: An OR gate can have any number of inputs but only one output. It gives higher output (1) if either input A or B or both are high (1), otherwise the output is low (0).

    A + B = Y

    Which is read as ‘A or B equals Y’.

    • AND gate: An AND gate can have any number of inputs but only one output. It gives a high output (1) if inputs A and B are both high (1), or else the output is low (0). It is described by the Boolean expression-
    • A . B = YWhich is read as ‘A and B equals Y’.
      • NOT Gate: A NOT gate is the simplest gate, with one input and one output. It gives as high output (1) if the input A is low (0), and vice versa.
        Whatever the input is, the NOT gate inverts it. It is described by the Boolean expression:
      • A=YWhich is read as ‘not A equals Y’.
        • NAND (NOT + AND) gate: It is obtained by connecting the output of an AND gate to the input of a NOT gate. Its output is high if both inputs A and B are not high.
        •  ‘A AND B negated equals Y’.
        • NOR (NOT + OR) Gate: It is obtained by connecting the output of an OR gate to the input of a NOT gate. Its output is high if neither input A nor input B is high.                   ‘A OR B negated equals Y’.