Types of Semiconductors

Introduction of Semiconductor:

Depends on the resistivity, the materials can be classified as Conductors,  Semiconductors and Insulators.

  1. Conductors are usually having  resistivity in the range of 10−7 to 10−8 m. A good example is metal.
  2. Semiconductors are having resistivity in the order of 10−3 to 3×103 m.
  3. The insulators are having resistivity in the order of 104 to 1014 m.

From this we can define the semiconductors as below:

A semiconductor material is one whose electrical properties lie in between those of insulators and good conductors. 

Under normal conditions, the Insulators does not have free charge carriers with them , where as conductors have plenty of free electrons for electrical conduction. 

Examples of semiconductors:
Gallium arsenide(GaAs)
Gallium phosphide
Indium Arsenide (InAs)

  • Semiconductors like Silicon(Si), Germanium(Ge) have crystalline structure.
  • Their atoms are arranged in an ordered array which is known as crystal lattice.
  • Both Si, Ge are tetravalent. ie, each has four valance electrons in their outer most shell.
  • The neighbouring atoms form covalent bonds by sharing four electrons with each other. Thus a stable structure is formed.

How to increase the conductivity in the semiconductor?
The conductivity of the semiconductor can be increased by

  1. Increasing the temperature
    When the temperature of semiconductors is increased above the room temperature, the resistivity is decreased.
    After reaching at some temperature the semiconductors effectively become conductors.
    Due to this reason, silicon(Si) must not operate more than of 150°C to 200°C(depending on its purity). 
    Similarly Germanium should not operate more 75°C to 90°C (depending on its purity). 
    In some semiconductors which has positive temperature co-efficient of resistance, increasing the temperature will decrease the conductivity.
  2. By applying external potential
  3. By applying the light ( in some particular semiconductors)

Types of semiconductors

  1. Intrinsic or pure semiconductors
  2. Extrinsic or impure semiconductors

Intrinsic semiconductors
An intrinsic semiconductor is one which is made up of the semiconductor material in its extremely pure form.

  • Pure Silicon and germanium have forbidden energy gaps of 1.1eV and 0.72 eV respectively.
  • As the energy gap is so small, even at normal room temperature, there are many electrons which posses sufficient energy to jump across from the valence band to the conduction band.
  • Remember that for each electron liberated into the conduction band, a positively charged hole is created in the valence band.
  • When an electric field is applied,  electrons move towards the anode and the holes in the valence band move towards the cathode. ie, the current flow happens in the semiconductor.

Extrinsic semiconductors:
When a suitable metallic impurity is added to an intrinsic semiconductor, it is known as extrinsic semiconductor.  Depends on the type of impurity added, the extrinsic semiconductors are divided into;

  1. N-type (Negative type) semiconductor.
  2. P-type (Positive type) semiconductor.

Let us see in detail about the extrinsic semiconductors. The semiconductor material is not a good insulator nor is it a good conductor. They are somewhere in between them. They can not be used in real time applications.

  • Arsenic and Phosphorus are called as N-type impurities.
  • When these impurities are added to pure semiconductor material like silicon(Si) or germanium(Ge), they form an n-type material.
  • Usually the amount of impurity added varies from 1 part impurity in 105 parts semiconductor material to 1 part impurity to 108 parts semiconductor material.
  • The amount of impurity added depends upon the resistivity required.
  • Indium, aluminium and boron are called as P-type impurities. When these impurities are added to pure semiconductor material they form a P-type material. 
  • The process of inserting impurity element (atom) into the lattice of an pure material is called doping.
  • When the pure semiconductor is doped with an impurity which have five electrons in its valence shell (ie. pentavalent impurity) it will become an n-type semiconductor material.
  • Remember that the N-type semiconductor material contains an excess of negative charge carriers.
  • Similarly, when the pure material is doped with an impurity which have three electrons in its valence shell (i.e. a trivalent impurity) it will become a p-type (i.e. positive type) semiconductor material.
  • Remember that the P-type material contains an excess of positive charge carriers. 

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