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Semiconductors and Insulators


- Semiconductors are materials with electrical conductivity levels between conductors like metals (copper, iron, etc.) and insulators like ceramics, wood, etc.
- The resistance offered by a semiconductor goes down as the temperature goes up. The conductivity level of a semiconductor can be changed by means of adding external impurities into its crystal structure. This method of adding impurities to alter their conductivity is called doping.
- In the same semiconductor crystal structure, if there are two differently doped regions, it creates a semiconductor region. The behavior of charged carriers like electrons and holes at this semiconductor region forms the foundation of modern electronics.
- Silicon is the most popularly used semiconductor, and gallium arsenide follows as the second most popularly used semiconductor. These two are the most commonly used semiconductors because the energy required to break their covalent bonds and release a free electron from their valence bands is lesser than that of other semiconductors.
- The energy needed for releasing an electron from the valence band is 1.08 eV for silicon, 0.66 eV for germanium, and 1.58 eV for gallium arsenide. Even though germanium requires lesser energy than silicon, it is not preferred over silicon due to its low energy gap.

Classification of Semiconductors

Semiconductors are classified into two types, namely intrinsic and extrinsic semiconductors. 

  • Intrinsic semiconductors: An intrinsic semiconductor is also called a pure semiconductor because they do not have any impurities in them. But still, at room temperature and under the influence of an external electric field, some of its valence electrons may acquire enough energy to enter the conduction band. The movement of these electrons produces an electric current.
  • Extrinsic semiconductors: Some intrinsic semiconductors have poor conductivity at room temperature without the addition of impurities in them. After the process of doping, these intrinsic semiconductors will have an increased conductivity. The quantity of the impurities added is extremely minute, to the likes of 1 for every 106 intrinsic atoms. Depending on the kind of impurities added, extrinsic semiconductors are further classified into N-type semiconductors and P-type semiconductors.
    - N-type semiconductor: When small amounts of pentavalent impurities like arsenic, antimony or phosphorous are added to a pure semiconductor, an N-type semiconductor is formed. Here the number of electrons is more than the number of holes.
    - P-type semiconductor: Unlike the N-type semiconductors, when small amounts of trivalent impurities like boron, gallium or indium are added to a pure semiconductor, a P-type semiconductor is obtained. Here the number of holes is more than the number of electrons.


You may want to learn about types of semiconductor devices


-Insulators are materials that offer very high resistance to the flow of electricity through them.
- They are such bad conductors of electricity because the electrons in their atoms are much more strongly packed and cannot escape their bonds easily, quite the opposite of conductors and semiconductors.
- However, if a very large electric voltage is applied to an insulator, the sheer magnitude of the electric field will attract a few electrons from the insulators and make them conduct electricity to some extent.
- When the insulator is subjected to an electric field that exceeds the threshold energy of the material, the free charge carriers are accelerated to high values of velocity, which leads to the material breaking off the electrons upon impact, thus ionizing them. This is called the breakdown voltage of an insulator.
- The breakdown voltage of a material is directly proportional to the bandgap energy levels. Because of their excellent resistivity, insulators are used as insulation in wires and cables. This layer of insulation is very useful in cases where a bunch of wires are in close proximity to each other, and contact between them would cause short circuits and even lead to fire hazards.


To better understand the differences between insulators and semiconductors, let us summarize using a table.

Semiconductors Insulators
As the temperature increases, the conductivity of the semiconductor also increases. With an increase in temperature, the resistivity of the insulator decreases slightly.
The reason for conduction is the movement of free electrons and holes. Insulators offer high resistivity because the electrons in them cannot be broken off easily.
The bandgap of semiconductors is 1 eV meaning it takes relatively less energy for electrons to reach the conduction band. The bandgap of insulators is huge at 5 eV, and this means insulators require enormous amounts of energy to conduct electricity.
Semiconductors have four valence electrons in their outer shell. Insulators have eight valence electrons in their outer shell.
They are used in most electronic devices. They are used as safety precautions in electrical wires and components.
Examples: silicon, germanium, boron etc. Examples: rubber, Teflon, paper etc.

NCERT Solutions For Class 10 Science Chapter 12 Electricity throws light on important concepts like Conductors, Semi-Conductors, and Insulators, along with some fundamental laws like Ohms Law.

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