Well, semiconductor is the category of conductors which besides being a conductor of current is also an insulator!!

(Image: Google Images)

Above image illustrates the various classes of conductors.
As evident from the diagram, the energy band gap between valence band and conduction band is:

• large in case of insulator
• overlap in case of metal
• moderate in case of semiconductor

What are they you ask?

1. Valence Band – Band in which electrons reside.
2. Conduction Band – Band to which electrons jump and conduct electricity.
3. Energy Band – Band which does nothing!

Imagine a situation where there are thousands of villagers waiting on one side of river (Valence Band) to get to the opposite bank (Conduction Band). (Why? None of your business)
But the river is acting as obstruction(Band Gap).
Now if the width of river is too large, none of the villagers would be able to gather guts to jump to opposite side (Insulator).
If the width is too small, villagers would be able to easily jump over the river.(Metal)
If the width is moderate, energetic villagers would be able to jump through.(Semiconductors)

Similarly for an electron to jump to the conduction band, it requires energy.
Such an amount of energy is almost impossible to provide to insulators but easily achievable in case of metals.

In the case of semiconductors, energy can be tuned so as to make it work like metal or an insulator.
The energy provided must be greater than the energy of the band gap (>1eV).

This feature enables semiconductors to be used as Switch.
For switching ON the circuit you just need to provide energy greater than 1eV to the semiconductor device.

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