Semiconductor Devices – An Introduction to Semiconductors

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We’ve often heard of terms like the Silicon Valley & the Silicon Economy. What do they really refer to?

All modern day electronics are build using a special class of materials called semiconductors. These materials have an electrical resistivity between a conductor & an insulator.

They are the foundations of all electronics which are computerized(computers, ipods, etc) & ones which use radio waves(radio, cell phones, etc), silicon being the heart of all these devices.

The elements like Silicon & Germanium having 4 valence electrons are elemental semiconductors. The 4 valence electrons can easily bond with 4 neighbouring electrons to give rise to a lattice structure with no free electrons(at zero temperature).

silicon_lattice

Since, there are no free electrons at zero temperature, Intrinsic(pure/elemental) Semiconductors behave as insulators at zero temperature.

Then how do they differ from insulators? Well, the difference is in terms of the energy gap between the valence & conduction bands.

This energy gap is zero in case of conductors, very high for insulators & very small for semi conductors(about 1 eV)

semiconductors

Hence, on increasing the temperature, the electrons in the valence band of the semiconductor gain energy & some of them get sufficient energy to move to the conduction band.

valence_band

This is what happens physically inside the lattice. In terms of the energy bands, we could show this as follows…

energy_bands

These electrons leave behind empty spaces called holes. The holes appear to move in a direction opposite to that of the electron & hence, are the positive charge carriers of the semiconductor.

hole_current

Hence, a semiconductor conducts only at high temperatures & the conduction is due to both electrons & holes, also, the electrons & holes are equal in number.

However, the conductivity of the semiconductors can be changed drastically by adding certain impurities to the semiconductor materials. This process is called doping & is explained in the next post.

Semiconductors find their major application in manufacturing transistors. The first transistor was made of Germanium. Germanium, in fact, would have more free electrons at a particular temperature than silicon. But Silicon is preferable as it can be used at extremely high temperatures.

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