Electricity & Megnetism

Semiconductors and superconductors

Difference between semiconductors and super conductors is that semiconductors are the materials that have properties lie between conductors and insulators,while super conductors are those materials that act strangely when cooled down.

Semiconductors

Semiconductors are those materials which are neither good conductors nor good insulators.Their conductivity is intermediate between conductors and insulators.Silicon and germanium are semiconductor elements, at room temperature.The conducting of semiconductors in charged by temperature applied voltage or incident light.Pure silicon or germanium are poor conductors but by doping with impurities charge the conductivity at a lot.The temperature coefficient of resistivity is negative for semiconductors.

The energy states of electrons in any material are quantized and group together in bands. Between the band, there is an energy gap which contains no states that an electron can occupy. This is called forbidden energy gap.An electron can jump from valence band the outer most band to conduction band.

A conductor the valence band and conduction bands are overlapped. A small applied electric field causes electrons to contribute electric current in conductors. In insulators, this forbidden energy gap is more than 3ev and no electron can jump from valence band to conduction band. So no free electron available in the insulator to contribute the electric current in insulators.

In semiconductor show, the energy gap is very narrow less than 2ev. In silicon, it is only 0.7 ev. So at the ordinary temperature, some electrons can jump from valence band to conduction band and contribute current.

The resistivity of conductors increases with increases in temperature so they have the temperature coefficient resistivity. White in temperature, more electrons acquire energy to jump from valence band to conduction band so increase the conductivity and decrease the resistivity of semiconductor material by the increase in temperature.

What is a Super Conductor?

If the resistivity of a material is zero it is called superconductor, and if a current is established in a superconductor material, it should persist forever, even with no electric field present.

In an ordinary conductor the resistivity increase with an increase in temperature, so by decreasing temperature the resistivity should decrease. But there is a finite value of resistivity at the lowest temperature. The superconductivity was first discovered in mercury at 4k. It is not a gradual decrease in resistivity but at 4k the material resistivity becomes suddenly zero and becomes a superconductor. Now some ceramic materials are discovered which are the superconductor at a relatively high temperature at almost 90k.

Microscopic pairs of superconductivity

The best room temperature conductors like gold, copper do not show any superconductivity at all.

These materials have a single weakly bound electron that contributes to conductivity. Superconductors depend on the motion of highly correlated pairs of electrons. Generally, electrons do not form pairs, a special circumstance is required two electrons each interact strongly with a lattice and with each other while in order any conductor, the electrons interact with lattice weakly.

Applications of superconductors in physics

These are following applications of superconductor material is suggested.

  1. Energy can be transported and stored in wires without resistance loads.
  2. Superconductors electromagnetic can produce large magnetic fields than conventional electromagnets. They can use in large accelerators and levitated trains.
  3. As superconductor components in electronic circuits do not produce Joule heating. So smaller circuits can be produced by the use of superconductors.

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