Semiconductors and Superconductors
By Vinooja Thurairethinam 13 V
As a conductor is cooled, its electrical resistance decreases. The colder the material, the less the atoms in the conductor vibrate and thus there is less resistance to the flow of electrons through it. However, when the temperature is below the critical temperature, the resistance of some materials falls to zero. The materials become superconductors and the process is known as superconductivity.
History of superconductors
Superconductivity was first observed by the Dutch physicist Heike Kamerlingh Onnes in 1911. He liquefied helium by reducing its temperature to 4 degrees Kelvin, prepared a very pure sample of the metal Mercury and put it into the liquid helium and measured the usual slow drop in resistance as the temperature decreased. However, after a while the resistance unexpectedly dropped to zero. Omnes later won a Nobel Prize in physics for his research in this area.
It took another 46 years from the discovery of superconductivity to achieve the first generally acceptable theory. This theory of superconductivity was known as BCS theory by American physicists John Bardeen, Leon Cooper, and John Schrieffer in 1957.
https://en.wikipedia.org/wiki/John_Bardeen (above left)
https://en.wikipedia.org/wiki/Leon_Cooper (above centre)
https://en.wikipedia.org/wiki/John_Robert_Schrieffer (above right)
The BCS theory of superconductivity
The individual electrons of a conductor are subjected to resistance which slows them down. Usually each electron travels through the metal entirely independently of all other electrons. Although positive metal ions are much larger than negative electrons, the attractive electrical force between an electron and a metal ion is enough to tug the ion towards the electron as it passes by. The ion, in turn, tugs on another electron. In this way, one electron attracts another with the help of the metal ion.
This process alters the way the current flows through the metal. Instead of being composed of single electrons, it is composed of paired up electrons known as Cooper pairs. According to the theory, the members of the pair transfer energy between them but they do not lose any to resistance. However the electrons in each Cooper pair spin in an opposite manner and cancel out. Therefore, Cooper pairs are bosons (force carriers).
The electrons that make up the pair may be very far away from each other in the metal with thousands of other electrons between one member of a Cooper pair and its partner.
At the critical temperature of the superconductor all the bosons crowd into the same state and once they are flowing as a single entity, it is extremely difficult to stop them. In a normal conductor an electric current is resisted by atoms which can easily obstruct the progress of electrons through the metal. However, it is nearly impossible for them to hinder a Cooper pair in a superconductor since each Cooper pair is in the same state as billions of others. Once started, the current in a superconductor will flow forever. This is known as a persistent current.
The Meissner Effect
When a superconducting material is placed on a magnet at a temperature above the critical temperature it is an ordinary conductor and the magnetic field from the magnet passes straight through it. When the temperature of the material is reduced below the critical temperature, it becomes superconducting and expels the magnetic field. This expulsion causes the superconductor to levitate and the effect is known as the Meissner effect.
The above diagram shows the Meissner effect. Magnetic field lines, represented as arrows, are excluded from a superconductor when it is below its critical temperature.
The above diagram shows a magnet levitating above a superconductor cooled by liquid nitrogen.
Applications of superconductors
Superconductivity offers a range of practical applications. Magnetic-levitation is an application where superconductors perform very well. Transport vehicles such as trains can be made to “float” on strong superconducting magnets, eliminating friction between the train and its tracks. However, this only works if the train moves fast enough (above 100 kmh^-1), since the levitation coils have to be scanned by the magnetic field of the superconducting coils: the faster the train moves, the faster the magnetic field passes in front of the levitation coils, the stronger the inducted currents and the better the levitation. If the train moves slowly, or stops, the inducted currents become too weak and the train stops levitating.
An area where superconductors can perform a life-saving function is in the field of biomagnetism. Doctors need a non-invasive means of determining what’s going on inside the human body. By impinging a strong superconductor-derived magnetic field into the body, hydrogen atoms that exist in the body’s water and fat molecules are forced to accept energy from the magnetic field. They then release this energy at a frequency that can be detected and displayed graphically by a computer.
Probably the one event, more than any other, that has been responsible for research into superconductors is high-energy particle research which hinges on being able to accelerate sub-atomic particles to nearly the speed of light. Superconductor magnets make this possible. CERN, a consortium of several European nations relies on them for the Large Hadron Collider (LHC).
Other uses of superconductor technology include electric generators made with superconducting wire and energy storage to enhance power stability.
Recently, power utilities have also begun to use superconductor-based transformers and “fault limiters”.
The General Atomics/Intermagnetics General superconducting. Fault Current Controller, employing HTS superconductors.
An idealised application for superconductors is to employ them in the transmission of commercial power to cities. However, due to the high cost and impracticality of cooling miles of superconducting wire to cryogenic temperatures, this has only happened with short “test runs”. In May of 2001 some 150,000 residents of Copenhagen, Denmark, began receiving their electricity through HTS (high-temperature superconducting) material. That cable was only 30 meters long, but proved adequate for testing purposes.
IT is hoped that superconductors will provide strong magnets for containing the plasma for nuclear fusion, which would drive highly efficient generators and load up superconducting storage rings. These storage rings could then release electricity whenever it is needed and it could be carried over great distances without today’s expensive losses due to electrical resistance.
The use of superconductors will greatly increase if we can produce superconductors with higher critical temperatures that don’t rely on the use of liquid nitrogen for cooling.
Semiconductors are materials with electrical conductivities that are higher than those of insulators but lower than those of conductors. Widely used semiconductors include silicon, gallium arsenide and germanium. Although all of these materials are solids, some liquids are also considered to be semiconductors. The electrical resistance of a semiconductor decreases with increasing temperature. Thus these materials have negative temperature coefficients of resistance.
History of semiconductors
The effect of semiconductors was first observed by Michael Faraday in 1833 as he saw that the electrical resistance of silver sulfide decreased with temperature.
Later in 1874, Karl Braun documented the first semiconductor diode effect. He observed that the electrical current flows freely in one direction only when a metal point came into contact with a galena crystal.
In 1901, Jagadis Chandra Bose invented a device called “cat whiskers”, which was a point-contact semiconductor rectifier used for detecting radio waves and it became known as the first semiconductor device.
Semiconductors can be intrinsic or extrinsic
As the temperature of a semiconductor is increased, the number of free electrons per unit volume also increases. Thus the electrical resistance decreases with increasing temperature.
When an electron escapes from a bond, due to thermal excitation, it leaves behind a “hole” in the atom lattice. Thus a hole is a region in which there is an excess of positive charge. If an electron comes near a hole, it is likely to be captured and which case the hole no longer exists. As holes are continually being filled by electrons and as electrons are being freed from their bonds, it appears as though the holes are moving through the structure.
Thus, in semiconductors there are two types of charge carriers – holes and electrons, where the holes are positive charge carries and electrons are negative charge carriers.
A process known as doping can be used to considerably increase the conductivity of a semiconductor. This is done by adding impurities.
Donor impurities can donate conduction electrons to the structure and materials which are doped in this way are known as n-type semiconductors since it is primarily negative charge carriers (electrons) which are involved in conducting electricity.
On the other hand, acceptor impurities accept electrons in such a way that more holes are created in the lattice. In this case the majority of the charge carries are holes and this type of semiconductor is known as a p-type semiconductor.
Uses of semiconductors
Semiconductor charge carriers are highly mobile and so semiconductors are the best materials for use in advanced electronics and communications. They are used to fabricate chips for every electronic device, including computers, cell phones, iPods, BlackBerrys and GPSs.
Semiconductors also have very optical properties and thus they are used to make lasers and light emitting diodes (LEDs). Due to the dopants they contain, semiconductors can emit light of a specific colour when a voltage is applied.
Other resources consulted
Stanley Thornes (Publishers) Limited, 1993
Page 837, 838, 839
Secret of record-breaking superconductor explained
IOP Physics World
24 April 2015