Semi-+and+Super-conductors

= **__ Semiconductors and Superconductors __** =

By Naima Shire, Alec Spera, Sarah Ibrahim

 * Prequisites:**

To fully understand superconductors and semiconductors, some knowledge above the grade 12 level is required and thus will briefly be explained here; note that this information is incomplete and there are exceptions to what is stated. Band gap: In essence this is a region within the atom of a solid in which no electron can exist (or at least is extremely unlikely to exist), generally referring the space in between the valence band and the conduction band. The size and even existence of this gap is dependent on many factors such as temperature, material and ionization.

Electron hole: An empty space in an atom or compound where one could exist, these often function conceptually as positive versions of electrons.

Perfect conductor: A substance that has zero electrical resistance; a property required for a substance to be called a superconductor. Thus far all known perfect conductors are superconductors, so the term is often used interchangeably, but theoretically they have two distinct meanings.

Band: essentially a primary energy level

Valence band: the highest band that electron(s) are normally present at absolute zero temperatures

Conductance band: the band above the valence band, is responsible for the conductance of the material and is usually empty at absolute zero, but as temperature and other factors change, it may contain electrons.

= **What are Semiconductors?** =

Semiconductors are one the greatest and most revolutionary inventions in the last half a century. They can be found in almost all electronics from a television to a car. Semiconductors are materials that are able to conduct electricity with some resistance. Hence the name “semiconductor.” This resistance just means that electrons will not flow through the conductor efficiently. Semiconductors lie midway between a metal, which is a good conductor, and an insulator. Silicon is one of the best known semiconductors. Silicon is known as a semi conductor because of its special properties. It could essentially act as a conductor or an insulator all depending on how it is treated. There are also other materials used such as geranium, gallium arsenide, silicon carbide, etc.



= **History** =

Semiconductors have been used for a long time. First semiconductors used were seen in wireless radios. In 1899, it was founded by a German scientist named Ferdinand Braun. The device was called a crystal detector which was made with one wire against a semiconductor crystal. This specific wire was known as a cat’s whisker. The consequence was the current flowed in one direction but couldn’t flow in the opposite direction. This was called a rectifying diode. Later in 1930, the crystal detectors were replaced by vacuum-tube diodes.

However, the modern semiconductors were said to be invented when transistors were invented by scientists at Bell Telephone. Before transistors, vacuum tubes were used.To replace this, thecientists John Bardeen, William Shockley, and Walter Brattain observed crystals, specifically germanium as semiconductors. Transistors acted like a conductor and a silicon, hence why they were semiconductors. They invented the point contact transistor on 17 November 1947. This could not have been achieved without the help of scientists in the past such as Julius Edgar Lilienfeld and Dr. Oskar Heil.

= **How Semiconductors Work** =

Semiconductors exhibit properties inherent in both insulators and conductors, thus various controllable conditions may cause them to act either, which is useful in many applications. This usually occurs because as a substance's temperature increases above absolute zero, electrons in the valence band are promoted to the conductance band or higher up the valence band and, if in the conductance band, can move easily from one atom or molecule to another because of the molecule's very weak grasp on the electron, allowing electron flow and thus conducting. However if these conditions are not met then the electrons fall back to the valence band, thus the conductance band is empty and the substance acts as an insulator. For example, silicon's electron configuration is 3s^2 3p^2 at absolute zero, but enough energy, such as heat or certain light, will cause electrons to move to the p orbital, so that it looks more like 3s^0 3p^4, but with even more energy (usually light is used in electronics for various reasons) one or more electrons are promoted further, so the electron configuration becomes 3s^0 3p^3 4s^1, thus because one electron is in the conductance band, the silicon acts as a conductor.

= **Applications** =

= **Effects on Society, The Environment, and The Economy** =

Superconductors are important in several fields for several reasons, the most important being that they are the strongest magnets and have the longest longevity. For these reasons they are used in MRI and NMR machines in medicine, thus creating jobs and saving lives and hence are of great importance. Their use in many scientific instruments have also allowed several discoveries in recent years that may have been otherwise impossible or further delayed. Thus their importance is difficult to measure or describe, as they have indirectly allowed many inventions which further affect society. Lastly, superconductors are still a developing field and many promising possibilities are still open, such as extremely powerful supercomputers, thus their full effect on society is yet to be realized. Furthermore their uses are highly specialized and only require a small amount of material, so their current effect on the environment is negligible, but may become important in future years as more technology based on superconductors becomes more widely available. Similarly, although superconducting technology is beneficial to the economy, the number of jobs and amount of money it creates is negligible or near negligible to a nation's economy, and thus is of little benefit.

= **What are Superconductors?** =

Superconductors are materials that are able to conduct electricity with close to zero or zero resistance. The flow of electrons in a superconductor is greater because they are able to pass through the material efficiently. Therefore, more electricity is able to pass through. The material is able to conduct electricity without resistance when it’s brought to very low temperatures.

= **History** =

A Dutch physicist, Heike Kamerlingh Onnes first examined superconductivity in 1911. He noticed superconductivity in mercury when he cooled it to very low temperatures of 4 degrees Kelvin (i.e. -269 degrees Celcius). He recorded that as the temperature decreased to extreme lows, resistance disappeared. ==

Later on in 1933, Walther Meissner and Robert Ochsenfeld both discovered that a superconductor when cooled to low temperatures could repel and attract a magnet at the same time causing it to levitate. This was called the Meissner Effect.

As time progressed, more superconductors were found. In 1944, niobium-nitride was found to superconduct at 16 Kelvin and in 1953, vanadium-silicon was found to super conduct at 17 Kelvin. This was followed by the BCS theory named after American physicists John Bardeen, Leon Cooper, and John Schrieffer. Their theory was founded in 1957 to further explain superconductivity. In the 1980’s, Alex Muller and Georg Bednorz discovered a ceramic compound that was able to become superconductive at 30 Kelvin. This was the highest temperature known at that time. This was a great discovery because ceramics are used as insulators and were never thought to be used as superconductors. As the years went by, higher temperatures in superconductors were found. The highest record was 138 Kelvin.

= **How Superconductors Work?** =

The properties inherent of superconductors go beyond zero electrical resistance as aforementioned, they must also go through a superconducting phase transition, the Meissner effect, Meissner effect breakdown and London moment.

First, zero electrical resistance occurs because of the absence of a band gap in substances at certain temperatures (depending on the substance) allows electrons to move through the electron holes without having to move through an area where it theoretically cant exist, preventing the loss in energy from the jump from one band to another; this happens because as a substance cools the amount of charge on the protons and electrons decreases, thus the distance from the nucleus to which the orbitals exist decreases also shortening the band gap and eventually allowing "contact" and overlap between the conduction and valence bands.

Second, Phase Transition is simply the temperature at which a substance becomes superconducting, there is much variation to these temperatures, some are so low they have not yet been reached by scientists, as they are extremely close to absolute zero. On the other hand some substances may reach transition around -150 degrees Celsius, a temperature easily reachable with proper laboratory equipment. The temperature at which a substance reaches transition depends on the size of the band gap and the density of the substance. The reasoning behind the size of the band gap has already been explained, and the reasoning behind the density of the substance is similar. Simply put the conducting bands of two atoms must overlap in order to prevent the jump and subsequent loss of energy, thus as atoms move closer together from cooling the conducting bands move closer together and eventually overlap, thus the closer the bands were to begin with, the higher the temperature allowed before they overlap.

Third, the Meissner Effect can be used by scientists to make certain things levitate off the ground. For example, maglev trains use this phenomenon to allow the train to float. This occurs because of diamagnetism. Diamagnetism is when a superconductor eliminates the magnetic force of a magnet causing it to float during its conversion to a superconducting state. Before liquid helium temperatures was used then later liquid hydrogen in order to cool down the superconductor to low temperatures. It wasn’t until after that liquid nitrogen was used, this was more efficient because it was cheaper. It became easier to demonstrate the effect as seen in the image on the right. Before this process, the superconductor acts like a stone with no magnetic forces. When liquid nitrogen is poured onto the superconductor, it begins to boil because the liquid nitrogen is absorbing heat from the superconductor. The liquid nitrogen thereafter turns into nitrogen gas as the superconductors temperature begins to decrease progressively. After the superconductor has cooled, the magnet begins to give an effect to the superconductor that it didn’t before. As the magnet draws near the superconductor it begins to repel. This is what the Meissner Effect is. By pushing the magnet close to the superconductor and allowing the magnetic fields from the magnet go through the superconductor, it causes what is called a flux trapping effect. The magnet and the superconductor will now attract and repel causing the magnet to levitate.This is different from two magnets attracting because the superconductor and the magnet will always have a specific distance separating them when they attract. This is due to the fact that it is also repelling at the same time.

When a superconductor is spinning, it creates a magnetic field that closely aligns to the spin axis. This quantum mechanical occurrence is referred to as London Moment. Named after the two German brothers, Fritz and Heinz London.

= **Effects on Society, The Environment, and The Economy** =

Semiconductors are extremely important in almost all fields of science, given that their use in modern electronics has allowed for transistors, thus allowing for the computer and furthermore any discoveries or inventions as a result of it. Their effect on society has been extreme, causing a complete transformation. They allow us to make products faster and more efficiently, make calculations faster and some possible, among many others. Thus semiconductors are indirectly responsible for nearly everything that computers are responsible for, and their effect on society has been near infinite, it is also notable that many solar panels use silicon, but much more costly and destructive refining processes are used, thus an increase in green energy production and it's effects also result. Furthermore, their impact on the environment is difficult to determine as everything computers have done or allowed also falls on semiconductors, however their extraction and refining can be examined in detail. Their extraction can be devastating to the natural environment, Silicon, the most common semiconductor, typically exist naturally as a dust or sand and thus extraction involves surface mines simply taking the silica (silicon oxide), which destroys the surface habitat. Thankfully though, most surface silica is found in the sand of deserts where destruction is minimized, although other locations for surface mines and other types of mines exist. The most common refining process for silicon is simple but also destructive, involving the extracted silica and either coal or charcoal as a reducing agent. The reactants are heated to approximately 2 500 degrees Celsius so that they melt and react with one another to form elemental silicon, which is then drained from the furnace, and CO2 and CO, which simply evaporate and leave the furnace after the reaction is complete. There are problems with this method though, first that oxygen gas must not be allowed to enter the furnace or silica may result as well, second is that silicon carbide (SiC) may also form, although this may also be purified by substituting it for the coal or charcoal that is normally used, thus the same products, but in different proportions, result.

Ex. Refinement of Silicon from Silica and charcoal (elemental Carbon) SiO2 + C --> Si + CO2

Botched refinement causing production of Silicon Carbide SiO2 + 2C --> SiC + CO2

Refinement of Silicon from Silica and Silicon Carbide SiO2 + SiC --> 2Si + CO2

The reason that this is so destructive, is sourcing the charcoal or coal. These typically destroy the environment that they were sourced from, forests in the case of charcoal or various other environments in the case of coal, both of which tend to release plenty of CO2 into the atmosphere as well as other pollutants. Finally, just as it's societal effects are numerous, so are it's economical ones. The computer has opened up several industries, such as computer hardware production and development, and improved many others, such as car manufacturing. Thus the computer has caused a huge boom in national economies and created hundreds of thousands, if not millions, of jobs.

= **Applications** =

http://www.howstuffworks.com/diode.htm http://www.pbs.org/transistor/science/info/conductors.html http://nanohub.org/topics/EduSemiconductor http://www.magnet.fsu.edu/education/tutorials/magnetacademy/superconductivity101/page7.html http://www.renewableenergyworld.com/rea/news/article/2010/10/superconducting-seatitan-opens-path-to-10-mw [] [] http://www.solarbotics.net/bftgu/starting_elect_semic.html
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