Solid State Physics
Solid state physics is the study of how solids behave, in particular, how the presence of other atoms effect properties of other nearby atoms and the applications of these properties. Band theory, an important aspect of solid state physics, explains how electrons behave in solids. Much like the energy levels of a single atom, the structure of a solid requires that electrons can only be in specific "bands" of energy. Each band can only hold a specific number of electrons and those bands with the least energy associated with them fill up first. The highest, unfilled band is called the conduction band. The highest filled band is called the valence band. The energy difference between the conduction and valence bands is called the energy gap. If the highest band is only partially filled, the material is a conductor. If the energy gap is large, the material is an insulator. If the energy gap is small, the material is a semiconductor.
"Doping" a solid is a process that involves adding impurities to a solid, usually to silicon, so that there are either extra electrons or fewer electrons (adding "holes") in the solid. The extra electrons and holes effect the conduction band of the material. Those with extra electrons are called "n-type" solids; those with holes are called "p-type" solids. When these solids are sandwiched together, it creates n-p junction. Combinations of these junctions are used to create solid state components such as the diode and the transistor.
A diode consists of one n-p junction and acts as a one-way valve for electrical current. When you have an n-p junction, the electrons can drop into the "holes" (lack of electron) until there is an area where there are no free charges. This area is called a depletion zone. Applying a potential difference in one way gives the electrons the energy they need to cross the zone; this is called forward biased. When the diode is reverse biased, the potential difference makes the zone harder to cross and so there is no current flow.
The Hall effect is seen when a solid conducting material, such as a strip of copper, has a current running through it while it is in a magnetic field. The charge carriers, usually electrons, are deflected by the magnetic field and a potential difference is set up on the sides of the strip transverse to the current. This allows the sign (either positive or negative) of the charge carriers to be determined.
Superconductors are another development from solid state theory. These substances usually show little or no conductivity at room temperature. Some, such as ceramics, are actually used as insulators at room temperature. However, when they are cooled to very low temperatures, all the resistivity of the material vanishes, and so the material can super-conduct current without resistance. A magnet set spinning over a cooled superconductor will induce an opposing magnetic field inside the material so that the magnet is levitated just above the surface. This is known as the Meissner effect. Superconductors have applications in many areas; they can be used in levitating trains, medical imaging, and high-energy particle research.
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