Basic Semiconductor Theory

Semiconductor is an appropriate name for the device because it perfectly describes the material from which it's made not quite a conductor, and not quite an insulator. To produce a useful device, semiconductor material is processed into MOSFET, IGBT, SCR, or diode devices, etc.

All IR semiconductor products originate from silicon wafers. Silicon is one of the most common elements on earth. It is the basis of sand and glass in the form of silicon dioxide (SiO₂). After being refined, silicon is supplied as amorphous silicon which means that the atoms are randomly arranged in the material. Under the proper conditions, silicon can be manufactured into epitaxial chunks which basically means a single crystal. Most gems are examples of single crystals. Diamonds, for example, are merely carbon atoms arranged in a particular 3-D or lattice structure shown in Figure 9. We make use of silicon in a similar lattice form to manufacture semiconductors.

A pure silicon lattice is a good insulator because the atoms are arranged so that all electrons are bonded to a silicon nucleus. In order to change the resistivity of the silicon, it is necessary to introduce impurities, which changes the number of electrons in the lattice structure. This is called doping, and may result in extra electrons (called n-type), or missing electrons (called p-type) in the lattice. Typical dopants include boron and phosphorous.

Missing electrons are also called holes. There is no physical basis for this nomenclature, but it is easy to understand how it came about. Assume that you have six paper cups and five balls. Line the cups in a row, and put balls in the five right cups. Now move the ball in the second cup to the first, the ball in the third to the second cup and so on. It appears that the empty cup is moving to the right when in reality, the balls are merely shifting to the left. This movement is possible because of the different number of cups and balls. The significance of this is that electrons can move approximately two times faster than holes, and as such, n-type material is much preferred to p-type material.

Semiconductor devices can also be divided into two groups based on how current is conducted through the material. A device in which the current is conducted by the charges dominant in the lattice is called a majority carrier device (e.g., electrons in n-type material, or holes in p-type material. If the current is conducted by charges not dominant in the lattice, the resulting device is called a minority carrier device (e.g., electrons in p-type material, or holes in retype material). This very important distinction has a large bearing on the device's operation, especially the recovery characteristics. A fish tank can be used to illustrate how current is conducted through silicon. If you put an air hose onto the bottom of the fish tank, it takes some time for the air to bubble out of the tank. This is analogous to a minority carrier device (current carrier is different from the bulk media). On the other hand, if you put a water hose onto the bottom of the tank, it doesn't take any time for the water to reach equilibrium. This is analogous to a majority carrier device (current carrier is the same as the bulk media).

Basic Semiconductor Processing

Wafers are sliced from a single silicon crystal which has to be "grown." This is done by melting silicon in a crucible. Pure silicon occurs in two forms - either as a single crystal, or as a collection of atoms with no particular arrangement, called polysilicon. A "seed" or a small silicon crystal is inserted into the crucible holding the molten polysilicon. As the seed is slowly drawn out, the molten silicon aligns with the crystal lattice in the seed. As it cools, the molten silicon expands on this crystal lattice forming an ingot as shown in Figure 1. The entire ingot is drawn out as a single crystal made up of many silicon atoms. This ingot is then sliced into thin wafers, and each wafer is polished to a mirror-like finish. The mirror-like finish of the silicon wafer needs to have a pattern etched into it to make a useful circuit, or circuit element (discrete).

A class of materials, such as silicon and germanium, whose electrical properties lie between those of conductors (such as copper and aluminum) and insulators (such as glass and rubber). A material that exhibits relatively high resistance in a pure state and much lower resistance when it contains small amounts of certain impurities. The term is also used to denote electronic devices made from semiconductor materials. See semiconductor device. semiconductor device An electronic device whose essential characteristics are governed by the flow of charge carriers within a semiconductor.

By adding some impurities to a semiconductor its electrical properties can be changed. Impurities that cause the increase of electron concentration are called donors. A semiconductor in which concentration of electrons is higher than the concentration of holes is said to be an (extrinsic) n-type semiconductor. The concentration of electrons in silicon and germanium (which are tetra-valent elements, i.e. have four valent electrons) can be increased by doping with penta-valent elements, such as phosphorus and arsenic. These elements have five valence electrons, but only four are necessary to form a covalent bond with the host semiconductor. The extra electron will be loosely bound to its parent atom and a very small amount of energy (referred to as ionization energy) will be sufficient to tear it off.

Compound semiconductors consist of a tri-valent and a penta-valent element (III-V compounds, such as gallium arsenide GaAs, gallium phosphide GaP) or a di-valent and a hexa-valent element (II-VI compounds, such as zinc sulfide ZnS). The chemical bond is formed by the component with higher valence lending some electron(s) to the component with lower valence. Donor impurities in compound semiconductors are elements with valence higher than that of the component they substitute, and acceptor impurities are elements with valence lower than that of the component they substitute. Sulfur's sixth valence electron is not necessary for the chemical bond with the surrounding gallium atoms; it is loosely bound to its parent sulfur atom and can easily become a free electron. It is interesting to notice that a tetra-valent element, such as silicon and germanium, in III-V compound can be both, a donor impurity (if it substitutes a tri-valent component) or an acceptor impurity (if it substitutes a penta-valent component).

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