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  • Stu WØSTU

Semiconductor Materials

Many modern electronic components are manufactured from semiconductor materials. A semiconductor material has an electrical conductivity somewhere between a great conductor like copper and a great insulator like glass – hence the name semiconductor. However, under properly applied electrical conditions a semiconductor may behave as either a conductor or as an insulator. Most modern electronics operate by leveraging the unique electrical properties of semiconductors.

An array of silicon atoms in a crystal, sharing electrons.
A silicon (Si) crystal lattice has a regular arrangement of silicon atoms each with 4 outer shell electrons that are shared among neighboring atoms to form strong bonds.

Semiconductor materials have a regular molecular structure called a lattice, and strong bonding pairs of shared electrons (valence electrons) among neighboring atoms in the lattice result in a strong, solid, crystalline material. Electronics integrated from semiconductor devices are said to be solid state electronics, contrasted with older thermionic-based electronics using vacuum tubes and other technology. Two common semiconductor materials are the elements silicon (Si) and germanium (Ge).

When used to fabricate electronic components, semiconductor materials are manufactured to be very pure, but with small amounts of precisely added impurities of other elements or compounds called dopants. Doping is the process of adding a dopant to a semiconductor material, and the type and amount of dopant helps to determine the electrical characteristics of a semiconductor device produced from the material.

An N-type crystal lattice of silicon doped with arsenic atoms, injecting an extra electron into the sharing arrangement.
An N-type semiconductor contains dopant molecules such as arsenic (As) that add an extra valence electron (small brown circles). Since it does not fit into the electron sharing pattern of the lattice, the extra electron is a free electron that may move through the lattice when voltage is applied.

When a semiconductor material is doped the regular crystal lattice structure’s electron sharing perfection is slightly disrupted. Some types of dopant atoms insert extra electrons into the latticework, while other types of dopant atoms result in the absence of an electron in their vicinity where the lattice structure would normally desire them. Semiconductor materials with extra electrons are called N-type materials, while those with a shortage of electrons are called P-type materials. You may note that extra electrons tend to provide a negatively charged ion state (N-type material) while an electron shortage results in a positively charged ion state (P-type material).

A silicon crystal lattice doped with germanium, injecting charge holes into the sharing arrangement.
A P-type semiconductor contains dopant molecules that leave holes (small open black circles) in the regular electron sharing structure of the lattice. Holes seem to cascade through the semiconductor as positive charges when a voltage is applied.

Since doped semiconductor materials have some level of electrical charge inherently, an applied voltage across the material will cause these charges to flow. The amount of current that results is a function of the voltage applied, but it is also dependent upon the quantity of extra electrons or absent electrons in the semiconductor material. By cleverly layering and joining these two different types of semiconductor materials, a variety of solid state electronic components may be constructed, such as diodes and transistors.

The charged subatomic entities that flow in doped semiconductors are called charge carriers. In the N-type material the extra electrons, or free electrons, are the majority charge carrier, and when voltage is applied they move readily through the crystal lattice without forming any strong bonds to hold them in place. In the P-type material the “holes” left in the molecular lattice seem to cascade and flow like positively charged particles. As such, these holes and the net positive charge of the atomic structures near them are said to be the majority charge carrier for P-type semiconductors.

-- Stu WØSTU


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