Semiconductor materials have physical characteristics that are totally different from those of metals. Whereas metals conduct electricity at all temperatures, semiconductors conduct well at some temperatures and poorly at others.

In the preceding section, it was shown that semiconductors are covalent solids. That is, the atoms form covalent bonds with themselves, the most important being silicon and germanium.

Others may form semiconductor compounds where two or more elements form covalent bonds, such as gallium (Group III) and arsenic (Group V), which combine to form gallium arsenide.

Typical semiconductor materials used in the fabrication of IC chips are

■ Elemental semiconductors
– Silicon
– Germanium
– Selenium

■ Semiconducting compounds
– Gallium arsenide (GaAs)
– Gallium arsenide–phosphide (FaAsP)
– Indium phosphide (InP)

Germanium is an elemental semiconductor that was used to fabricate the first transistors and solid state devices. But, because it is difficult to process and inhibits device performance, it is rarely used now.

The other elemental semiconductor, silicon, is used in approximately 90 percent of the chips fabricated. Silicon’s popularity can be attributed to its abundance in nature and retention of good electrical properties, even at high temperatures. In addition, its silicon dioxide (SiO2) has many properties ideally suited to IC manufacturing.

Gallium arsenide is classified as a semiconducting compound. Some of its properties, such as faster operating frequencies (two to three times faster than silicon), low heat dissipation, resistance to radiation, and minimal leakage between adjacent components, makes GaAs an important semiconductor for use in high-performance applications. Its drawbacks are the difficulty of growing the ingots and fabricating the ICs.

An elemental or compound semiconductor that was not contaminated by the introduction of impurities is called an intrinsic semiconductor. At an absolute zero temperature, intrinsic semiconductors form stable covalent bonds that have valence shells completely filled with electrons.

These covalent bonds are very strong, so that each electron is held very strongly to the atoms sharing it. Thus, there are no free electrons available, and no electrical conduction is possible. As the temperature is raised to relatively high temperatures, the valence bonds sometimes break, and electrons are released.

The free electrons behave in the same way as free electrons in a metal; therefore, electrical conduction is now possible when an electric field is applied.

If an impurity, such as phosphorus or boron, is introduced into the crystal structure of an intrinsic semiconductor, its chemical state is altered to where the semiconductor will have an excess or deficiency of electrons, depending on the impurity type used. The process of adding a small quantity of impurities to an intrinsic semiconductor is called doping.

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