Semiconductor Devices - Introduction
It is widely seen that the distance of a nucleus from the electron of a particular atom is not equal. Normally, electrons rotate in a well-defined orbit. A particular number of electrons can only hold by outer shell or orbit. The electrical conductivity of an atom is influenced mainly by the electrons of the outer shell. These electrons have a great deal to do with the electrical conductivity.
Conductors and Insulators
Electrical conduction is the result of irregular or uncontrolled movement of electrons. These movements cause certain atoms to be good electrical conductors. A material with such type of atoms has many free electrons in its outer shell or orbit.
Comparatively, an insulating material has a relatively small number of free electrons. Consequently, the outer shell electrons of insulators tend to hold their place firmly and hardly allow any current to flow through it. Therefore, in an insulating material, very little electrical conductivity takes place.
Semiconductors
In between conductors and insulators, there is a third classification of atoms (material) known as semiconductors. Generally, the conductivity of a semiconductor lies in between the conductivities of metals and insulators. However, at absolute zero temperature, the semiconductor also acts like a perfect insulator.
Silicon and germanium are the most familiar semiconductor elements. Copper oxide, cadmium-sulfide, and gallium arsenide are some other semiconductor compounds that are frequently used. These kinds of material are generally classified as type IVB elements. Such atoms have four valence electrons. If they can give up four valence electrons, stability can be accomplished. It can also be achieved by accepting four electrons.
Stability of an Atom
The concept of stability of an atom is an important factor in the status of semiconductor materials. The maximum number of electrons in the valence band is 8. When there are exactly 8 electrons in the valence band, it can be said that the atom is stable. In a stable atom, the bonding of valence electrons is very rigid. These types of atoms are excellent insulators. In such atoms, free electrons are not available for electrical conductivity.
Examples of stabilized elements are gases such as Argon, Xenon, Neon, and Krypton. Due to their property, these gases cannot be mixed with other material and are generally known as inert gases.
If the number of valence electrons in the outer shell is less than 8, then the atom is said to be unstable i.e., the atoms having fewer than 8 valence electrons are unstable. They always try to borrow or donate electrons from the neighboring atoms to become stable. Atoms in the outer shell with 5, 6, or 7 valence electrons tend to borrow electrons from other atoms to seek stability, while atoms with one, two, or three valence electrons tend to release these electrons to other nearby atoms.
