Semiconductor Basics

    • Electrons that are in orbits further from the nucleus have higher energy & are less tightly bound to the atom.
    • The force attraction between the positive charged nucleus & the negatively charged electron decreases with increasing distance from the nucleus.
    • This outermost shell is known as the valence shell & electrons in this shell are called valence electronics.

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    • When a valence electron absorb energy from a heat or light source it can actually escape from the outer shell and the resulting is a positive charged (more protons than electrons) atom is called positive ion i.e. H+. The escaped valence electron is called a free electron.
    • When a free electron loses energy and falls into the outer shell of an atom, the atom becomes negatively charged (more electrons than protons) and is called a negative ion i.e. H– .
    • Materials can be categorized into conductors, semiconductors or insulators by their ability to conduct electricity.
    • Conductors: Metals conduct electricity easily because there is no band gap since the conduction overlaps the valence band.
    • Semiconductors: The band gap is small enough that electron that absorb thermal energy can bridge the gap to the conduction band.
    • Insulators: Very large band gap between the valence & conduction bands makes it hard for electrons to bridge the gap.

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    • Two types of semi conductive materials are silicon and germanium. Both have four valence electrons.
    • The valence electrons in germanium are in 4th shell while the ones in silicon are in 3rd shell, closer to the nucleus.
    • This means that the germanium valence electrons are at higher energy levels than those in silicon. Thus require a smaller amount of additional energy to escape from the atom.
    • This property makes germanium more unstable than silicon at high temperatures, which is the main reason silicon, is the most widely used semi conductive material.

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