atomic structure of silicon
As indicated in Fig. 3, silicon has 14 orbiting electrons, germanium has 32 electrons, gallium has 31 electrons, and arsenic has 33 orbiting electrons.
For germanium and silicon there are four electrons in the outermost shell, which are referred to as valence electrons.
Valence electrons are electrons in the outer shells that are not filled. Because valence electrons have higher energy than electrons in inner orbits, they are involved in the majority of chemical processes. They assist us in determining the chemical properties of an element, such as its valency or how it forms bonds with other elements.
Gallium has three valence electrons and arsenic has five valence electrons. Atoms that have four valence electrons are called tetravalent, those with three are called trivalent, and those with five are called pentavalent.
The term valence is used to indicate that the potential (ionization potential) required to remove any one of these electrons from the atomic structure is significantly lower than that required for any other electron in the structure.
Ionization energy, also called ionization potential, is the energy necessary to remove an electron from the neutral atom. X + energy → X+ + e− where X is any atom or molecule capable of being ionized, X + is that atom or molecule with an electron removed (positive ion), and e − is the removed electron.
In a pure silicon or germanium crystal the four valence electrons of one atom form a bonding arrangement with four adjoining atoms, as shown in Fig. 4.
This bonding of atoms, strengthened by the sharing of electrons, is called covalent bonding.
Because GaAs is a compound semiconductor, there is sharing between the two different atoms, as shown in Fig. 5. Each atom, gallium or arsenic, is surrounded by atoms of the complementary type. There is still a sharing of electrons similar in structure to that of Ge and Si, but now five electrons are provided by the As atom and three by the Ga atom.
Although the covalent bond will result in a stronger bond between the
valence electrons and their parent atom, it is still possible for the
valence electrons to absorb sufficient kinetic energy from external
natural causes to break the covalent bond and assume the “free” state.
The term free is applied to any electron that has separated
from the fixed lattice structure and is very sensitive to any applied
electric fields such as established by voltage sources or any difference
in potential. The external causes include effects such as light
energy in the form of photons and thermal energy (heat) from the
surrounding medium. At room temperature there are approximately
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