Semiconductor Basics
Semiconductors materials like germanium (Ge), silicon (Si), and gallium arsenide (GaAs), have electrical properties somewhere in the middle, in between those of a "conductor" and an "insulator". They are neither good conductors nor good insulators (thus their name "semi"- conductors). They have a few "fee electrons" because their atoms are closely grouped together in the crystalline pattern called as "crystal lattice". But, their ability to conduct electricity can be improved greatly by adding certain "impurities" to this crystalline structure thus, producing more free electrons than the holes or vice versa. By controlling amount of the impurities added to the semiconductor material it is possible to control the conductivity of it. These impurities are called as donors or acceptors depending on whether they are producing electrons or holes. This method of adding impurity atoms to the semiconductor atoms (order of 1 impurity atom per 10 million (or more) atoms of semiconductor) is called as Doping.
The most commonly semiconductor material used is silicon. It has 4 valence electrons in its outer most shell which it shares with the adjacent atoms of it in forming covalent bonds. The structure of the bond in between two silicon atoms is such that each atom shares one electron with its neighbor making the bond quite stable. As there are a few free electrons available to move from place to place producing an electrical current, crystals of pure silicon (or germanium) are
hence good insulators, or at the very least very high value resistors. The Silicon atoms are arranged in a specific symmetrical pattern giving them a crystalline solid structure. A crystal of pure silicon (silicon dioxide or glass) is generally said to be an intrinsic crystal (which has no impurities).
The diagram drawn above shows the structure and lattice of the 'normal' pure crystal of Silicon.
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