Germanium & Silicon: Diodes and Transistors

• Germanium and silicon atoms are very important elements in the manufacture of diodes and transistors.
• Diagrams of atomic structures are two-dimensional, but orbiting electrons do not rotate in perfect circles or rotate in a flat plane in reality.
• Each atom has four electrons in its outer shell, called valence electrons, which are farthest from the nucleus and least tightly bound.
• Valence electrons play the active part in electrical conduction.
• Silicon and germanium are crystalline substances and the valence electrons of individual atoms link up and arrange themselves with valence electrons in adjacent atoms to form covalent bonds.
• Each atom has a half-share in eight valence electrons, creating a stable, regularly repeating three-dimensional structure called a crystal lattice.
• Pure silicon and germanium are very good insulators.
• At room temperatures, atoms vibrate enough in the lattice for some bonds to break, freeing valence electrons and leaving a "hole."
• Free electrons are attracted to these holes.
• An atom short of an electron is positively charged.
• If a battery is placed across a pure semiconductor, electrons are attracted to the positive terminal.
• Free electrons travel through semiconductor, hopping from one hole to another, making it appear as if positive holes are moving to the negative terminal.
• This small current flow is intrinsic conduction.
• Using the cinema analogy illustrates how electrons moving one way is like holes moving the other way.
• Vacancy (hole) appears to have moved along the row in one direction while the occupants (electrons) have move in the opposite direction.
• As temperature rises, more bonds break, increasing conduction and generating more heat.
• This leads to thermal runaway which eventually destroys the crystal structure.
• Semiconductors have a negative temperature coefficient, meaning resistance decreases with increasing temperature.
• Insulators are changed into conductors by doping, achieved by mixing a tiny amount of a selected impurity atom into the semiconductor material.(Typically 1 part in 10^10)
• The material transformed becomes an extrinsic semiconductor.

Types of Extrinsic Semiconductors

• N-Type and P-Type

N-Type Semiconductor

• Impurities like phosphorus or arsenic with five (pentavalent) electrons in the outermost orbit are used for doping.
• When introduced into the basic material, four electrons from the doping material join with the co-valent bonding, and one electron is left 'free'.
• The number of free electrons can be strictly controlled by this doping.
• Free electrons migrate through the inter-atomic space and act as current carriers when a low voltage is applied.
• Each impurity atom is neutral, and the overall N-type material is neutral.
• Majority carriers are electrons (negative), and minority carriers are holes (due to intrinsic conduction).

P-Type Semiconductor

• Impurities like indium or aluminum with three (trivalent) electrons in the outermost orbit are used to make this material
• When introduced, all three electrons link into the crystal structure, leaving a "hole," which acts as a positive current carrier.
• Applying a small voltage causes electrons to fill the holes, forming fresh holes, leading to a general drift of holes from positive to negative (opposite to electron flow in N-type).
• Each impurity atom is neutral, and the overall P-type material is neutral.
• Majority carriers are holes (positive), and minority carriers are electrons (due to intrinsic conduction).