Semiconductor

Semiconductor —I
• Materials that permit flow of electrons are called conductors (e.g., gold, silver, copper, etc.).
• Materials that block flow of electrons are called insulators (e.g., rubber, glass, Teflon, mica, etc.).
• Materials whose conductivity falls between those of conductors and insulators are called semiconductors.
• Semiconductors are “part-time” conductors whose conductivity can be controlled.



Semiconductor —II
• Silicon is the most common material used to build semiconductor devices.
• Si is the main ingredient of sand and it is estimated that a cubic mile of seawater contains 15,000 tons of Si.
• Si is spun and grown into a crystalline structure and cut into wafers to make electronic devices.

Semiconductor —III
• Atoms in a pure silicon wafer contains four electrons in outer orbit (called valence electrons).
– Germanium is another semiconductor material with four valence electrons.
• In the crystalline lattice structure of Si, the valence electrons of every Si atom are locked up in covalent bonds with the valence electrons of four neighboring Si atoms.
– In pure form, Si wafer does not contain any free charge carriers.
– An applied voltage across pure Si wafer does not yield electron flow through the wafer.
– A pure Si wafer is said to act as an insulator.
• In order to make useful semiconductor devices, materials such as phosphorus (P) and boron (B) are added to Si to change Si’s conductivity.