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. germanium Semiconductors silicon
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. 4 valence electrons