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
N-Type Silicon • Pentavalent impurities such as phosphorus, arsenic, antimony, and bismuth have 5 valence electrons. • When phosphorus impurity is added to Si, every phosphorus atom’s four valence electrons are locked up in covalent bond with valence electrons of four neighboring Si atoms. However, the 5th valence electron of phosphorus atom does not find a binding electron and thus remains free to float. When a voltage is applied across the silicon-phosphorus mixture, free electrons migrate toward the positive voltage end. • When phosphorus is added to Si to yield the above effect, we say that Si is doped with phosphorus. The resulting mixture is called N-type silicon (N: negative charge carrier silicon). • The pentavalent impurities are referred to as donor impurities. 5 valence electrons