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• Pure (intrinsic) semiconductors (Silicon is the most common one) do not allow electrons to flow at room temperature.
• Silicon can be doped. This is the addition of controlled amounts of impurities.
• Doped silicon has interesting properties and many semiconductor devices with different properties can be manufactured.
• Each silicon atom has four electrons in the outer shell.
• In pure silicon, the outer electrons become free to move if the silicon is very hot. All the other electrons are always tightly bound to the nucleus.
• Beware: Silicone (with an E) is used for breast implants and bathroom sealants. It's not used in electronic circuits. It might be used to make a circuit box waterproof.

Dope

• Dope is the addition of tiny and carefully controlled amounts of impurities.
• N type silicon is doped with atoms that have five electrons in the outer shell. This means that there are extra electrons. These can flow at room temperature.
• P type silicon is doped with atoms that have only three electrons in the outer shell. This leaves holes where an electron would normally be expected. These holes can move at room temperature.

Silicon doped with Boron and Phosphorous

• By fabricating devices with suitable zones of P and N type silicon, it is possible to manufacture the devices familiar to electronic engineers.
• The Diode has a PN junction. This is a layer of P type and a layer of N type silicon. The useful property is that current can only flow in one direction. You need to study quantum physics to really understand why this is. A layman's explanation is that electrons can flow into holes. If a potential difference is applied in the other direction, a region is created where there are no charge carriers so no current flows. This is called a depletion zone.

• The Bipolar NPN transistor has three layers of doped silicon. It is useful because a small base current can control a much larger collector/emitter current.
• Microchips are complex circuits built from many transistors all fabricated onto one silicon wafer. Many millions of devices can be deposited onto one wafer.