Semiconductors are substances with properties lies somewhere between insulators and conductors .
Integrated circuits and electronic components such as diodes and transistors are made of semiconductors. Common semiconductors are silicon and germanium. Silicon is well-known of these, forms most of ICs.
■ Semiconductors's conductivity lies between conductors and insulators . The conductivity of semiconductors increases with the increase in temperature and vice-versa. Here current flows due to free electrons and holes.
■ There are two types of semiconductors - main difference between intrinsic and extrinsic semiconductor is that intrinsic semiconductors are pure in form, while extrinsic semiconductors are impure form contains the doping of trivalent or pentavalent elements
■ Extinsic semiconductors are two types- P type and N type
When a trivalent impurity like boron is added to the semiconductor is termed as P-type whereas I when the pentavalent impurity is added. Impurities such as Arsenic, Antimony, Phosphorous and Bismuth, the semiconductors are called N type semiconductors
■ In P-type semiconductor ,Group III element like boron (B) or indium (In) is doped in Group IV element like Silicon has four valence electrons and boron of Group III has three valence electrons. When a small amount of boron is doped to a single crystal of silicon, valence electrons will be insufficient at one position to bond silicon and boron, resulting in holes that lack electrons. When a voltage is applied in this state, the neighboring electrons move to the hole, so that the place where an electron is present becomes a new hole, and the holes appear to move .
■ In N- type semiconductors, pentavalent impurities like phosphorus or arsenic are added. Four of the impurities' electrons form bonds with the surrounding silicon atoms, leaves one electron free. The resulting material has a large number of free electrons.
■ A PN Junction Diode is one of the simplest semiconductor devices around, and which has the characteristic of passing current in only one direction only.
• PN junction diode can be used as a photodiode as the diode is sensitive to the light .
• It can be used as a solar cell.
• it can be used in LED lighting applications
■ NPN and PNP transistors are bipolar junction transistors, and it is a basic electrical and electronic component which is used to build many electrical and electronic devices ..
- In PNP transistors, majority charge carriers are holes, whereas in NPN transistors, electrons are the majority charge carriers.
- Transistors are used in digital and analog circuits as a switch.
- uses in signal amplifier devices.
- Has applications in Cellular phones .
- uses in power regulator and controllers
