BySquadron Leader Zahid Mir

CS&IT Department , Superior University

PHY-BE -03 Semiconductors (Continued)

Extrinsic Semiconductors

• If in an intrinsic semiconductor some suitable impurity or doping agent (dopent) is added in extremely small amounts (about 1 part in 108) then the semiconductor is called extrinsic semiconductor.

• Doping agents are normally from group III or group V of periodic table.– Pentavalent (having 5 valance electrons)– Trivalent (having 3 valance electrons)

Doping

The conductivity of silicon and germanium can be drastically increased by the controlled addition of impurities to the intrinsic semiconductor material. This process, called doping, increases the number of current carriers (electrons & holes).

N-Type Semiconductors

• Dopent is pentavalent atoms.• Arsenic (As), Antimony (Sb), Phosphorus (P) and Bismuth (Bi).• Dopent atom forms covalent bond with the surrounding four Ge

or Si atoms with the help of four of its five electrons.• The fifth electrons is superfluous and is loosely bound to the

dopent. Hence it can be easily excited from the valence to the conduction band by the application of electric field or increase in thermal energy.

• Dopent is a doner impurity, because it donates or contributes one electron in the conduction band of pure semiconductor.

N-Type Semiconductors

• The dopent giving away its one valance electron becomes positively charged ion. But it cannot take part in conduction because it is firmly fixed or tied into the crystal lattice.

• In n-type semiconductors, electrons are the majority carriers.

• However there are also few holes that are created when electron-hole pairs are thermally generated. Holes are minority charge carriers in n-type semiconductor.– Note These thermally generated holes are not produced by

the pentavalent impurity atoms.

P-Type Semiconductors• Dopent is trivalent atoms.• Boron (B), Indium (In), and Gallium (Ga).• Each trivalent atom forms covalent bonds with four adjacent silicon or

germanium atoms.• All three of the dopent’s valence electrons are used in the covalent

bonds; and since four electrons are required, a hole results when each trivalent atom is added.

• The number of holes ca be carefully controlled by the number of trivalent impurity atoms added.

• The trivalent atom can take an electron , it is often referred to as an acceptor atom.

• Note A hole created in this process is not accompanied by a conduction electron.

Majority & Minority Charge Carriersin P-Type Semiconductors

• Holes are the majority charge carriers in p-type semiconductor.

• Although holes are the majority carriers, there are also a few free electrons that are created when electron-hole pairs are thermally generated.

• Thermally generated electrons are the minority charge carriers in p-type semiconductor.– Note These thermally generated free electrons are

not produced by the addition of trivalent impurity atoms

Practice Questions

1. Electron-hole pairs are produced bya) Recombinationb) Thermal energyc) Ionizationd) Doping

2. Recombination is whena) An electron falls into a holeb) A positive and negative ion bond togetherc) A valance electron becomes a conduction electrond) A crystal is formed

3. The current in semiconductor is produced bya) Electron onlyb) Holes onlyc) Negative ionsd) Both electrons and holes

Practice Questions

4. The purpose of a pentavalent impurity is to a) Reduce the conductivity of siliconb) Increase the number of holesc) Increase the number of free electronsd) Create minority carriers

5. The majority carriers in n-type semiconductors area) Holesb) Valance electronsc) Conduction electronsd) Protons

6. Electrons in p-type semiconductors area) Minority charge carriers that are thermally produced b) Majority charge carriers that are produced by dopingc) Minority charge carriers that are produced by dopingd) Majority charge carriers that are thermally produced

References

• “A Textbook of Electrical Engineering” by

B L Theraja

• “Electronic Devices”by

Thomas L. Floyd (7th Edition)