Semiconductor

Semiconductors are useful in electronics because their electronic properties can be greatly altered in a controllable way by adding small amounts of impurities. These impurities, called dopants, add extra electrons or holess. A semiconductor with extra electrons is called an n-type semiconductor, while a semiconductor with extra holes is called a p-type semiconductor. Semiconductors are fundamental materials in modern electronic devices (e.g. diodes, transistors, and integrated circuits) and electro-optic devices (e.g. laser diodes and LEDs).

For more information on how semiconductors are used in devices, see the article semiconductor devices.

Table of contents

1 Intrinsic semiconductors 2 Doping and extrinsic semiconductors 2.1 N-type doping 2.2 P-type doping 2.3 P-n junctions 3 See also 3.4 Encompassing fields 3.5 Sub-fields 3.6 Concepts 4 External Links

Intrinsic semiconductors

An intrinsic semiconductor is one which is pure enough that impurities do not appreciably affect its electrical behavior. In this case, all carriers are created by thermally or optically exciting electrons from the full valence band into the empty conduction band. The band gap, or energy spacing between the valence band and the conduction band, corresponds to the energy necessary to free charge carriers in this way. Note that both the electron and the hole it leaves behind are charge carriers, the hole having a charge of the same magnitude (1.6×10⁻¹⁹ C) but opposite sign of the electron. Their effective masses, i.e. their resistance to acceleration, may differ considerably. Electrons and holes flow in opposite directions in an electric field. Equal numbers of electrons and holes are present in an intrinsic semiconductor.

The concentration of carriers is strongly dependent on the temperature. At low temperatures, the valence band is completely full, making the material an insulator (see electrical conduction for more information). Increasing the temperature leads to an increase in the number of carriers and a corresponding increase in conductivity. This principle is used in thermistors. This behavior contrasts sharply with most conductors, which tend to become less conductive at higher temperatures due to increased carrier scattering.

Doping and extrinsic semiconductors

An extrinsic semiconductor is one that has been doped with impurities to modify the number and type of free charge carriers.

N-type doping

The purpose of n-type doping is to produce an abundance of carrier electrons in the material. To help understand how n-type doping is accomplished, consider the case of silicon (Si). Si atoms have four valence electrons, each of which is covalently bonded with one of four adjacent Si atoms. If an atom with five valence electrons, such as those from group VA of the periodic table (eg. phosphorus (P), arsenic (As), or antimony (Sb)), is incorporated into the crystal lattice in place of a Si atom, then that atom will have four covalent bonds and one unbonded electron. This extra electron is only weakly bound to the atom and can easily be excited into the conduction band. At normal temperatures, virtually all such electrons are excited into the conduction band. Since excitation of these electrons does not result in the formation of a hole, the number of electrons in such a material far exceeds the number of holes. In this case the electrons are the majority carriers and the holes are the minority carriers. Because the five-electron atoms have an extra electron to "donate", they are called donor atoms.

P-type doping

The purpose of p-type doping is to create an abundance of holes. In the case of silicon a trivalent atom, such as boron, is substituted into the crystal lattice. The result is that an electron is missing from one of the four possible covalent bonds. Thus the atom can accept an electron from the valence band to complete the fourth bond, resulting in the formation of a hole. Such dopants are called acceptors. When a sufficiently large number of acceptors are added, the holes greatly outnumber the excited electrons. Thus, the holes are the majority carriers, while electrons are the minority carriers in p-type materials. Blue diamonds (Type IIb), which contain boron impurities, are an example of a naturally occurring p-type semiconductor.

P-n junctions

A p-n junction may be created by doping adjacent regions of a semiconductor with p-type and n-type dopants. If a positive bias voltage is placed on the p-type side, the dominant positive carriers (holes) are pushed toward the junction. At the same time, the dominant negative carriers (electrons) in the n-type material are attracted toward the junction. Since there is an abundance of carriers at the junction, current can flow through the junction from a power supply, such as a battery. However, if the bias is reversed, the holes and electrons are pulled away from the junction, leaving a region of relatively non-conducting silicon which inhibits current flow. The p-n junction is the basis of an electronic device called a diode, which allows electric current to flow in only one direction. Similarly, a third region can be doped n-type or p-type, to form a three-terminal device. These n-p-n and p-n-p junction devices form the basis for most semiconductor devices including the transistor.

See also

Encompassing fields

• Solid state physics
• Electronics
• Solid state chemistry
• Electrical engineering

• Semiconductor device fabrication
• Semiconductor devices
  • Transistors
  • Diodes
  • Microprocessors
  • Thermistors
• Semiconductor materials
  • Aluminium arsenide
  • Aluminium gallium arsenide
  • Boron nitride
  • Diamond
  • Gallium arsenide
  • Gallium nitride
  • Germanium
  • Indium phosphide
  • Silicon
  • Silicon carbide
  • Silicon germanide
• Wide bandgap semiconductors
• Spintronics

• Conduction band
• Effective mass
• Electron hole
• Exciton
• Quantum tunneling
• Valence band

• Electrical Engineering Training Series A set of articles on Semiconductors and Transistors
• Semiconductor Technology Information on the Semiconductor Industry.
• Semiconductor and Device Information on semiconductor properties and correlation with electronic device.
• NSM-Archive Physical Properties of Semiconductors (such as Si, GaAs and others), including band structure, mechanical, electrical, thermal and optical properies
• Organic Semiconductor Technology Group discussing about organic semiconductor technology.
• Principles of Semiconductor Devices
• Britney Spears Guide to Semiconductor Physics
• Semiconductor resource launch page
• Semiconductor glossary

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