Semiconductor

Semiconductor is a nonmetallic element (e.g. Si or Ge) that contains both electrons and holes as charge carriers in contrast to an enormous number of electrons only as in metals. A hhole is essentially a “half broken) covalent bond which has a missing electron and therefore behaves effectively as if positively charges. Under the action of an applied field the hole can move by accepting an electron froma neighboring bond thereby passing on the “hole” . Electron and hole concentrations in a semiconductor are generally many orders of magnitude less than those inmetals, thus leading to much smaller conductivities.

Extrinsic semiconductor

Doped semiconductor featuring either p- or n- type conductivity.

Intrinsic semiconductor

Undoped semiconductor material i.e. semiconductor which does not display either n- or p-type conductivity; free electrons and holes result only from band to band generation and hence feature the same concentration (n=p).

n-type semiconductor

semiconductor in which concentration of electrons is much higher than concentration of holes; electrons are majority carriers and dominate conductivity.

p-type semiconductor

a semiconductor for which the predominant charge carriers responsible for electrical conduction are holes. Normally, acceptor impurity atoms give rise to the excess holes.

Resistivity

Resistance per unit area and unit length; expressed in ohm-cm; the reciprocal of conductivity

Doping

Introduction of dopant into semiconductor for the purpose of altering its electrical properties; allows control of resistivity/conductivity of semiconductor by several orders of magnitude; also used to convert p-type material into n-type material and vice versa.

Four-point probe

The most common techniques for measuring resistivity of semiconductors; uses four equally placed collinear metal probes touching semiconductor, the two outside probes are used to flow the current in semiconductor while two inside probes measure resulting voltage drop which is proportional to semiconductor resistivity (doping level).

Hall effect

Hall effect is a phenomenon that occurs in a conductor carrying a current when it is placed in a magnetic field perpendicular to the current. The charge carriers in the conductor become deflected by the magnetic field and give rise to an electric field (hall field) that is perpendicular to both the current and magneteic field. If the current density J_x is along x and the magnetic field, Bz is along z, then the Hall field is either along +y or –y depending on the polarity of the charge carriers in the material.

Hall coefficient

Hall coefficient (R_H) is a parameter that gauges the magnitude of the Hall effect. If Ey is the electric field set up in the y- direction due to a current density, Jx along x and a magnetic field, Bz along z, then

Drift mobility

Drift mobility is the drift velocity per unit applied field. If m_d is the mobility then the defining equation is where is the drift velocity and E is the electric field.

Drift velocity

Drift velocity is the average velocity, over all the conduction electrons in the conductor, in the direction of an applied electrical force (F=-eE for electrons). In the absence of an applied field, all the electrons are moving around randomly and the average velocity, over all the electrons, in any direction is zero. With an applied field, E_x via where is the drift mobility.

Lorentz force

Lorentz force is the force experienced by a moving charge in a magnetic field. When a charge q is moving with a velocityv in a magnetic field B , then it experiences a force, F that is proportional to the magnitude of its charge, q, its velocity v and the field B such that

Mean free path

The average length covered by a charge carrier between two successive collisions in the solid lattice; term applies also to the motion of species in the gas.

Electron mobility

Electron mobility is a measure of electron scattering in semiconductor; proportionality factor between electron drift velocity and electric field as well as between carrier concentration and conductivity of semiconductor; unit cm²/V s; the same way as effective mass of an electron, electron mobility is different for different semiconductors; electron mobility at 300 K for three key semiconductors: Si – 1500 cm²/Vs , GaAs – 7500 cm²/Vs, 6H-SiC - 400 cm²/V s; higher mobility of charge carriers makes semiconductor better suited for high speed applications.

Hole mobility

A parameter which is a measure of hole scattering in a semiconductor; proportionality factor between hole drift velocity and electric field as well as conductivity and hole concentration in semiconductor; due to its higher effective mass, hole mobility is typically significantly lower than electron mobility

Band gap

Energy gap (E_g) in semiconductors and insulators; forbidden energy levels separating valence band and conduction band; no electrons are allowed at these levels; no bandgap in metals.

Band gap energy

For semiconductors and insulators, the energies that lie between the valence and conduction bands

Energy gap, forbidden band, bandgap Eg

Energy band separating conduction and valence banks in the solid; no electron energy levels are allowed in the forbidden band; no energy gap in metals in which case conduction and valence banbds overlap; solids featuring energy gap are defined as eithr semiconductors or insulators based on trhe width of energy gap; values of Eg (300K) for common semiconductors: InSb-0.17 eV, Ge – 0.67eV, Si – 1.12eV, GaAs – 1.43 eV, GaP – 2.26 eV, GaN – 3.5 eV and insulators Ta₂O₅ – 4.2 eV, TiO₂ – 5 eV, Si₃N₄ - 5.1 eV, Al₂O₃ ~ 5 eV

Conduction band

The upper energy band in semiconductor separated by the energy gap (bandgap) from the valence band; c.b. is not completely filled with electrons, hence electrons can conduct in the conduction band.

Valence band

Energy band in semiconductor that is filled with electrons at 0 K; electrons cannot conduct in valence band.

Elastic collision

An ideal elastic collision is the one during which there is no loss of kinetic energy and momentum is conserved; with an exception of special cases of scattering, collision between species in solids are never perfectly elastic.

Inelastic collision

An inelastic collision is the one during which there is a loss of kinetic energy (i.e part of the kinetic energy is converted into some other form of energy) and momentum is not conserved; collisions between species in solids are almost always inelastic.

Ferromagnetism

Ferromagnetic materials have atomic fields that align themselves parallel with externally applied fields creating a total magnetic field much greater than the applied field. Ferromagnetic materials have permeabilities much greater than 1. Above the Curie temperature, the ferromagnetic materials become paramagnetic.

Flux Density

Magnetic (B)- The fundamental magnetic force field. “flux” means to flow (around a current carrying conductor, for example) and “density” refers to its use with an enclosed area and Faraday’s Law to determine induced voltage. Also called the “induction field”. From Faraday’s law, the MKSA unit of flux density is a volt-second per square meter per turn or “Tesla”. (The CGS unit of flux density is the Gauss. There are 10,000 Gauss per Tesla)

Electromagnet

A magnet formed by current flowing through a conductor. The electrical conductor may be wire, copper plate or strips of foil and exist with a permeable material such as steel to conduct the field to desired areas. The magnetic field exists only so long as current flows through the coil.

Soft magnetic material

Shaped piece of ferromagnetic material that onc having been magnetized is very easily demagnetized, i.e. requires a slight coercive force to remove the resultant magnetism. Generally accepted as having a coercivity of less than 300 oersteds (24 kA/m) though most soft materials used in inductors have coercivities of under 10 oersteds.

Air gap

A non-magnetic discontinuity in a ferro-magnetic circuit. For example, the space between the poles of a magnet, although filled with brass or wood or any other non-magnetic material, is nevertheless called an air gap.

Gauss

The unit of magnetic induction, B in the CGS electromagnetic system. One gauss is equal to one Maxwell per square centimeter or 10^-4 Tesla.

Gaussmeter

An instrument that measures the instantaneous value of magnetic induction, B. Its principle of operation is usually based on one fo the following: the Hall effect, nuclear magnetic resonance (NMR) or the rotating coil principle.

Hysterisis and Hysterisis loss

Hysterisis is the tendency of a magnetic material to retain its magnetization. Hysterisis causes the graph of magnetic flux density versus magnetizing force to form a loop rather than a line. The area of the loop represents the difference between energy stored and energy released per unit volume of material per cycle. This difference is called hysterisis loss. It is one of two major loss mechanisms in inductor cores; the other is eddy current loss. Hysterisis losss is measured at low frequency to distinguish it from eddy current loss.