of Semiconductor Physics, Optoelectronics and Photonics
Physical Properties of Semiconductors
More Properties of Semiconductor Materials
Acceptor -a dopant atom with a lower valence than the host material atoms. Acceptor atoms add energy levels above the valence band.
Alloyed Semiconductor - semiconductor material where proportion of the atoms occupying the lattice sites are replaced with another element from the same group in order to obtain more desirable material properties such as lattice matching or energy gap. More than one element may be substituted, allowing control of lattice spacing and energy gap.
Active Region - layer of semiconductor material where radiative recombination takes place. Also ACTIVE LAYER.
Amphoteric Impurity - an element added as an impurity that can act as a donor or an acceptor depending on the semiconductor material it is added to. For example, Si is a group IV element. In Ga, a group III element, it acts as a donor. While in As, a group V element, it acts as an acceptor.
Auger Recombination- non-radiative recombination involving the interaction of at least three particles. The energy released by a recombining electron is immediately absorbed by another electron which then dissipates the energy by emitting phonons. Most common processes for III-V semiconductors, CCCH, CHHS, CHHL.
Basis - the arrangement of atoms in the UNIT CELL.
Bloch Function - a wavefunction for an electron in a crystal having periodic structure; the function has the form y=u(r) exp(i k.r), where k is the wave vector, r is a position vector, and u(r) is a function of the position that has the lattice periodicity.
Bloch Theorem - a theorem stating that every electronic wave in a periodic crystal lattice can be represented by a Bloch function.
Bose-Einstein Distribution - a function that specifies the number of particles in each of the allowed energy states for an assembly of independent bosons.
Boson - a particle which obeys the Bose-Einstein distribution function. Boson have integer spin. Examples include photons, phonons and composite particles have an even number of member particles. See also FERMIONS
Boule - a large, cylindrical, single crystal of semiconductor material pulled from the melt which is cut into semiconductor wafers.
Bravais Lattice - any one of the fourteen possible arrays of points repeated periodically in three-dimensional space such that the arrangement of points about any one of the points is identical in every respect to that about any other point in the array.
Brillouin Zone - a fundamental polyhedron in wave vector space (k-space) whose geometry plays an important role in band theory and the specification of diffraction condition; it is bounded by a Wigner-Seitz primitive cell in the reciprocal lattice.
Carrier Density - the volume density of mobile charge carriers in a semiconductor material.
Carrier Mobility - a measure of the average speed of a carrier in a given semiconductor, given by the average drift velocity of the carrier per unit electric field.
CCCH -Auger recombination process. Two electrons collide in the conduction band. The energy of the collision sends one electron into a higher energy level in the conduction ban. The excited electron loses energy by release of phonons and the other electron is knocked into a hole in the heavy-hole band
CHHS - Auger recombination process, involving two electron collision in the heavy-hole band. One electron is excited to a higher energy in the conduction band and decays by emitting phonons. The second electron falls into a hole in the spin-off valence band.
CHHL - Auger recombination process. Two electrons in the heavy-hole valence band collide. One electron is excited to a higher energy level in the conduction band and decays by emitting phonons. The second electron falls into a hole in the light-hole valence band.
Chemical Potential - alternative name for the FERMI ENERGY.
Charge Neutrality - the ground state of matter in which there is an exact balance between all positive and negative charges in a system.
Coherence Length - the distance over which a wave train exhibits coherence. a characteristic scale of the distance over which the order parameter describing the superconducting state can vary in a superconductive material.
Coherent Radiation - the radiation that emanates from a monochromatic source, having a definite phase relationship
Compound Semiconductor - a semiconductor substance that is composed of two or more elements that are chemically combined such as GaAs, InP, GaN, GaNAs, GaInNAs
Conduction Band - an energy band in a solid in which electrons are freely mobile and can produce a net electric current.
Conductor - a material that has suffient free electrons in the conduction band to allow an electrical current to flow when a potential difference is applied. Conductors are usually metallic in nature, but not always, Graphite for example. See also SEMICONDUCTOR and INSULATOR
Current Confinement - method employed to reduce threshold currents in semiconductor lasers by restricting the cross-sectional area in which the current is allowed to flow through the active region. See ION-IMPLANTATION,OXIDE CONFINEMENT.
Current Density - Current per unit area
De-Broglie Wavelength - Prince Louis De-Broglie, proposed that since light displayed particle-like properties in addition to wave-like properties at the atomic scale, perhaps matter could display wave-like properties as well as particle-like properties. The De-Broglie wavelength is defined by λ
=h/p. Where h is Planck's constant and p is the momentum of the particle.
Degenerate Semiconductor - a semiconductor in which the conduction approaches that of a simple metal.
Density of States - a function of the energy in a solid given by the number of permitted quantum states in the energy range between E and E+dE, per unit volume of the material.
Debye Temperature - a characteristic temperature denoted by Q and defined by the relation kbQ=hn, where kb is the Boltzmann constant, h is Planck's constant, and n is the Debye frequency; used in computing the Debye specific heat of a crystal lattice.
DFB-Laser - (Distributed Feedback Laser). Laser with an wavelength selecting grating grown within the cavity. The grating selects a single lasing mode.
Diffusivity - a measure of the rate at which electrons or holes diffuse across a concentration gradient.
Distributed Bragg Reflector (DBR) - many layered structure of consisting of semiconductor material with contrasting refractive index and thickness such that reflected component of the electromagnetic wave from the layers interfaces arrive in phase with each other and superpose to create a highly reflective mirror.
Direct Gap - a semiconductor in which the location of the minimum in the conduction band coincides with that of the maximum in the valence band in reciprocal space, so that radiation due to recombination of carriers occurs at high intensity.
Donor - dopant atom with a valence higher than of the host material atoms. Donor atoms add energy levels below the conduction band.
Donor Level - an intermediate energy level close to the conduction level in a semiconductor; filled at absolute zero, the electrons in this level can acquire energies corresponding to the conduction level at other temperatures.
Doping - The lacing of an intrinsic semiconductor material with another type of atom to change the propulation of either positive or negative charge carriers, see also N-TYPE and P-TYPE
Double-Heterostructure - layer of semiconductor material with a low energy gap sandwiched between two layer of a different semiconductor material of a greater band gap. Double heterostructure because at each semiconductor/semiconductor interface is separate by different semiconductor materials.
Drift Mobility - a measure of how readily charge carriers can move under the influence of an electric field in a semiconductor or metal, given by the average drift velocity of the carrier per unit electric field. Also, MOBILITY.
Drift Velocity - the average velocity of charge carriers subjected to an electric field within a semiconductor, a conductor, or an evacuated region.
Effective Mass - a parameter for electrons or holes in a semiconductor that differs from the mass of a free electron and depends to some extent on the particle's position in the energy band, affecting the mobility and the resulting current.
Electron Affinity - The energy require to move an electron from the bottom of the conduction band to the energy level of an electron at rest in vacuum outside the solid.
Electron-Hole Pair - a conduction electron in the conduction band and an accompanying electron hole in the valence band, which result when an electron jumps the gap in an intrinsic semiconductor.
Electron-Hole Recombination - a process in which a hole is recombined with an electron within a doped semiconductor, accompanied by a release of energy, typically in the form of radiation.
Electronic Work Function - the energy required to remove an electron with the Fermi energy in a solid to the energy level of an electron at rest in vacuum outside the solid.
Elemental Semiconductor - a semiconductor material formed by a single type of atom. Examples include, Silicon and Germanium
Einstein Coefficients - a set of probability coefficients that express the probabilities of stimulated and spontaneous radiative transitions between stationary energy levels
E-k Diagram - representation of the band structure of a semiconductor material that relates the quasi-continuous energy levels to the quasi-momentum or k-space.
Energy gap - a range of forbidden energy levels between two permitted bands.
Epitaxial layer - a layer of semiconductor material having the same crystalline orientation as that of the substrate upon which it is grown.
Exciton - a bound hole-electron pair in a semiconductor, having a definite half-life during which it migrates through the crystal, and releasing its eventual recombination energy as a photon or photons.
Extended Zone Scheme - method of displaying the E-k diagram where higher energy bands are displayed in successively higher Brillouin zones. Also: REDUCED-ZONE SCHEME.
External Quantum Efficiency - the number of photons emitted per radiative electron-hole pair recombination above threshold.
Extrinsic Semiconductor - semiconductor material intentionally doped with impurity atoms to create vacancies for electrons known as holes or add electrons to material.
Fermi-Dirac Distribution - an electron energy-state distribution in a semiconductor as given by the Fermi-Dirac distribution function, in which nearly all energy levels below the Fermi level are filled and nearly all levels above the Fermi level are empty.
Fermi Level - the Fermi level is the energy at which the probability of occupation by an electron is exactly one half.
Fermion - a particle which obeys the Fermi-Dirac distribution function. Such particles have half integer spin. Examples of Fermions include electrons, protons, muons, pions.
Filled Band - an energy-level band in which there are no vacancies, and the electrons do not contribute to valence or conduction processes.
Forbidden Band - a gap between two bands of allowed energy levels in a crystalline solid. Also ENERGY GAP
FP Laser - Fabry-Perot Laser
Free-Electron Theory of Metals - an early theory of metallic conduction based on the concept that outer valence electrons, which do not form crystal bonds, are free to migrate through crystal, and so form electron gas. Also, SOMMERFELD THEORY.
Gain Guiding - see stripe geometry laser
GRIN-SCH - GRaded INdex Separate Confinement Heterostructure. A type of double heterostructure cavity structure, characterised by linearly graded refractive index in the cladding layers to the lower band gap material. Multiple Quantum Wells are separated by a number of cladding layers.
Hall Effect - the build-up of charge on a semiconductor block perpendicular to current flow and applied magnetic field. See section on Hall Effect
Hall mobility - a quantity given by the product of the Hall coefficient and the conductivity of a conductor or semiconductor; used to quantify electron mobility of a conductor or semiconductor.
Hole - Vacancy created by the addition of acceptors to the host material to generate a weakly attractive trap for an electron.
Hydrogenic Impurity- when the valence of the impurity atom is enough to netralise the charge of the surrounding atoms, additional electrons can be modeled using a simple Hydrogen like model in which we consider a single electron traveling around a positive nucleus.
Impurity - a substance that is incorporated into a semiconductor material and provides free electrons (n-type impurity) or holes (p-type impurity).
Impurity Band - an energy level, associated with the energy of a set of impurities in a solid, that is not part of the normal band scheme.
Impurity Scattering - non-radiative scattering mechanism where electrons lose energy in the form of phonon by scattering off impurity atoms in the semiconductor crystal.
Insulator - a material in which all the electrons lie in the valance band and the energy gap is too large for electrons to occupy available higher energy levels when a potential difference is applied. See also CONDUCTOR and SEMICONDUCTOR
Indirect Gap Semiconductor - a semiconductor in which the minimum in the conduction band and the maximum in the valence band occur at different locations in reciprocal space; their energy difference defines the band-gap energy.
Intrinsic Semiconductor - a semiconductor that has no impurities to change the conduction properties of the material. Also, I-TYPE SEMICONDUCTOR
Interstitial Atom - an atom occupying a region between lattice points.
Interstitial Impurity - an atom of a species not normally found in a crystal, located between normal atomic sites.
Internal Differential Quantum Efficiency - the ratio of the number of radiatively recombinant electron-hole pairs to the number of electron hole pair generated.
Ion Implantation - By selectively implanting ions into a semiconductor it can be turned into an insulating material. Protons are most often used, however other ion species including F+, O+, N+ and H+ have been tried. The bombardment of semiconductor material with ions tends to damages the semiconductor crystal structure and so it must be used with caution within close proximity to the active layer.
Kronig-Penney Model - an idealised one-dimensional model of a crystal for which the wave equation can be solved exactly; the potential energy in the model is represented by an infinite sequence of periodic square wells.
Lattice - a regularly space array of points that represents the structure of a crystal. Crystals are composed of groups of atoms repeated at regular interval in three dimensions with the same orientation. Each such group of atoms is replaced by a representative point; the collection of points so formed is the space lattice or lattice of the crystal. Each crystal lattice is a Bravais lattice.
Lateral - of semiconductor laser cavity, direction perpendicular to the length of the cavity and in the plane of the semiconductor junction. (Usually the width of the cavity.)
Lattice-matching - of alloyed semiconductors. For epitaxial growth of layers of differing semiconductor material without dislocations, it is necessary to choose an alloy composition with a similar lattice spacing to that of the substrate material.
Lattice Scattering - the electron scattering that occurs within a crystal due to vibrations in the lattice structure.
Lattice Vibration - a regular vibration of atoms in a crystal lattice about their equilibrium positions.
LASER - Light Amplification by the Stimulated Emission of Radiation
Longitudinal - of semiconductor laser cavity, direction parallel to the length.
Maxwell Boltzmann distribution - relation describing the distribution of energies for distinguishable particles in thermal equilibrium.
Mean-free path - the average distance travelled by charge carriers in a semiconductor before a collision by scattering mechanism.
Mean-free time - the average time between collisions for charge carriers in a semiconductor.
Miller Indices - the plane with Miller indices h, k, and l makes intercepts a/h, b/k and c/l with the unit cell axes a, b and c. The law of rational indices states that the indices of the faces of a crystal are usually small integers, seldom greater than three.
Mobility - in an electrical conductor, the ratio of the drift speed and the applied electric field.
Modulation-Doped Structure - a semiconductor heterostructure in which conduction electrons are spatially separated from their parent donor or acceptor impurity atoms.
Molecular-Beam Epitaxy (MBE) - a technique in crystal growth in which a molecular beam is directed at a crystalline substrate in a vacuum, resulting in the formation of crystals whose orientation is related to the orientation of the substrate.
MOCVD - Metal Organic Chemical Vapour Deposition. Growth technique for the creation of epilayers of semiconductor material.
MOPVE - Metal Organic Phase Vapour Epitaxy. See also MOCVD.
Multiphonon Emission - a non-radiative recombination process wherein the energy of an electron-hole recombination is dissipated in the form of lattice vibrations.
MQW- Multiple Quantum Well.
n-type Semiconductor - material doped with donors to increase the population of free electrons.
Optical Confinement Factor - Ratio of the emission profile within the active region to the emission profile within the cavity.
Optical Path Length - In a medium of constant refractive index, n, the geometric distance and the refractive index. In a medium of varying refractive index, the integral of nds, ds is an element of length along the path and n is the local refractive index. Optical path length is proportional to the phase shift that a lightwave undergoes along a path.
Oxide Confinement - technique for lowering the threshold current of VCSELs by producing a layer of aluminium oxide in between the cavity layer and the one of the Bragg reflectors. To grow an oxide layer one first forms a layer of AlGaAs at the location for the current apertures. In a steam environment of 350°C to 500°C the AlGaAs is converted into an insulating oxide layer. The rate of formation of the oxide layer is proportional to the content of Al in the material; thus the oxide forms first in those layers with the highest aluminium content. The oxide layer has a low refractive index compared with the semiconductor and thus also acts as a waveguide for the emitted light.
Phonon - a quantum of vibration excited by the acoustic mode of a crystal lattice; the vibration is usually thermally excited.
Phonon Emission - the generation of phonons in response to electron scattering within the crystal lattice, to anharmonic lattice forces due to the interaction with other phonons, or to X-ray or high-energy particle bombardment.
Photonic Crystal - ordered variations in refractive index, in 1-d, 2-d or dimensions, designed to create a bandstructure with forbidden regions and allowed energies that can select or confine electromagnetic waves.
p-n junction - semiconductor material doped in with donors and acceptors to form a sharply defined region with over which the doping type changes from p to n. A single p-n junction acts as a diode. More complicated arrangements of p-n junctions form the basis of many semiconductor devices such as transistors, LED's and the first inefficient semiconductor lasers.
Polarisation - a phenomenon associated with wave propagation in certain media, by which the displacement vector of the wave disturbance varies in time in a definite way.
Population Inversion - a dynamic, non-equilibrium situation in which there is a higher electron density in the conduction band than in the valence band.
p-type Semiconductor - semiconductor material doped with acceptors to create 'holes'
Quantum Well - semiconductor layer within the laser cavity with a width comparable with the De-Broglie wavelength, in which electron-hole recombination occurs.
Quaternary-Semiconductor - A semiconductor compound made of four elements (e.g., GaInAsP).
Quasi-Fermi Level - the energy level at which the occupation probability is equal to1/2 within a single energy band. Used when modelling the non-equilibrium situation of current injection within a device.
Quasi-Free Electron Theory - a modification applied to the free electron theory by which a conduction electron in a metal is described by an effective mass different from its free mass; this modification accounts for the presence of the periodic variation in the electrical potential throughout the crystal.
Quasi-Momentum - another name for the wave-vector.
Reciprocal Lattice / Reciprocal Space - the lattice with axes a*, b* and c*, related to the crystal lattice or direct lattice (with axes a, b and c) in such a way that a* is perpendicular to b and c; b* is perpendicular to a and c; c* is perpendicular to a and b. a*, b* and c* are related to a, b and c, as for example by a* = (b x c)/(a b x c)
Recombination Radiation - the electromagnetic radiation released when a conduction electron recombines with a hole in the valence band of a semiconductor.
Recombination Time - the average time for an electron-hole pair to recombine. Recombination may be radiative or non-radiative.
Reduced-Zone Scheme - method of representing the E-k diagram with higher energy bands being displayed in the same Brillouin zone. Also EXTENDED ZONE SCHEME.
Relaxation Time - in any of several physical systems, the e-folding time that is characteristic of a system when it is disturbed suddenly and then allowed to return to its steady-state condition; i.e., the average length of time that is required for a system to move toward its equilibrium state. specifically, the average length of time an electron travels through a metal before it changes its momentum upon scattering
Semiconductor - a material that is characterised by ability to conduct a small a electrical current. In terms of the band model, the lower energy levels are filled at low temperatures. Forbidden regions of energy prevent most electrons from filling upper energy levels and carrying a current.
Shubnikov-de Haas Effect - an effect due to the quantised energy levels of electrons in a metal or semiconductor, manifested by oscillations of the resistance values or Hall coefficients when the magnetic field acting on the material is increased.
Spin-off Split Orbital Band - a valence band in semiconductor materials. See also HEAVY-HOLE BAND, LIGHT-HOLE BAND.
Spontaneous Emission - the relaxation of an excited state to a ground state with the emission of a photon.
Stimulated Absorption - an incident photon of light is absorbed by an electron and excited to a higher energy state. Depends on the photon density, number of available states in the excited level and number of electrons in the ground state.
Stimulated Emission - incident photon interacts with an excited electron and loses its energy by emission of a photon of identical phase direction and energy.
Stripe Geometry Laser - semiconductor laser with current confinement provided by a stripe metallic contact region over the active region.
Substitutional Impurity - an impurity atom in a semiconductor that occupies the site of a host atom in the lattice.
Ternary Semiconductors - A semiconductor compound made of three elements (e.g., GaAlAs).
Threshold - Condition met when the gain in the laser cavity is equal to the losses
Transverse - of semiconductor laser cavity, direction perpendicular to the length of the cavity and also the junction. (Usually the thickness of the cavity.) See also LONGITUDINALand LATERAL
TE, TM - Transverse Electric/ Transverse Magnetic.
TEM Modes - Electromagnetic field variations perpendicular to the direction of travel of the wave are called transverse electromagnetic modes, or TEM modes
Unit Cell - is a volume that fills up space with an integer multiplicity, if shifted by each of the lattice vectors. It contains an integer number of lattice points. See also LATTICE, BASIS.
Vacuum Level- The energy at which electron becomes free from its host ion.
Valence Band - the highest occupied energy level in a solid crystal semiconductor or insulator that is occupied by electrons at T= 0K.
VCSEL - Vertical Cavity Surface Emitting Laser.
Vapour-Phase Epitaxy - the formation of a thin solid film on a crystal surface as the result of a thermo-chemical vapour-solid reaction, in which the deposited film crystal orientation is determined by the substrate crystal orientation.
Wave-Vector - a vector whose direction is the direction of propagation and whose magnitude is given by the wave number, 2π/λ
, where λ is the wavelength.
Wave-Vector Space - the space of wave vectors of the state functions in a system; used in calculations of energies of electrons, X-ray diffraction, and thermal vibrations in a crystal lattice. Also, K-SPACE, RECIPROCAL SPACE.
Wigner-Seitz Cell - is a volume made up of space which is closer to a given lattice point than to any other point. There is a practical recipe for the construction of this cell: Select a lattice point, draw the lines connecting it to its neighbours (nearest and next-nearest is usually sufficient), and draw the perpendicular bisecting planes to these lines. The smallest volume enclosed within these planes is the WS cell. The WS cell is a primitive unit cell that preserves the symmetries of the Bravais lattice.
Wurtzite Structure - wurtzite structure is based on the hexagonal close packed lattice (hcp), where every other atom is A or B. For example GaN
X Y Z
Zincblende - structure of tetrahedral crystals also known as sphalerite consists of two interpenetrating face-centred cubic lattices (fcc) with one of the lattices displaced in each direction by 1/4 of a lattice parameter. Compound semiconductors with a Zincblende structure include GaAs, InP, InAs, AlAs, AlSb, SiC, CdS, BN and HgTe.