Wide bandgap semiconductors
Encyclopedia
Wide bandgap semiconductors are semiconductor materials with electronic band gap
s larger than one or two electronvolt
s (eV). The exact threshold of "wideness" often depends on the application, such as optoelectronic and power
devices. Wide bandgap materials are often utilized in applications in which high-temperature operation is important.
s can be used to create light throughout the visible spectrum
.
Wide bandgap semiconductors can also be used in RF signal processing. Silicon-based power transistors are reaching limits of operating frequency, breakdown voltage
, and power density
. Wide bandgap materials can be used in high-temperature and power switching applications.
and silicon carbide
(SiC).
Aluminium nitride
(AlN) can be used to fabricate ultraviolet
LEDs with wavelengths down to 200-250 nm
.
Gallium nitride (GaN) is used to make blue LED
s and lasers.
Boron nitride
(BN) is used in Cubic boron nitride.
Elements high on the periodic table are more likely to be wide bandgap materials. With regard to III-V compounds, nitrides are associated with the largest bandgaps, and, in the II-VI family, oxides are generally considered to be insulators.
Bandgaps can often be engineered by alloying, and Vegard's Law
states that there is a linear relation between lattice constant
and composition of a solid solution
at constant temperature.
The position of the conduction band
minima versus maxima in the band diagram
determine whether a bandgap is direct or indirect. Most wide bandgap materials are associated with a direct bandgap, with SiC
and GaP as exceptions.
energy that is needed to excite an electron into the conduction band
is associated with the bandgap of a material. When electron-hole pairs undergo recombination
, photons are generated with energies that correspond to the magnitude of the bandgap.
A phonon
is required in the process of absorption or emission in the case of an indirect bandgap. There must be a direct bandgap in applications of optical devices.
is often attributed to be the cause of breakdown. At the point of breakdown, electrons in a semiconductor are associated with sufficient kinetic energy
to produce carriers when they collide with lattice atoms.
Wide bandgap semiconductors are associated with a high breakdown voltage. This is due to a larger electric field required to generate carriers through impact mechanism.
At high electric field
s, drift velocity
saturates
due to scattering from optical phonons. A higher optical phonon energy results in fewer optical phonons at a particular temperature, and there are therefore fewer scattering centers, and electrons in wide bandgap semiconductors can achieve high peak velocities.
The drift velocity reaches a peak at an intermediate electric field and undergoes a small drop at higher fields. Intervalley scattering is an additional scattering
mechanism at large electric fields, and it is due to a shift of carriers from the lowest valley of the conduction band
to the upper valleys, where the lower band curvature raises the effective mass of the electrons and lowers electron mobility
. The drop in drift velocity at high electric fields due to intervalley scattering is small in comparison to high saturation velocity that results from low optical phonon scattering. There is therefore an overall higher saturation velocity.
.
s, band discontinuities formed at equilibrium can be a design feature, although the discontinuity can result in complications when creating ohmic contact
s.
and zincblende structures characterize most wide bandgap semiconductors. Wurtzite phases allow spontaneous polarization in the (0001) direction. A result of the spontaneous polarization and piezoelectricity
is that the polar surfaces of the materials are associated with higher sheet carrier density than the bulk. The polar face produces a strong electric field, which creates high interface charge densities.
can be considered to be secondary properties that are essential in processing, and these properties are related to the bonding in wide bandgap materials. Strong bonds result in higher melting temperatures and lower thermal expansion coefficients. A high Debye temperature results in a high thermal conductivity. With such thermal properties, heat is easily removed.
of wide bandgap semiconductors is a useful property in high power applications that require large electric fields.
Devices for high power and high temperature applications have been developed. Both gallium nitride and silicon carbide
are robust materials well suited for such applications. Cubic boron nitride
is used as well. Most of these are for specialist applications in space programmes and military
systems. They have not begun to displace silicon
from its leading place in the general power semiconductor market.
s may replace incandescent bulbs in many situations. The next generation of DVD
players (The Blu-ray and HD DVD
formats) uses GaN based blue laser
s.
s.
Due to its cost, aluminum nitride is so far used mostly in military applications.
Band gap
In solid state physics, a band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference between the top of the valence band and the...
s larger than one or two electronvolt
Electronvolt
In physics, the electron volt is a unit of energy equal to approximately joule . By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electric potential difference of one volt...
s (eV). The exact threshold of "wideness" often depends on the application, such as optoelectronic and power
Power semiconductor device
Power semiconductor devices are semiconductor devices used as switches or rectifiers in power electronic circuits . They are also called power devices or when used in integrated circuits, called power ICs....
devices. Wide bandgap materials are often utilized in applications in which high-temperature operation is important.
Motivation Driving Utilization in Devices
Solid state lighting could reduce the amount of energy required to provide lighting as compared with incandescent lights, which are associated with a light output of less than 20 lumens per watt. The efficiency of light emitting diodes is on the order of 160 lumens per watt. Wide bandgap semiconductorSemiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
s can be used to create light throughout the visible spectrum
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...
.
Wide bandgap semiconductors can also be used in RF signal processing. Silicon-based power transistors are reaching limits of operating frequency, breakdown voltage
Breakdown voltage
The breakdown voltage of an insulator is the minimum voltage that causes a portion of an insulator to become electrically conductive.The breakdown voltage of a diode is the minimum reverse voltage to make the diode conduct in reverse...
, and power density
Power density
Power density is the amount of power per unit volume....
. Wide bandgap materials can be used in high-temperature and power switching applications.
Materials
There are many III-V and II-VI compound semiconductors with high bandgaps. The only high bandgap materials in group IV are diamondDiamond
In mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is less stable than graphite, but the conversion rate from diamond to graphite is negligible at ambient conditions...
and silicon carbide
Silicon carbide
Silicon carbide , also known as carborundum, is a compound of silicon and carbon with chemical formula SiC. It occurs in nature as the extremely rare mineral moissanite. Silicon carbide powder has been mass-produced since 1893 for use as an abrasive...
(SiC).
Aluminium nitride
Aluminium nitride
Aluminium nitride is a nitride of aluminium. Its wurtzite phase is a wide band gap semiconductor material, giving it potential application for deep ultraviolet optoelectronics.-History:...
(AlN) can be used to fabricate ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
LEDs with wavelengths down to 200-250 nm
Nanometre
A nanometre is a unit of length in the metric system, equal to one billionth of a metre. The name combines the SI prefix nano- with the parent unit name metre .The nanometre is often used to express dimensions on the atomic scale: the diameter...
.
Gallium nitride (GaN) is used to make blue LED
LEd
LEd is a TeX/LaTeX editing software working under Microsoft Windows. It is a freeware product....
s and lasers.
Boron nitride
Boron nitride
Boron nitride is a chemical compound with chemical formula BN, consisting of equal numbers of boron and nitrogen atoms. BN is isoelectronic to a similarly structured carbon lattice and thus exists in various crystalline forms...
(BN) is used in Cubic boron nitride.
Material Properties
Wide bandgap materials are defined as semiconductors with bandgaps greater than 1.7 eV.Bandgap
The magnitude of the coulombic potential determines the bandgap of a material, and the size of atoms and electronegativities are two factors that determine the bandgap. Materials with small atoms and strong, electronegative atomic bonds are associated with wide bandgaps. Smaller lattice spacing results in a higher perturbing potential of neighbors.Elements high on the periodic table are more likely to be wide bandgap materials. With regard to III-V compounds, nitrides are associated with the largest bandgaps, and, in the II-VI family, oxides are generally considered to be insulators.
Bandgaps can often be engineered by alloying, and Vegard's Law
Vegard's law
In metallurgy, Vegard's law is an approximate empirical rule which holds that a linear relation exists, at constant temperature, between the crystal lattice parameter of an alloy and the concentrations of the constituent elements....
states that there is a linear relation between lattice constant
Lattice constant
The lattice constant [or lattice parameter] refers to the constant distance between unit cells in a crystal lattice. Lattices in three dimensions generally have three lattice constants, referred to as a, b, and c. However, in the special case of cubic crystal structures, all of the constants are...
and composition of a solid solution
Solid solution
A solid solution is a solid-state solution of one or more solutes in a solvent. Such a mixture is considered a solution rather than a compound when the crystal structure of the solvent remains unchanged by addition of the solutes, and when the mixture remains in a single homogeneous phase...
at constant temperature.
The position of the conduction band
Conduction band
In the solid-state physics field of semiconductors and insulators, the conduction band is the range of electron energies, higher than that of the valence band, sufficient to free an electron from binding with its individual atom and allow it to move freely within the atomic lattice of the material...
minima versus maxima in the band diagram
Band diagram
This article refers to the electronic bandgap found in the semiconductors; for discussion of the photonic band gap, see Photonic Crystal article....
determine whether a bandgap is direct or indirect. Most wide bandgap materials are associated with a direct bandgap, with SiC
Silicon carbide
Silicon carbide , also known as carborundum, is a compound of silicon and carbon with chemical formula SiC. It occurs in nature as the extremely rare mineral moissanite. Silicon carbide powder has been mass-produced since 1893 for use as an abrasive...
and GaP as exceptions.
Optical properties
The minimum photonPhoton
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
energy that is needed to excite an electron into the conduction band
Conduction band
In the solid-state physics field of semiconductors and insulators, the conduction band is the range of electron energies, higher than that of the valence band, sufficient to free an electron from binding with its individual atom and allow it to move freely within the atomic lattice of the material...
is associated with the bandgap of a material. When electron-hole pairs undergo recombination
Carrier generation and recombination
In the solid state physics of semiconductors, carrier generation and recombination are processes by which mobile charge carriers are created and eliminated. Carrier generation and recombination processes are fundamental to the operation of many optoelectronic semiconductor devices, such as...
, photons are generated with energies that correspond to the magnitude of the bandgap.
A phonon
Phonon
In physics, a phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, such as solids and some liquids...
is required in the process of absorption or emission in the case of an indirect bandgap. There must be a direct bandgap in applications of optical devices.
Breakdown field
Impact ionizationImpact ionization
Impact ionization is the process in a material by which one energetic charge carrier can lose energy by the creation of other charge carriers...
is often attributed to be the cause of breakdown. At the point of breakdown, electrons in a semiconductor are associated with sufficient kinetic energy
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...
to produce carriers when they collide with lattice atoms.
Wide bandgap semiconductors are associated with a high breakdown voltage. This is due to a larger electric field required to generate carriers through impact mechanism.
At high electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
s, drift velocity
Drift velocity
The drift velocity is the average velocity that a particle, such as an electron, attains due to an electric field. It can also be referred to as Axial Drift Velocity since particles defined are assumed to be moving along a plane. In general, an electron will 'rattle around' in a conductor at the...
saturates
Velocity saturation
In semiconductors, when a strong enough electric field is applied, the carrier velocity in the semiconductor reaches a maximum value, saturation velocity. When this happens, the semiconductor is said to be in a state of velocity saturation...
due to scattering from optical phonons. A higher optical phonon energy results in fewer optical phonons at a particular temperature, and there are therefore fewer scattering centers, and electrons in wide bandgap semiconductors can achieve high peak velocities.
The drift velocity reaches a peak at an intermediate electric field and undergoes a small drop at higher fields. Intervalley scattering is an additional scattering
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...
mechanism at large electric fields, and it is due to a shift of carriers from the lowest valley of the conduction band
Conduction band
In the solid-state physics field of semiconductors and insulators, the conduction band is the range of electron energies, higher than that of the valence band, sufficient to free an electron from binding with its individual atom and allow it to move freely within the atomic lattice of the material...
to the upper valleys, where the lower band curvature raises the effective mass of the electrons and lowers electron mobility
Electron mobility
In solid-state physics, the electron mobility characterizes how quickly an electron can move through a metal or semiconductor, when pulled by an electric field. In semiconductors, there is an analogous quantity for holes, called hole mobility...
. The drop in drift velocity at high electric fields due to intervalley scattering is small in comparison to high saturation velocity that results from low optical phonon scattering. There is therefore an overall higher saturation velocity.
Saturation velocity
High effective masses of charge carriers are a result of low band curvatures, which correspond to low mobility. Fast response times of devices with wide bandgap semiconductors is due to the high carrier drift velocity at large electric fields, or saturation velocitySaturation velocity
Saturation velocity is the maximum velocity a charge carrier in a semiconductor, generally an electron, attains in the presence of very high electric fields. Charge carriers normally move at an average drift speed proportional to the electric field strength they experience temporally. The...
.
Bandgap discontinuity
When wide bandgap semiconductors are used in heterojunctionHeterojunction
A heterojunction is the interface that occurs between two layers or regions of dissimilar crystalline semiconductors. These semiconducting materials have unequal band gaps as opposed to a homojunction...
s, band discontinuities formed at equilibrium can be a design feature, although the discontinuity can result in complications when creating ohmic contact
Ohmic contact
An ohmic contact is a region on a semiconductor device that has been prepared so that the current-voltage curve of the device is linear and symmetric. If the I-V characteristic is non-linear and asymmetric, the contact is not ohmic, but is a blocking or Schottky contact...
s.
Polarization
WurtziteWurtzite
Wurtzite is a zinc iron sulfide mineral a less frequently encountered mineral form of sphalerite. The iron content is variable up to eight percent. It is trimorphous with matraite and sphalerite....
and zincblende structures characterize most wide bandgap semiconductors. Wurtzite phases allow spontaneous polarization in the (0001) direction. A result of the spontaneous polarization and piezoelectricity
Piezoelectricity
Piezoelectricity is the charge which accumulates in certain solid materials in response to applied mechanical stress. The word piezoelectricity means electricity resulting from pressure...
is that the polar surfaces of the materials are associated with higher sheet carrier density than the bulk. The polar face produces a strong electric field, which creates high interface charge densities.
Thermal properties
Melting temperatures, thermal expansion coefficients, and thermal conductivityThermal conductivity
In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....
can be considered to be secondary properties that are essential in processing, and these properties are related to the bonding in wide bandgap materials. Strong bonds result in higher melting temperatures and lower thermal expansion coefficients. A high Debye temperature results in a high thermal conductivity. With such thermal properties, heat is easily removed.
High Power Applications
The high breakdown voltageBreakdown voltage
The breakdown voltage of an insulator is the minimum voltage that causes a portion of an insulator to become electrically conductive.The breakdown voltage of a diode is the minimum reverse voltage to make the diode conduct in reverse...
of wide bandgap semiconductors is a useful property in high power applications that require large electric fields.
Devices for high power and high temperature applications have been developed. Both gallium nitride and silicon carbide
Silicon carbide
Silicon carbide , also known as carborundum, is a compound of silicon and carbon with chemical formula SiC. It occurs in nature as the extremely rare mineral moissanite. Silicon carbide powder has been mass-produced since 1893 for use as an abrasive...
are robust materials well suited for such applications. Cubic boron nitride
Boron nitride
Boron nitride is a chemical compound with chemical formula BN, consisting of equal numbers of boron and nitrogen atoms. BN is isoelectronic to a similarly structured carbon lattice and thus exists in various crystalline forms...
is used as well. Most of these are for specialist applications in space programmes and military
Military
A military is an organization authorized by its greater society to use lethal force, usually including use of weapons, in defending its country by combating actual or perceived threats. The military may have additional functions of use to its greater society, such as advancing a political agenda e.g...
systems. They have not begun to displace silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
from its leading place in the general power semiconductor market.
Light-Emitting Diodes
In the future, high brightness, long life white LEDLEd
LEd is a TeX/LaTeX editing software working under Microsoft Windows. It is a freeware product....
s may replace incandescent bulbs in many situations. The next generation of DVD
DVD
A DVD is an optical disc storage media format, invented and developed by Philips, Sony, Toshiba, and Panasonic in 1995. DVDs offer higher storage capacity than Compact Discs while having the same dimensions....
players (The Blu-ray and HD DVD
HD DVD
HD DVD is a discontinued high-density optical disc format for storing data and high-definition video.Supported principally by Toshiba, HD DVD was envisioned to be the successor to the standard DVD format...
formats) uses GaN based blue laser
Blue laser
A so-called blue laser is a laser that emits electromagnetic radiation at a wavelength of between 360 and 480 nanometres, which the human eye sees as blue or violet. Diode lasers which emit light at 445 nm are becoming popular as handheld lasers. Light of a shorter wavelength than 400 nm is...
s.
Transducers
Large piezoelectric effects allow wide bandgap materials to be used as transducerTransducer
A transducer is a device that converts one type of energy to another. Energy types include electrical, mechanical, electromagnetic , chemical, acoustic or thermal energy. While the term transducer commonly implies the use of a sensor/detector, any device which converts energy can be considered a...
s.
HEMT
Very high speed GaN utilizes the phenomenon of high interface charge densities.Due to its cost, aluminum nitride is so far used mostly in military applications.
Important wide bandgap semiconductors
- Silicon carbideSilicon carbideSilicon carbide , also known as carborundum, is a compound of silicon and carbon with chemical formula SiC. It occurs in nature as the extremely rare mineral moissanite. Silicon carbide powder has been mass-produced since 1893 for use as an abrasive...
- Aluminium nitrideAluminium nitrideAluminium nitride is a nitride of aluminium. Its wurtzite phase is a wide band gap semiconductor material, giving it potential application for deep ultraviolet optoelectronics.-History:...
- Gallium nitride
- Boron nitrideBoron nitrideBoron nitride is a chemical compound with chemical formula BN, consisting of equal numbers of boron and nitrogen atoms. BN is isoelectronic to a similarly structured carbon lattice and thus exists in various crystalline forms...
- DiamondDiamondIn mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is less stable than graphite, but the conversion rate from diamond to graphite is negligible at ambient conditions...
See also
- Band gapBand gapIn solid state physics, a band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference between the top of the valence band and the...
- Direct bandgap
- SemiconductorSemiconductorA semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
- Semiconductor deviceSemiconductor deviceSemiconductor devices are electronic components that exploit the electronic properties of semiconductor materials, principally silicon, germanium, and gallium arsenide, as well as organic semiconductors. Semiconductor devices have replaced thermionic devices in most applications...
s - Semiconductor materialsSemiconductor materialsSemiconductor materials are nominally small band gap insulators. The defining property of a semiconductor material is that it can be doped with impurities that alter its electronic properties in a controllable way...