Wide-bandgap semiconductor

Wide-bandgap semiconductors are semiconductor materials that permit devices to operate at much higher voltages, frequencies and temperature than conventional semiconductor materials allowing more powerful electrical mechanisms to be built which are cheaper and more energy efficient. "Wide-bandgap" refers to higher-energy electronic band gaps, usually significantly larger than one electronvolt (eV). The exact threshold of "wideness" depends on the context, relative to what is considered a normal bandgap for the particular application at the particular time, but in contemporary usage, "wide" bandgap typically refers to material with a band gaps of at least two[1] or three eV,[2] significantly greater than that of the commonly used semiconductors, silicon (1.1 eV) or gallium arsenide (1.4 eV). WBG semiconductors are considered by the US Department of Energy to be a foundational technology that will transform multiple markets and industries, resulting in billions of dollars of savings for businesses and consumers when use becomes widespread.[3] Applications include optoelectronic devices such as those for LED lighting and power components needed for higher efficiency transformers for grid and alternative energy generation as well as the robust and efficient power components used in high energy vehicles from electric trains to plug-in electric vehicles. Wide bandgap materials are often utilized in applications in which high-temperature operation is important.

Use in devices

Wide bandgap materials have several characteristics that make them useful compared to lower bandgap materials. The higher energy gap gives devices the ability to operate at higher temperatures,[4] and for some applications, allows devices to switch larger voltages. The wide bandgap also brings the electronic transition energy into the range of the energy of visible light, and hence light-emitting devices such as light-emitting diodes (LEDs) and semiconductor lasers can be made that emit in the visible spectrum, or even produce ultraviolet emission.

Solid-state lighting could reduce the amount of energy required to provide lighting as compared with incandescent lights, which are associated with a luminous efficacy of less than 20 lumens per watt. The efficacy of LEDs is on the order of 160 lumens per watt.

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.

Materials

Further information: List of semiconductor materials

There are many III–V and II–VI compound semiconductors with high bandgaps. The only high bandgap materials in group IV are diamond and silicon carbide (SiC).

Aluminum nitride (AlN) can be used to fabricate ultraviolet LEDs with wavelengths down to 200–250 nm.

Gallium nitride (GaN) is used to make blue LEDs and lasers.

Boron nitride (BN) is used in cubic boron nitride.

Material properties

Wide bandgap materials are semiconductors with bandgaps greater than 3 eV.[2]

Bandgap

Main article: Band gap

The magnitude of the potential well (see Coulomb's Law) 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 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 structure determine whether a bandgap is direct or indirect. Most wide bandgap materials are associated with a direct bandgap, with SiC and GaP as exceptions.

Optical properties

The bandgap determines the wavelength at which LEDs can emit light and the wavelength at which photovoltaics operate most efficiently. Wide-bandgap devices therefore are useful at shorter wavelengths than other semiconductor devices. The bandgap for GaAs of 1.4 eV, for example, corresponds to a wavelength of approximately 890 nm, which is invisible infrared light (the equivalent wavelength for light energy can be determined by dividing the constant 1240 nm-eV by the energy in eV, so 1240 nm-eV/1.4 eV=886 nm). Therefore, GaAs photovoltaics are not ideal for converting shorter-wavelength visible light into electricity. Silicon at 1.1 eV (1100 nm) is even worse. For solar-energy conversion using a single junction photovoltaic cell, the ideal bandgap has been variously estimated from around 1.0 eV up to around 1.5 eV[5] (depending on various assumptions) because that low wavelength threshold covers nearly the entire solar spectrum that reaches the Earth's surface, but a lower-bandgap single-junction cell wastes a large portion of that power by inefficiently converting the shorter-wavelength parts of the solar spectrum. Because of this, a major area in solar energy research is developing multi-junction solar cells that collect separate parts of the spectrum with more efficiency, and wide bandgap photovoltaics are a key component for collecting the part of the spectrum beyond the infrared.

For LED lighting, obviously the capacity to emit visible light rather than invisible infrared light is very necessary, and the rise of LEDs for lighting applications depended particularly on the development of wide-bandgap nitride semiconductors.

The connection between the wavelength and the bandgap is that the energy of the bandgap is the minimum energy that is needed to excite an electron into the conduction band. In order for an unassisted photon to cause this excitation, it therefore must have at least that much energy. In the opposite process, when excited 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 semiconductor, so indirect bandgap semiconductors are usually very inefficient emitters, though they work reasonably well as absorbers (as with silicon photovoltaics).

Breakdown field

Impact ionization 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 fields, 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.

Saturation velocity

Main article: 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 velocity.

Bandgap discontinuity

When wide bandgap semiconductors are used in heterojunctions, band discontinuities formed at equilibrium can be a design feature, although the discontinuity can result in complications when creating ohmic contacts.

Polarization

Wurtzite 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.

Thermal properties

Melting temperatures, thermal expansion coefficients, and thermal conductivity 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.

Applications

High power applications

The high breakdown voltage of wide bandgap semiconductors is a useful property in high-power applications that require large electric fields.

Devices for high power and high temperature[4] applications have been developed. Both gallium nitride and silicon carbide are robust materials well suited for such applications. Due to its robustness and ease of manufacture, semiconductors using silicon carbide are expected to be used widely, create simpler and higher efficiency charging for hybrid and all-electric vehicles, reduced energy lost and longer life solar and wind energy power converters, and elimination of bulky grid substation transformers.[6] 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.

Light-emitting diodes

In the near future, white LEDs with the features of more brightness and longer life may replace incandescent bulbs in many situations. The next generation of DVD players (The Blu-ray and HD DVD formats) uses GaN based blue lasers.

Transducers

Large piezoelectric effects allow wide bandgap materials to be used as transducers.

HEMT

Main article: HEMT

Very high speed GaN uses 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

See also

References

  1. ↑ Yoshikawa, A. (2007). "Development and Applications of Wide Bandgap Semiconductors". In Yoshikawa, A.; Matsunami, H.; Nanishi, Y. Wide Bandgap Semiconductors. Springer. p. 2. ISBN 978-3-540-47235-3.
  2. 1 2 Shen, Shyh-Chiang, Wide-bandgap device research and development at SRL, Georgia Institute of Technology Semiconductor Research Laboratory, retrieved 2014-09-03
  3. ↑ DOE Advanced Manufacturing Office (April 2013), Wide Bandgap Semiconductors: Pursuing the Promise (DOE/EE-0910) (pdf), United States Department of Energy, retrieved 2014-09-03
  4. 1 2 Kirschman, Randall, ed. (1999), High-Temperature Electronics, NY: IEEE Press, ISBN 0-7803-3477-9
  5. ↑ Ahmed, Samir A. (1980). "Prospects for Photovoltaic Conversion of Solar Energy". In Manassah, Jamal T. Alternative Energy Sources. Elsevier. p. 365.
  6. ↑ Ozpineci, Burak; Tolbert, Leon (September 27, 2011), "Silicon Carbide: Smaller, Faster, Tougher", IEEE Spectrum, retrieved 2014-09-03
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