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Wide bandgap semiconductors

Wide bandgap semiconductors are semiconductor materials with electronic band gaps larger than one or two electronvolts (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.

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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 semiconductors 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.

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 SiC.

Aluminium nitride 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.

Material Properties

Wide bandgap materials are defined as semiconductors with bandgaps greater than 1.7 eV.

Bandgap

The magnitude of the columbic 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 lattic 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 maximas 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.

Optical Properties

The minimum photon 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.

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 undergos 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 bnd curvature raises the effective mass of the electrons and lowers 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 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.

Processing

Epitaxial Growth

Doping

Etching

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 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.

Light-Emitting Diodes

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

Transducers

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

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.