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.
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).
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 LEDs and lasers.
Wide bandgap materials are defined as semiconductors with bandgaps greater than 1.7 eV.
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 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.
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.
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 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.
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.
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.
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