Wide-bandgap semiconductors (also known as WBG semiconductors or WBGSs) are semiconductor materials which have a larger band gap than conventional semiconductors. Whereas conventional semiconductors like silicon, germanium (due to a slightly wider bandgap than silicon), and selenium have a bandgap in the range of 0.7 – 1.5 electronvolt (eV), wide-bandgap materials have bandgaps in the range above 2 eV. Generally, wide-bandgap semiconductors have electronic properties which fall in between those of conventional semiconductors and electrical (not thermal) insulators.
Wide-bandgap semiconductors allow devices to operate at much higher voltages, frequencies, and temperatures than conventional semiconductor materials like silicon, germanium, and gallium arsenide. They are the key component used to make short-wavelength (green-UV) LEDs or lasers, and are also used in certain radio frequency applications, notably military radars. Their intrinsic qualities make them suitable for a wide range of other applications, and they are one of the leading contenders for next-generation devices for general semiconductor use.
The wider bandgap is particularly important for allowing devices that use them to operate at much higher temperatures, on the order of 300 °C. This makes them highly attractive for military applications, where they have seen a fair amount of use. The high temperature tolerance also means that these devices can be operated at much higher power levels under normal conditions. Additionally, most wide-bandgap materials also have a much higher critical electrical field density, on the order of ten times that of conventional semiconductors. Combined, these properties allow them to operate at much higher voltages and currents, which makes them highly valuable in military, radio, and power conversion applications. The US Department of Energy believes they will be a foundational technology in new electrical grid and alternative energy devices, as well as the robust and efficient power components used in high-power vehicles from plug-in electric vehicles to electric trains. Most wide-bandgap materials also have high free-electron velocities, which allows them to work at higher switching speeds, which adds to their value in radio applications. A single WBG device can be used to make a complete radio system, eliminating the need for separate signal and radio-frequency components, while operating at higher frequencies and power levels.
Research and development of wide-bandgap materials lags behind that of conventional semiconductors, which have received massive investment since the 1970s. However, their advantages in many applications, combined with some unique properties not found in conventional semiconductors, has led to increasing interest in their use in everyday electronic devices instead of silicon. Their ability to handle higher power density is particularly attractive for attempts to sustain Moore's law – the observed steady rate of increase in the density of transistors on an integrated circuit, which has, over decades, doubled roughly every two years. Conventional technologies, however, appear to be reaching a plateau of transistor density.
Use in devices Wide-bandgap materials have several characteristics that make them useful compared to narrower bandgap materials. The higher energy gap gives devices the ability to operate at higher temperatures, as bandgaps typically shrink with increasing temperature, which can be problematic when using conventional semiconductors. For some applications, wide-bandgap materials allow 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 radiation.
Solid-state lighting using wide-bandgap semiconductors has the potential to reduce the amount of energy required to provide lighting compared with incandescent lights, which have 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
The only wide bandgap materials in group IV are diamond and silicon carbide (SiC).
The only semiconducting material in group III is boron, with a wider bandgap than silicon, selenium, and germanium.
There are many III–V and II–VI compound semiconductors with wide bandgaps. In the III-V semiconductor family, aluminium nitride (AlN) is used to fabricate ultraviolet LEDs with wavelengths down to 200–250 nm, gallium nitride (GaN) is used to make blue LEDs and laser diodes, and boron nitride (BN) is proposed for blue LEDs.
Table of common wide-bandgap semiconductors Group Elem. Material Formula Band gap (eV) Gap type Description IV 1 Diamond C 5.47 indirect Excellent thermal conductivity. Superior mechanical and optical properties. III 2 Boron B >2 indirect Pure crystalline boron is a black, lustrous semiconductor; i.e., it conducts electricity like a metal at high temperatures and is almost an insulator at low temperatures. It is hard enough (9.3 on Mohs scale) to scratch some abrasives, such as carborundum, but too brittle for use in tools. It constitutes about 0.001 percent by weight of Earth's crust. Boron occurs combined as borax, kernite, and tincalconite (hydrated sodium borates), the major commercial boron minerals, especially concentrated in the arid regions of California, and as widely dispersed minerals such as colemanite, ulexite, and tourmaline. Sassolite—natural boric acid—occurs especially in Italy. IV 2 Silicon carbide SiC 2.3-3.3 indirect Bandgap varies depending on the crystal structure, 3C-SiC, 4H-SiC, or 6H-SiC. Used for high-voltage and high-temperature applications, and for early yellow and blue LEDs. III-V 2 Boron nitride BN 5.96-6.36 indirect Bandgaps listed are for cubic or hexagonal crystal structures respectively. Potentially useful for ultraviolet LEDs. III-V 2 Aluminium phosphide AlP 2.45 indirect III-V 2 Aluminium arsenide AlAs 2.16 indirect III-V 2 Aluminium nitride AlN 6.0 direct AlN is one of the few materials that have both a wide and direct bandgap (almost twice that of SiC and GaN) and large thermal conductivity. This is due to its small atomic mass, strong interatomic bonds, and simple crystal structure. III-V 2 Gallium nitride GaN 3.44 direct p-doping with Mg and annealing allowed first high-efficiency blue LEDsJohn Dakin, Robert G. W. Brown Handbook of optoelectronics, Volume 1, CRC Press, 2006 p. 57 and blue lasers. GaN transistors can operate at higher voltages and temperatures than GaAs, used in microwave power amplifiers. When doped with, e.g., manganese, it becomes a magnetic semiconductor. III-V 2 Gallium phosphide GaP 2.26 indirect Used in early low- to medium-brightness cheap red/orange/green LEDs. Employed standalone or with GaAsP. Transparent for yellow and red light, used as a substrate for GaAsP red/yellow LEDs. Doped with S or Te for n-type, with Zn for p-type. Pure GaP emits green; nitrogen-doped GaP emits yellow-green; ZnO-doped GaP emits red. II-VI 2 Cadmium sulfide CdS 2.42 direct Used in photoresistors and solar cells; CdS/Cu2S was the first efficient solar cell. Also used in solar cells with CdTe. Common as quantum dots. Crystals can act as solid-state lasers. Electroluminescent. When doped, can act as a phosphor. II-VI, oxide 2 Zinc oxide ZnO 3.37 direct Photocatalytic. Band gap is tunable from 3 to 4 eV by alloying with magnesium oxide and cadmium oxide. Intrinsic n-type; p-type doping is difficult. Heavy aluminium, indium, or gallium doping yields transparent conductive coatings; ZnO:Al is used as window coatings transparent in visible and reflective in infrared regions, and as conductive films in LCD displays and solar panels as a replacement for indium tin oxide. Resistant to radiation damage. Possible use in LEDs and laser diodes; potential for random lasers. II-VI 2 Zinc selenide ZnSe 2.7 direct Used for blue lasers and LEDs. Easy to n-type dope; p-type doping is difficult but achievable with, e.g., nitrogen. Common optical material in infrared optics. II-VI 2 Zinc sulfide ZnS 3.54/3.91 direct Band gap 3.54 eV (cubic), 3.91 (hexagonal). Can be doped both n-type and p-type. Common scintillator/phosphor when suitably doped. II-VI 2 Zinc telluride ZnTe 2.3 direct Can be grown on AlSb, GaSb, InAs, and PbSe. Used in solar cells, components of microwave generators, blue LEDs and lasers, and in electrooptics. Combined with lithium niobate to generate terahertz radiation. Oxide 2 Copper(I) oxide Cu2O 2.17 One of the most studied semiconductors. Many applications and effects were first demonstrated with it. Formerly used in rectifier diodes before silicon. Oxide 2 Tin dioxide SnO2 3.7 Oxygen-deficient n-type semiconductor. Used in gas sensors and as a transparent conductor. Layered 2 Gallium selenide GaSe 2.1 indirect Photoconductor. Utilized in nonlinear optics and as a 2D-material. Air sensitive.
Materials properties
Bandgap
Quantum mechanics gives rise to a series of distinct electron energy levels, or bands, that vary from material to material. Each band can hold a certain number of electrons; if the atom has more electrons then they are forced into higher energy bands. In the presence of external energy, some of the electrons will gain energy and move back up the energy bands, before releasing it and falling back down to a lower band. With the constant application of external energy, like the thermal energy present at room temperature, an equilibrium is reached where the population of electrons moving up and down the bands is equal.
Depending on the distribution of the energy bands, and the "band gap" between them, the materials will have very different electrical properties. For instance, at room temperature most metals have a series of partially filled bands that allow electrons to be added or removed with little applied energy. When tightly packed together, electrons can easily move from atom to atom, making them excellent conductors. In comparison, most plastic materials have widely spaced energy levels that require considerable energy to move electrons between their atoms, making them natural insulators. Semiconductors are those materials that have both types of bands, and at normal operational temperatures, some electrons are in both bands.
In semiconductors, adding a small amount of energy pushes more electrons into the conduction band, making them more conductive and allowing current to flow like a conductor. Reversing the polarity of this applied energy pushes the electrons into the more widely separated bands, making them insulators and stopping the flow. Since the amount of energy needed to push the electrons between these two levels is very small, semiconductors allow switching with very little energy input. However, this switching process depends on the electrons being naturally distributed between the two states, so small inputs cause the population statistics to change rapidly. As the external temperature changes, due to the Maxwell–Boltzmann distribution, more and more electrons will normally find themselves in one state or the other, causing the switching action to occur on its own, or stop entirely.