(Invited) Challenges the UWBG Semiconductors AlGaN, Diamond, and Ga2O3 Must Master to Compete with SiC and GaN HPE Devices
Authors: Jones Kenneth A. , Tompkins R. P. , Graziano M. B. , Derenge M. A. 
Published: February 2020

Ultra-wide bandgap (UWBG) semiconductor devices have the potential to operate more efficiently than, and could enable applications that cannot be realized with, SiC or GaN devices because the larger bandgap, EG, provides a larger critical electric field, ξC. This enables the semiconductor to be doped more heavily for a given breakdown voltage, VB, so the internal resistance, RON, and therefore heat loss, will be smaller provided that the probability of premature breakdown has not been significantly enhanced, mobility of the carrier has not been substantially degraded, charge carriers from the dopant states can easily access a conducting band at room temperature, and dopant and electrically active impurity and point defect concentrations can be adequately controlled. If these problems can be addressed, then devices used for continuous power applications such as high power electronic inverters can be built to run more efficiently, or RF HEMTs with more output power can be fabricated. In addition, pulsed power devices such as those that could be used for electronic armor, that do not currently exist, could be built.
GaN transistors should be able to operate more efficiently than those made from SiC primarily because its 2-dimensional electron gas, 2DEG, created at the AlGaN/GaN interface has a carrier concentration that is 10X larger and an electron mobility that is 50X larger than that of SiC/oxide inverted surface; the bulk mobility is also 20% larger. However, SiC can be controllably doped at ~5x1014 cm-3, whereas background Si, O, and C impurity densities often exceed 1016 cm-3 in GaN. Si and O are shallow donors, and it has been suggested that their sources are the NH3 reactant gas in CVD growth and the NH3 or N2 reactant gas in MBE growth. Given the higher growth temperature, the quartz reactor used in CVD growth is also thought to be a source. C is amphoteric, but for n-type material it is thought to occupy a N site where it is a deep acceptor. The reactant gases containing N are thought to be a source for all types of growth; the SiC platen is thought to be a source for all types of CVD, and the metal reactant in MOCVD growth is yet another source. Si forms a DX center, O forms a deep donor, and C forms a deep acceptor in AlN so it is unlikely that AlN will be an active semiconductor in HPE devices. However, the C background concentration will be a problem for AlGaN with enough Ga so that Si acts as a shallow donor, ~ 20%. Thus, AlGaN has the advantage that it can be “tuned” over a wide range of compositions for this Ga content and above. However, alloy scattering in the 2DEG and the bulk will have to be determined to discover if the increase in the allowed maximum carrier concentration enabled by the larger EG can be more than offset by the reduction in the mobility caused by the alloy scattering. Researchers will also have to deal with the problem of lattice mismatch between the substrate and device structure. The problem is made more challenging by the facts that for structures with low Al content in the bulk layer, will be in tension because the substrate of choice is GaN, and for the high Al content structures, the drift region will have an Al content at least 30% less than the AlN substrate.
Diamond is an attractive material because both electrons and holes have a large mobility, its ξC is almost as large as it is for AlN, and it has a larger thermal conductivity. However, both the P donor and B acceptor are very deep. Also, someone will have to identify a dielectric whose band structure properly matches it to create a 2DEG with a reasonable mobility. An alternative approach could be to fabricate a bipolar junction transistor, which is made more palpable by the fact diamond has an indirect bandgap Also, methods will have to be developed to grow high quality, larger area structures.
High quality, large area, relatively inexpensive wafers of β-Ga2O3 are already available, and Si or Sn form shallow donors. However, the facts that it cannot now be doped p-type, the electron mobility of 200 cm2/V·s is quite small, its thermal conductivity is ~100X less than the other UWBGs, and its complex rhombohedral structure could create complicated point defect structures and not be compatible with materials with simpler crystal structures. Currently, the best HPE device option appears to me a MESFET.