Bridging the Gap: Overcoming Materials and Device Challenges that Limit UWBG Semiconductor Progress

Ultrawide bandgap semiconductors (UWBGS) offer a myriad of electrical, optical, thermal, and mechanical properties that have the potential to unlock previously unattainable levels of performance in coupled-phenomena devices. Despite their known theoretical advantages, progress in UWBGS materials and devices has been inhibited by a number of challenges over the past few decades. Practical considerations such as cost, size and consistency of substrate material have made it difficult for the research community to systematically study technical questions that exist with the synthesis and processing of UWBGS. It is exciting to see recent progress in wafer size and affordability of UWBGS substrates such as single crystal diamond, AlN, and Ga₂O₃; however, proof-of-concept device demonstrations that clearly display the performance benefits of these materials are still needed to further attract interest in the technology.<br/><br/>While the breakdown strength advantages of UWBGS have been clearly verified in the lab by several groups, there are additional challenges that need to be overcome in order to establish this technology’s long-term applicability. This talk will discuss a few of these challenges along with potential approaches being explored to provide solutions:<br/><br/><i>Challenge #1: Realizing highly mobile electronic charge at sufficient density within UWBGS channels and thin films</i> – Conventional approaches such as impurity doping in UWBGS are difficult because many impurities form deep levels within the bandgap (typically 0.2 eV or deeper), resulting in low room-temperature ionization efficiency. Additionally, conventional thin film growth approaches may not provide an energetically favorable environment for the incorporation of dopant atoms on the desired lattice site. Lastly, UWBGS materials often have high levels of point or extended defects that can serve as compensation dopants, limiting the concentration of free carriers. A synthesis approach that has been studied by a few groups is the use of metal-modulated molecular beam epitaxy (MME) for the kinetically-controlled incorporation of impurities in AlN thin films. We will discuss our group’s efforts involving Si-doping of AlN with MME.<br/><br/><i>Challenge #2: Managing high electric fields present in UWBGS devices</i> – Common device passivation layers such as SiN_x and gate dielectrics such as HfO₂ have electric field breakdown strengths in the ~5-10 MV/cm range. As the breakdown strength of UWBGS materials exceeds this range and devices start to operate at 100’s and 1000’s of volts, passive dielectrics in the device will quickly become potential points for catastrophic failure. As much effort as the community is investing in UWBGS thin films, it should also be working to develop complementary dielectrics necessary for device passivation or gate insulation to support long-term reliability. In this talk, we will discuss the concept of relative permittivity engineering for dielectric superjunctions and their use to spread the peak electric fields in UWBGS devices.<br/><br/><i>Challenge #3: Improving the thermal management of high power UWBGS devices</i> – A potential future application of UWBGS is in the area of RF power transistors. The current state-of-the-art, GaN, is typically operated in a backed-off power state to prevent acceleration of failure mechanisms due to excessive self-heating during operation, limiting its ultimate RF output power density. While the physics of failure mechanisms in UWBGS devices are largely unknown at present, it is likely that thermal management of these devices will need to be co-designed alongside electrical considerations in order for the technology to realize performance benefits. This talk will discuss strategies for device integration with diamond for improved thermal management.