Non-technical:Heterojunction bipolar transistors based on the gallium nitride/aluminum gallium nitride {(Al,Ga)N} system have long been of interest for compact high frequency power devices for telecommunications and sensing. These devices are difficult to realize due to the inability to produce a thin, highly conductive base layer that is essential for high current gain and frequency of operation. Hot electron transistors (HETs) could circumvent this problem. Carrier transport in HETs occurs via the injection of hot electrons from an emitter to a collector modulated by a base electrode. An ultra-thin base is needed to enable ultra-short transit time for high performance. Atomically thin 2D materials such as transition metal dichalcogenides are ideal materials to serve as the base electrode in a HET. This will, however, require incorporating the 2D base layer within the nitride heterostructure while retaining high quality interfaces. Previous studies have used layer transfer methods to incorporate 2D layers in HETs, but this approach introduces interfacial impurities and is difficult to scale to large areas. This project will focus instead on direct epitaxial growth of ultra-thin TMD base layers on an (Al,Ga)N collector via metal organic chemical vapor deposition along with the use of ultra-wide bandgap hexagonal boron nitiride as the emitter layer. Fundamental issues concerning the epitaxial growth, material characterization and current transport will be investigated in a step-wise process focusing on the emitter-base and base-collector interfaces. This will provide insight into the overall HET device performance and suggestions for further optimization. Research outcomes will be incorporated into undergraduate and graduate course curriculum. The project will also provide research opportunities for undergraduates and high school students from diverse backgrounds.Technical:A Two-Dimensional (2D) layer base Hot Electron Transistor (HET) is proposed for overcoming the difficulties faced by III-Nitride-based Heterojunction Bipolar Transistors (HBTs). These are severely handicapped in demonstrating good electrical characteristics due to the limited doping values of their base. The 2D layer used in the transistor base will lead to low base resistance due to its high electrical conductivity while an ultrathin base should allow the electron carriers injected into it to travel without transit time limitations. 2D transition metal dichalcogenides (TMDs) such as WS2 and WSe2, which are nearly lattice-matched to GaN and AlGaN collectors will be explored for the base. To avoid introduction of interfacial impurities and the difficulties involved in scaling to large areas commonly encountered in 2D layer transfer techniques, the research will focus on direct growth by Metalorganic Chemical Vapor Deposition (MOCVD). This will be employed for growth of Hexagonal Boron Nitride (hBN) emitters and TMD monolayer and few-layer base films on Aluminum Gallium Nitride ((Al,Ga)N) collectors. It will also allow high quality 2D/nitride interfaces. The studies will allow understanding of the role of surface energy, lattice mismatch and defects on the nucleation and epitaxial growth of 2D films on (Al,Ga)N, as well as defects at the interfaces and their correlation to material properties and growth. Basic material and device blocks will be investigated to gain good understanding of the fundamentals of growth and transport mechanisms and allow optimization of HETs. Of major importance is the understanding of current transport through the various layers leading to HET devices and their optimization. The knowledge obtained from the project will impact the fields of telecommunications and sensing thereby contributing to improving quality of life. Educational/outreach activities will be targeted at graduate, undergraduate and high school students as well as K-12 and the public.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
