Objectives
Independent of the material system, it is difficult to adequately cool transistors in high-power Terahertz (THz) transmitters. In power amplifiers, transmission-line networks combine the outputs of high-power transistors. A high-power transistor contains many gate or emitter fingers, placed in a small area. The power density increases as frequency squared. Advanced semiconductors such as Gallium Nitride (GaN) provide high power density, but the waste heat must nevertheless be removed. Heat removal is critical in THz power amplifiers.
Approach
In our approach, owing to the excellent heat conductivity of diamond, a layer of polycrystalline diamond (PCD) on GaN is being used to spread the heat from the top of the device (as close as possible to the high temperature spot). We grow diamond on GaN using a microwave plasma chemical vapor deposition (MPCVD) system. In this approach, we are focusing on three main goals: maximizing the thermal conductivity of diamond, reducing the film stress, and minimizing damages to the underlying devices.
Accomplishments
During the first phase of the research program, we have grown poly-crystalline diamond directly on N-polar GaN, which resulted in a smooth interface and uniform diamond layer. We have also used a thin silicon mononitride (SiN) layer on top of the GaN prior to diamond growth to enhance the adhesion by making silicon carbide (SiC) interfacial layer and protect the GaN from H-plasma etching. We have successfully grown diamond with grain sizes up to 500 nm without any delamination from the surface. We have shown that the two-dimensional electron gas (2DEG) mobility stayed the same before and after the diamond growth. In addition, we have achieved a seeding method that allows us to uniformly cover the GaN wafer. Also, a low power, low temperature growth recipe has been established that minimizes the GaN etch in hydrogen.
Team Leader
Srabanti Chowdhury is an Associate Professor of Electrical Engineering (EE) at Stanford. Her research focuses on wideband gap (WBG) materials and device engineering for energy efficient and compact system architecture for power electronics, and RF applications. Besides Gallium Nitride, her group is exploring Diamond for various electronic applications. She received her B.Tech in India in Radiophysics and Electronics (Univ. of Calcutta) and her M.S and PhD in Electrical Engineering from University of California, Santa Barbara. She received the DARPA Young Faculty Award, NSF CAREER and AFOSR Young Investigator Program (YIP) in 2015. In 2016 she received the Young Scientist award at the International Symposium on Compound Semiconductors (ISCS). Among her various synergistic activities, she serves as the member of two committees under IEEE Electron Device Society (Compound Semiconductor Devices & Circuits Committee Members and Power Devices and ICs Committee). She has served the IEEE International Electron Devices Meeting (IEDM) technical sub committee on Power Devices & Compound Semiconductor and High Speed Devices (PC) sub-committee in 2016 and 2017. She was the PC subcommittee chair for IEDM-2018, and continues to serve the IEDM executive committee for 2019. She is a senior member of IEEE.
Publications
Material pending.