Gallium Nitride Superjunction Fin Field Effect Transistor

Project Overview 

The initial goal of this project is to develop the gallium vacancy assisted diffusion (GAID) method in GaN and apply it to demonstrate 2-terminal, 3-terminal, and large area Superjunction-enhanced modules. These devices will find use in high efficiency power conversion applications with the capability of operating at higher voltages, higher frequency, and less loss than comparable state-of-the-art devices. The approach of this project has been to utilize simulation tools to drive diffusion experiments and inform semiconductor device fabrication processes. VASP has been relied upon to simulate first principles diffusion mechanisms while SILVACO TCAD has been utilized to understand tradeoffs of the critical dimensions in GaN devices.

TCAD simulations have been performed to understand how an error function p-type profile, consistent with the diffusion process, in a Superjunction device affects the overall performance. Results show that tight control of the diffusion profile is necessary to achieve a device with both high breakdown voltage and low on-resistance capabilities but can ultimately lead to higher Baliga-Figure-of-Merit (BFOM) than an abrupt Superjunction device.

Driven by first principles simulations of atomic defect formation in GaN, we have selectively diffused Mg into GaN resulting in the first demonstration of selective area doping by diffusion at low temperatures. Confirmation was provided with the analysis of atomic composition measurements along with laterally mapped electrical characterization of prepared layers. Although magnesium has been observed in quantities consistent with highly doped GaN layers, hole concentrations necessary for demonstration of a high voltage PN diode have not been realized. Experiments designed to ‘trick' the GaN lattice into MgGa formation by varying the in-situ bias showed successful control of Mg incorporation, but PN diode formation was not realized.

To mitigate the risk of low hole concentration realized by the GAID process, external collaborators at UCSB have grown GaN structures which were subsequently processed at Lawrence Livermore National Laboratory (LLNL) for lateral Superjunction formation. In conclusion, we have utilized TCAD and microfabrication capabilities established on this project to form a 2-terminal half-step charge balanced diode. Additionally, work done at UCSB in collaboration with this project has demonstrated a 2-terminal charged balanced Schottky diode.

Mission Impact 

This project directly supports LLNL's Energy and Resource Security mission area. Because power electronic switches touch nearly aspect of electricity generation and consumption, even small gains in efficiency can drive outsized effects in energy consumption and improve United States energy security and reduce greenhouse gas emissions. It also falls with the Advanced Materials and Manufacturing Core Competency, as GaN is a next generation semiconductor material, and the project will be developing new manufacturing techniques for GaN devices.

Publications, Presentations, and Patents 

Varley, Joel Basile, Noah Patrick Allen, Clint Frye, Kyoung Eun Kweon, Vincenzo Lordi, and Lars Voss. "Field assisted interfacial diffusion doping through heterostructure design." U.S. Patent Application 17/166,962, filed August 19, 2021.

Voss, Lars F., Clint D. Frye, Noah A. Allen, Sarah E. Harrison, Kyoung Kweon, Joel Basile Varley, Vincenzo Lordi, Rebecca Nikolic, Travis J. Anderson, and Jennifer K. Hite. "Moderate Temperature Mg Diffusion Doping of GaN." In ECS Meeting Abstracts, no. 26, p. 1810. IOP Publishing, 2020.

Voss, Lars F., Clint D. Frye, Noah A. Allen, Sarah E. Harrison, Kyoung Kweon, Joel Basile Varley, Vincenzo Lordi, Rebecca Nikolic, Travis J. Anderson, Jennifer K. Hite., Jung Han, and Bingjun Li.  "Prospects for Magnesium diffusion doping of GaN" ECS Meeting Abstracts, 2021.

 Noah A. Allen, Clint D. Frye, Kyoung E. Kweon, Vincenzo Lordi, Qinghui Shao, Joel Basile Varley, Lars F. Voss "Superjunction Devices by Field-Assisted Diffusion of Dopants." Record of Invention, Accepted. 2022