Electrical and Thermal Characterisation of GaN-based Devices for RF and Power Electronics

Abstract

The rapid increase in global energy demand in the period of significant environmental concern requires us to look beyond standard Si based electronics and towards the wide bandgap semiconductors. AlGaN/GaN based heterostructures are capable of fast switching, larger breakdown voltages and higher power densities, offering a viable alternative to conventional Si solutions. GaN-based devices are predicted to dominate key automotive, consumer and industrial markets in the next decade however, to achieve widespread commercial application a number challenges relating to electrical and thermal reliability have to be overcome.
In this thesis, the relationship between vertical charge transport processes in GaN buffers is explored. By combining electrical characterisation of GaN-on-Si power HEMTs with TCAD simulations the dominant vertical charge transport processes are identifies as 3D variable range hopping most likely in the defect band in the carbon-doped GaN buffer and 1D hopping along the dislocation in the unintentionally doped GaN channel. The effect of these charge transport
process on dynamic On-resistance (RON) is studied by examining the temperature and field dependence of dynamic RON in the transfer length method (TLM) structures and in lateral Schottky barrier diodes grown on the identical epitaxy. The results indicate that the variation in dynamic RON is dominated by charge transport processes in GaN layers and not by trapping.
In addition, comprehensive electrical and thermal characterisation of early generation "bufferfree" GaN-on-SiC RF HEMTs (where thin GaN channel is grown directly on AlN nucleation layer) is combined with computational simulations to determine device design for optimised electrical and thermal performance. Removal of thick doped buffer eliminates the issue of trapping in this layer and brings the high thermal conductivity substrate closer to the source of Joule heating
during device operation. However, reduction in GaN thickness results in significant decrease in thermal conductivity of this layer offsetting some of the benefits of "buffer-free" design. The effects of GaN thickness and thermal interface between the GaN channel and SiC substrate are discussed in detail. Moreover, the position of Fermi level in the AlN nucleation layer becomes a critical parameter for carrier confinement, with thicker GaN layers showing increase in short
channel effects.
Finally, electrical and thermal characterisation of proof-of-concept GaN HEMTs bonded onto SiC substrate using novel low-temperature technique is demonstrated. The bonding process utilises thin water layers trapped between the AlGaN/GaN heterostructure and the bond substrate to create a covalent bond. The devices are characterised before and after bonding showing no evidence of compromised electrical performance, while exhibiting all the major benefits associated with new high thermal conductivity substrate. This bonding technique could provide a promising solution to hetero-integration of III-V micro-electronic devices.

Date of Award	27 Sept 2022
Original language	English
Awarding Institution	University of Bristol
Supervisor	Michael J Uren (Supervisor) & Martin H H Kuball (Supervisor)