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Clockwise from top left: optical microscopy image of the high-power GaN heterostructure field-effect transistor (HFET); schematic of the graphene- graphite quilt on top of the transistor structure for spreading the heat from the local hot spot near the transistor drain; coloured SEM image of the graphene quilt overlapping transistor; optical microscopy image of the graphene quilt on the device electrode illustrating its flexibility


GaN transistors have been commercially available since 2006. The problem with them, like all high power operating devices, is the significant amount of dissipated heat, which has to be fast and efficiently removed. Various thermal management solutions such as flip-chip bonding or composite substrates have been attempted. However, applications have still been limited because of increases in temperature due to dissipated heat. The breakthrough in thermal management of GaN power transistors was achieved by Balandin and three of his electrical engineering graduate students: Guanxiong Liu, Zhong Yan, and Javed Khan, who started working at Intel Corporation this year. Balandin has previously discovered that graphene is an excellent heat conductor.


Few-layer graphene films preserve their excellent thermal properties even when their thickness is only a few nanometres, which is unlike metal or semiconductor films. The latter makes them excellent candidates for applications as the lateral heat spreaders and interconnects. The Balandin group researchers designed and built graphene- graphite “quilts” on top of GaN transistors.


The graphene-graphite quilts’ function was to remove and spread the heat from the hot spots – the opposite of what you expect from the conventional quilts.


From left to right: Guanxiong Liu, Alexander Balandin and Zhong Yan, who worked on the project


Using micro-Raman spectroscopic thermometry the researchers demonstrated that temperature of the hot spots can be lowered by as much 20 degrees Celsius in transistors operating at the large power levels.


From left, cross-sectional schematic of the GaN HFET with graphene quilt for heat spreading; current-voltage characteristics of GaN HFET with and without graphene-based heat spreaders demonstrating improvement in the saturation current due to lower junction temperature in the device with graphene quilt


The computer simulations performed by the group suggested that graphene quilts can perform even better in GaN devices on more thermally resistive substrates. The Balandin group is also known in graphene community for their investigation of low-frequency noise in graphene transistors, development of the first large-area method for quality control of graphene and demonstration of the first selective gas sensor implemented with pristine graphene.


The work on thermal management of GaN transistors with graphene quilts was supported by the Office of Naval Research. Balandin’s research of the thermal properties of graphene was funded by the Semiconductor Research Corporation and the Defence Advanced Research Project Agency.


June 2012 www.compoundsemiconductor.net 69


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