Researchers Reduce Temperature of GaN Transistors by 20 Degrees

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

A research team at the University of California, Riverside Bourns College of Engineering have made a breakthrough in thermal management of Gallium Nitride (GaN) power transistors. The research group demonstrated that hot spots in GaN transistors can be lowered by as much 20 degrees Celsius through the introduction of alternative heat-escaping channels implemented with graphene multilayers. Gallium Nitride is used in wireless applications because of its high efficiency and high voltage operation. However, GaN electronics generate a lot of heat that, until now, has been difficult to remove quickly.

The research team 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. 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. The computer simulations performed by the group suggested that graphene quilts can perform even better in GaN devices on more thermally resistive substrates.

The Nano-Device Laboratory research group was led by Alexander Balandin, professor of electrical engineering and founding chair of Materials Science and Engineering program. The other members of team were Guanxiong Liu, Zhong Yan, both Ph.D. candidates, and Javed Khan, who earned his Ph.D. and is currently working at Intel. 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 Defense Advanced Research Project Agency.

The new approach to thermal management of power electronics with graphene was outlined in a paper: Graphene quilts for thermal management of high-power GaN transistors.” The paper was published May 8, 2012 in Nature Communications.

Graphene quilts for thermal management of high-power GaN transistors

Self-heating is a severe problem for high-power gallium nitride (GaN) electronic and optoelectronic devices. Various thermal management solutions, for example, flip-chip bonding or composite substrates, have been attempted. However, temperature rise due to dissipated heat still limits applications of the nitride-based technology. Here we show that thermal management of GaN transistors can be substantially improved via introduction of alternative heat-escaping channels implemented with few-layer graphene — an excellent heat conductor. The graphene-graphite quilts were formed on top of AlGaN/GaN transistors on SiC substrates. Using micro-Raman spectroscopy for in situ monitoring we demonstrated that temperature of the hotspots can be lowered by ~20°C in transistors operating at ~13 W mm-1, which corresponds to an order-of-magnitude increase in the device lifetime. The simulations indicate that graphene quilts perform even better in GaN devices on sapphire substrates. The proposed local heat spreading with materials that preserve their thermal properties at nanometre scale represents a transformative change in thermal management.

More info: University of California, Riverside

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