by Kevin Loutfy, President, Nano Materials International
GaN high-mobility transistors (HEMTs) have succeeded in producing pulsed or CW RF output power of 2 kW, an impressive record in the 15 years commercial devices have been available. And it’s almost certain that even higher levels will be achieved if the power density within a given amount of device gate periphery continues to increase.
This presents significant challenges because the amount of heat that must be dissipated increases as power density increases. Consequently, it is critical to remove it from the die level to the heat spreader and then to a heat sink or other thermal management solution. In short, the more efficiently this heat is dissipated, the greater the power density can be accommodated, the smaller the amplifier can be, the more reliable the devices will become, and the longer their operating lifetimes will be.
Although high-performance GaN devices today primarily use silicon carbide as a substrate material, much of what can be realized with GaN in the future rests on another material, synthetic diamond produced by chemical vapor deposition (CVD diamond). When used as a substrate material, GaN on diamond can more than triple the power density obtainable with SiC.
Diamond’s potential is derived from its formidable characteristics. Perhaps the most important is its thermal conductivity (TC), which is higher than any solid (and more than 2,300 W/mK), which is far higher than its most widely used substrate counterparts such as copper (401 W/mK), SiC (250 W/mK), aluminum nitride (170 w/mK), and silicon (150 W/mK).
In addition to its use as a substrate material, diamond has tremendous benefits when used as a heat spreader in aluminum-diamond metal matrix composites (MMCs). The concept of using MMCs based on diamond for these applications was conceived more than 25 years ago and substantial research in academic, industry, and government has been expended over the years, and they’re manufacturable in production volumes. When used with discrete and MMIC GaN devices, they can further increase cooling efficiency and increase GaN’s achievable power density, regardless of the substrate material.
While not retaining all of pure diamond’s unparalleled TC, aluminum-diamond MMCs have a TC of about 550 W/mK, which is higher than any other heat spreader material, and their coefficient of thermal expansion (CTE) can be matched to any GaN device.
In short, the future enhancement of GaN devices relies in considerable measure on getting closer to achieving their theoretical maximum power density, which is reportedly up to 40 W/mK.
Aluminum diamond MMCs employ an aluminum alloy composition that is infiltrated into a packing of industrial-grade diamond particles. Both the size and ratio of the diamond particles must be optimized to retain their high thermal conductivity, low coefficient of thermal expansion (CTE), and high mechanical strength. The diamond particles provide the material with high thermal conductivity and low coefficient of thermal expansion (CTE), while the aluminum matrix provides good strength and ductility.
Aluminum-diamond MMCs also have strength of up to 400 MPa, comparable to other common materials such as aluminum. This makes them suitable for applications where high strength and good thermal conductivity are required.
When GaN was first pressed into service by the Army around 2005, it was just emerging from labs sponsored by DARPA. There was considerable debate about whether it was reliable, especially when used in hostile operating environments. Needless to say, those concerns have disappeared. What’s next is the material’s continued advance, and when used in the substrate and as a heat spreader, diamond is the material that will allow it to get there.