Power delivery using semiconductor devices is increasing at a rapid pace. This is evident from different forms of power delivered, whether it is controlled power through power inverters, or RF power through amplifiers. Power is necessary to operate nearly everything, such as for alternative forms of energy generation, electric vehicles, radar systems, cellular base stations and even smartphones. However, semiconductor devices need to dissipate the heat they generate, and this poses a stringent challenge for power and thermal management.
Such high-power semiconductor devices are now using a new technology in the form of GaN-on-diamond wafers and synthetic diamond heat-spreaders. The reason behind this is the excellent thermal conductivity of diamond, the highest of any material. At room temperatures, diamond conducts heat about five times better than copper does.
Any semiconductor material can use diamond heat spreaders and these lower the temperature of the semiconductor gate junction by almost 30 percent. In addition, the use of GaN-on-diamond wafers helps lower the temperatures of GaN devices further. With the gate-junction temperature going down by almost 50 percent, GaN-on-diamond devices can handle more than three times the power density than similar GaN-on-SiC can.
Manufacturers use the technique of plasma-assisted microwave CVD or Chemical Vapor Deposition for synthesizing diamond heat spreaders. With this method of growing the synthetic diamond, manufacturers make freestanding diamond wafers up to 140 mm in diameter and nearly 1 mm thick. The wafers have thermal conductivities higher than 2000W/mK, which is five times that of copper. By using microwave CVD for growing diamonds, manufacturers can engineer the properties of the diamond wafers precisely, giving them a range of thermal conductivities. This allows them to offer different cost to performance ratios for matching the specific needs of any application.
When using metalized diamond heat spreaders, manufacturers attach them to the bottom of the semiconductor die. Since they use as thin a layer of solder as is possible for attaching the heat spreader, the diamond lies within 100 to 300 microns of the gate junctions of the device. The diamond heat spreader distributes the heat equally and effectively in both lateral and vertical directions. Heat spreading in the lateral direction is particularly important for RF power amplifiers, as they typically form hot spots of up to 1 micron in diameter with intense heat density.
Manufacturers need to keep the metallization of the die and the heat spreader thin – to the extent of a few hundreds of nanometers. Metallization of the diamond has to be done carefully using a carbide-forming metal as the first layer. The solder layer used to attach the heat spreader must also be thin, preferably lower than 10 microns. With optimal integration into a package, diamond heat spreaders typically help to reduce the gate junction temperatures by nearly 30 percent, when compared to what ceramic packages do that are not using diamond heat spreaders.
The GaN-on-diamond substrates now offer new thermal management tools for GaN semiconductor devices. The reduced thermal resistance of GaN-on-diamond and diamond heat spreaders allows simpler, less expensive thermal management systems. This has a favorable impact on cooling complexity and expenses involved, also leading to better lifetimes of the entire system.
