technology GaN transistors
to continuous wave mode, and PAE drops to about 55 percent due to thermal self-heating. To improve the thermal environment for these power bars, they were mounted on polycrystalline diamond tabs supplied by Element Six (see Figure 9 for an example).
Prior to mounting, the SiC substrates were thinned to 100 µm to improve thermal management: Thermal conductivity of SiC is 450 W m-1
K-1
, which is four times lower than that of high-quality polycrystalline diamond.
The benefit of a high conductivity platform is revealed with thermal infrared imaging. When a 36 mm bar was mounted on a diamond heat spreader and dissipated 3.5 Wmm-1
on a jig kept at 62 °C, its maximum channel temperature was 173 °C (see Figure 9 b). This increased to 214 °C for a similar 400 µm-thick power bar, not mounted on a diamond heat spreader.
. This incredibly low figure helps to dissipate hundreds of watts of heat from the HEMT.
It is possible to further improve the thermal management of the device with active heat exchangers, which can operate by pushing water into a cavity with a large surface area for heat exchange. Such systems have been designed by III-V Lab and Swerea IVF and produced by FCubic using their three-dimensions machining capabilities (see Figure 10). One of the latter firm’s contributions to the MORGaN project has been the development of a new process to make high precision copper parts using layer manufacturing. This has enabled the construction of heat exchangers with a thermal resistance, estimated by experiment, of 0.16 °C W-1
Armed with this heat exchanger, researchers have constructed a 2 GHz amplifier that includes two power bars of 36 mm periphery and produces 200 W. Switch to pulsed-mode operation, and a 250 W output is possible, which is a record for devices with an InAlN/GaN heterostructure. The limit of operation is 320 W, and PAE can hit 35
Figure 9.(a) Optical photography of a 36 mm InAlN/GaN HEMT thinned down to 100 µm thick SiC,mounted on diamond tab TM180 (appear black in image); The power bar was set to dissipate 3.5 W mm-1 in CW – the assembly is mounted on a jig at a temperature fixed at 62°C (b) Thermal infrared microscopy of the power bar shown in (a). (c) Infrared imaging of a similar dice but without a heatspreader and with a SiC substrate 400 µm-thick
percent, which equates to 55 percent at the device level, according to reversed analysis. These incredibly impressive results are just one of the highlights of the MORGaN project, which has also led to improvements in materials, sensors, microwave devices and packaging.
This article has only touched on some of the breakthroughs, and anyone seeking more details should take a look at the MORGaN public website:
http://www.morganproject.eu/
Further reading
Europe turns to AlInN to push the limits of transistor and sensor performance, Compound Semiconductor Nov&Dec 2009, p.27
© 2012 Angel Business Communications. Permission required.
Figure 10.(a) Copper heat exchanger made by three-dimensional machining (b) Partial view of 200 W L-Band using InAlN/GaN HEMT devices (c) Output power and PAE of amplifier including 2 devices of 36 mm at 2 GHz. Duty cycle measurements with 10 µs pulses were used as well as CW
operation.Top: Output power; bottom: PAE
50
www.compoundsemiconductor.net April / May 2012
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