This page contains a Flash digital edition of a book.
INDUSTRY ENGINEERED SUBSTRATES


to temperature, are in good agreement with simulations. They show that at a dissipated power of 4.2 W/mm, the peak junction temperature of the 10 mm gate-to-gate GaN-on-diamond HEMT is just 6.3 °C (6 percent) higher than the 30 mm gate-to-gate GaN-on-SiC device. And for 40 mm gate-to-gate spacings at 4.2 W/mm dissipated power, the GaN-on-diamond device has a junction temperature 8.5°C lower than the GaN- on-SiC equivalent. These efforts also reveal that switching from SiC to diamond delivers a spike in areal dissipation density from 140 W/mm2 420 W/mm2


to .


These results are for an early generation of GaN-on-diamond structures. Measurements on more recent material show that thermal performance has improved, which should result in even more impressive devices.


Is GaN-on-diamond reliable? The introduction of any new substrate will always bring concerns over reliability. With diamond, scepticism can stem from its significant differences, compared to GaN, in its thermal expansion coefficient, crystal structure, surface properties and internal stress. To put these concerns to bed, the authors have subjected GaN-on-diamond HEMTs to channel temperatures of up to 350°C using a constant source-drain voltage of 24 V.


A series of endurance tests involved monitoring the source-drain currents and gate leakage currents of batches of devices operated for thousands of hours at elevated channel temperatures. Currents for the GaN-on-diamond HEMTs deviated by less than 25 percent of their starting values after: 4,000 hours at 350°C, 9,000 hours at 290°C and


Figure 6. Temperature change measured for GaN-on-diamond and GaN-on-silicon HEMTs using a micro-Raman technique.


17,000 hours at 210°C. In contrast, all the control GaN-on-silicon devices, which share the same GaN epitaxy and device structure as their GaN-on-diamond cousins, catastrophically failed within a few hundred hours of the start of the tests.


Apparently, removal of the highly defective transition layers between GaN and silicon, prior to diamond deposition, contributed to the improvements resulting from the introduction of GaN-on- diamond. However, to confirm that this is the case, more research is warranted to better understand these results. Trimming the thermal resistance by


40 percent by switching from GaN-on- SiC to GaN-on-diamond should have two major benefits on the design of radar, electronic warfare, defence radio, communications and weather satellites, cellular base stations, and naval avionics systems: it should cut cooling complexity and cost, and it should deliver a three- fold increase in the areal power density of the GaN transistor.


Reductions in cooling complexity and cost are possible when thermal resistance falls, because less stringent demands are placed on the coolant temperature. This can open the door to simpler, cheaper thermal management


Figure 7. GaN HEMTs characterized by micro-Raman and gate thermometry techniques. From left: GaN-on-SiC 40 micron gate-to-gate spacing, GaN- on-SiC 30 micron gate-to-gate spacing, and GaN-on-diamond 10 micron gate-to-gate spacing.


Distribution Statement A (Approved for Public Release, Distribution Unlimited) October 2014 www.compoundsemiconductor.net 45


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76