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INDUSTRY ENGINEERED SUBSTRATES


devices, with a gate-to-gate spacing of 10 mm and 40 mm for GaN-on-diamond and 30 mm and 40 mm for GaN-on-SiC (see Figure 7). Gate temperatures were measured in all these devices, which had a common packaging configuration and operated at multiple power dissipation levels.


Figure 3. X-band load-pull measurements taken from GaN-on-diamond and GaN-on-silicon HEMTs across multiple levels of power dissipation.


Simulations suggest that for a similar peak channel temperature, GaN-on- diamond HEMTs can employ one-third of the gate-to-gate spacing of GaN-on- SiC equivalents, thanks to a 40 percent reduction in channel-to-substrate thermal resistance.


Figure 4. Current droop measured from GaN-on-diamond (left chart) and GaN-on-silicon HEMTs (right chart) across various duty-cycles.


and SiC foundations, respectively. Quantitative measurements of thermal resistance are possible with micro- Raman thermal analysis, which has a spot volume of about 1 mm3 GaN surface into the buffer.


from the


Temperatures measured between the gate and drain (nearer the former) as well as near the edge of the diamond substrate, led to values for thermal resistance of about 8.0 K W-1


mm-1


To verify this, engineers probed thermal characteristics with micro-Raman thermography and gate thermometry (see Figure 8). Both of these techniques are not capable of measuring the true peak temperature that occurs in the HEMT, so peak temperatures are simulated − these represent the hottest nodal temperature in the HEMT finite element model. Confidence in the peak- temperature calculations derives from a good agreement between the model, the gate thermometry and the micro-Raman measurements. HEMT temperatures determined via measurement of gate forward bias voltage, which is calibrated


for


the GaN-on-diamond HEMT (see Figure 6). In comparison, GaN-on-silicon equivalents exhibited thermal resistance of about 21 K W-1


mm-1 .


Comparisons with SiC For high-power GaN RF applications, the industry’s leading substrate is SiC. So, it is the performance of HEMTs built on this platform that set the benchmark against which GaN-on diamond devices should be judged. Engineers at Raytheon have compared these two classes of device, using 10 x 125 mm HEMTs formed with the company’s microwave GaN process. This study involved forming a portfolio of


Figure 5. An infrared image of GaN-on-diamond and GaN-on-silicon HEMTs under bias. The temperature difference between the device’s gate and the base of the substrate was used in the calculations.


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