Characterization of the Thermal Conductivity of CVD Diamond for GaN-on-Diamond Devices

Gallium nitride (GaN) based high electron mobility

transistors (HEMTs) have proven to have great potential

for RF devices and power electronics. The

development and fabrication of AlGaN/GaN HEMTs on

SiC substrates been the primary focus in industry in order

to produce reliable transistors. In spite of the high thermal

conductivity of SiC substrates, these devices are still

limited to DC power densities on the order of 7-10 W/mm

considering a maximum junction temperature of 200°C.

This limitation is a direct result of the thermal resistance

imparted by the SiC which must be addressed in order to

push the limits of the technology. Recently, the use of

CVD diamond in GaN HEMTs has shown promise in

increasing the power densities of these devices without

increasing the junction temperature. However, the

integration of CVD diamond into GaN HEMTs through

growth on the backside of the GaN buffer layer results in

the nucleation and columnar growth of diamond grains

with large gradients in thermal conductivity in the film

. Measurements of the thermal conductivity of 1 μm

diamond films have shown thermal conductivity values

that can be less than 100 W/mK while bulk films can have

thermal conductivities >2000 W/mK . Thus, this

strong gradient in thermal conductivity along with thermal

boundary resistance between the diamond and GaN are

expected to play a role in the success of using CVD

diamond as a thermal management solution in GaN.

However, accurate characterization of the thickness

dependence of the thermal conductivity in order to

analyze the impact on the performance of GaN-on-Diamond devices.

In this work, we utilize TDTR to measure the thermal

conductivity of CVD diamond films grown on Si

substrates from 5 – 13.8 μm in thickness by Element Six.

Additional measurements were also made on bulk samples

ranging from 300 – 550 μm in thickness also grown by

Element Six. The surface of the samples were polished to

a surface roughness less than 30 nm rms to facilitate

thermal conductivity measurements. Measurements were

performed at two different laboratories in order to

correlate thermal conductivity measurements and

investigate some sources of variability. The results were

then used to estimate the impact on the thermal response

of 10 finger AlGaN/GaN HEMTs with SiC and diamond

with a vertical gradient in thermal conductivity.