1. Diamond Substrates for High Power Density Electronics
George Davis

2. Improving thermal management techniques
Cutting edge AlGaN/GaN high electron mobility transistors (HEMTs) have achieved RF power densities reaching 40 W/mm. HEMT’s in production are typically limited to around 5-7 W/mm.
Current technology typically uses SiC substrates due to its relatively high thermal conductivity (~400 W/m-K)
Majority of junction to case resistance comes from ceramic insulating layers
Defects at the interface of GaN and SiC lessen thermal performance
Fig 1: Cross section of semiconductor
die on substrate

3. Why use diamond? Higher thermal conductivity (~2000 W/m-K)
Use of diamond can mitigate the near junction thermal resistance (limiting factor for GaN on SiC)
Dielectric strength
Hardness
Table 1: Physical properties of common substrate materials

4. Fabrication of diamond substrates
Diamond grown through chemical vapor deposition (CVD) process
Poor fabrication techniques will result in defects and increased thermal resistance
Primary method involves transferring pre-grown AlGaN/GaN structures to polycrystalline CVD diamond
Fig 2. Cross-section of GaN on diamond composite substrate

5. Supporting test data
For the same level of power dissipation, GaN-on-diamond HEMT shows over 25% lower temperature rise than GaN-on-SiC
Fig 3: Thermal distribution of SiC dies on AlN substrate (1.Dies (SiC), 2.Solder, 3.Copper, 4.AlN substrate, 5.Copper, 6.Heat sink)
Fig 4: Thermal distribution of SiC dies on diamond substrate (1.Dies (SiC), 2.Solder, 3.Copper, 4.AlN substrate, 5.Copper, 6.Heat sink)

6. Supporting test data (cont’d)
GaN-on-diamond HEMTs recorded a record of over 7 W/mm output power with a 40 V bias at 10 GHz.
Good power added efficiency (PAE) (~55%, comparable to GaN-on-SiC)
Low level of current collapse (13%, comparable to GaN-on- SiC)

7. Flaws/Future improvements
Large mismatch of crystalline properties (lattice constants, coefficients of thermal expansion) make fabrication of high quality GaN-on-diamond difficult
Further development needed to reduce gate leakage current. Power added efficiency will increase with improvement.
Increase in stress and decrease in safety factor caused by mismatch in coefficient of thermal expansion between materials