Diamond Substrates for High Power Density Electronics George Davis

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

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

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

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)

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)

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