Roland B. Simon

Reducing GaN-on-diamond interfacial thermal resistance for high power transistor applications

Integration of chemical vapor deposited polycrystalline diamond offers promising thermal performance for GaN-based high power radio frequency amplifiers. One limiting factor is the thermal barrier at the GaN to diamond interface, often referred to as the effective thermal boundary resistance (TBReff). Using a combination of transient thermoreflectance measurement, finite element modeling and microstructural analysis, the TBReff of GaN-on-diamond wafers is shown to be dominated by the SiNx interlayer for diamond growth seeding, with additional impacts from the diamond nucleation surface. By decreasing the SiNx layer thickness and minimizing the diamond nucleation region, TBReff can be significantly reduced, and a TBReff as low as 12 m2K/GW is demonstrated. This enables a major improvement in GaN-on-diamond transistor thermal resistance with respect to GaN-on-SiC wafers. A further reduction in TBReff towards the diffuse mismatch limit is also predicted, demonstrating the full potential of using diamond as the heat spreading substrate.

Contactless Thermal Boundary Resistance Measurement of GaN-on-Diamond Wafers

Low thermal resistance GaN-on-diamond wafers offer enhanced thermal management with respect to GaN-on-SiC devices. The GaN/diamond interfacial thermal resistance can contribute significantly to the total device thermal resistance and must therefore be minimized to gain the maximum benefit from GaN-on-diamond. A contactless thermoreflectance measurement technique has been developed, which can be used after wafer growth and before device fabrication, enabling rapid feedback about the influence of growth parameters on interfacial thermal resistance. A measured 2× reduction in the GaN/diamond interfacial resistance is achieved by reducing the dielectric thickness between the GaN and diamond from 90 to 50 nm, enabling a potential 25% increase in transistor power dissipation for GaN-on-diamond.

Thermal conductivity of bulk GaN—Effects of oxygen, magnesium doping, and strain field compensation

The effect of oxygen doping (n-type) and oxygen (O)-magnesium (Mg) co-doping (semi-insulating) on the thermal conductivity of ammonothermal bulk GaN was studied via 3-omega measurements and a modified Callaway model. Oxygen doping was shown to significantly reduce thermal conductivity, whereas O-Mg co-doped GaN exhibited a thermal conductivity close to that of undoped GaN. The latter was attributed to a decreased phonon scattering rate due the compensation of impurity-generated strain fields as a result of dopant-complex formation. The results have great implications for GaN electronic and optoelectronic device applications on bulk GaN substrates.

Temperature-Dependent Thermal Resistance of GaN-on-Diamond HEMT Wafers

The thermal properties of GaN-on-diamond high-electron mobility transistor (HEMT) wafers from 25 °C to 250 °C are reported. The effective thermal boundary resistance between GaN and diamond decreases at elevated temperatures due to the increasing thermal conductivity of the amorphous SiNx interlayer, therefore potentially counteracting thermal runaway of devices. The results demonstrate the thermal benefit of GaN-on-diamond for HEMT high-power operations, and provide valuable information for assessing the thermal resistance and reliability of devices.

Diamond micro-Raman thermometers for accurate gate temperature measurements

Determining the peak channel temperature in AlGaN/GaN high electron mobility transistors and other devices with high accuracy is an important and challenging issue. A surface-sensitive thermometric technique is demonstrated, utilizing Raman thermography and diamond microparticles to measure the gate temperature. This technique enhances peak channel temperature estimation, especially when it is applied in combination with standard micro-Raman thermography. Its application to other metal-covered areas of devices, such as field plates is demonstrated. Furthermore, this technique can be readily applied to other material/device systems.

Effect of grain size of polycrystalline diamond on its heat spreading properties

The exceptionally high thermal conductivity of polycrystalline diamond (>2000 W m−1 K−1) makes it a very attractive material for optimizing the thermal management of high-power devices. In this paper, the thermal conductivity of a diamond sample capturing grain size evolution from nucleation towards the growth surface is studied using an optimized 3ω technique. The thermal conductivity is found to decrease with decreasing grain size, which is in good agreement with theory. These results clearly reveal the minimum film thickness and polishing thickness from nucleation needed to achieve single-crystal diamond performance, and thus enable production of an optimal polycrystalline diamond for heat-spreading applications.

Novel Thermal Management of GaN Electronics: Diamond Substrates

Microwave and power electronics based on GaN enables the performance of systems and their safe operating area to be driven to ‘extremes’. One of the major issues that then arises is thermal management. This includes heat transfer limitations across interfaces, however also the need of incorporating novel high thermal conductivity materials such as diamond. Thermal parameters of these novel device systems and their implications on the near junction temperature in the devices are not well known. The role of interfaces between the GaN transistor and the diamond substrate, and of the diamond thermal properties themselves near this interface are discussed, and novel thermal characterization approaches, such as enabling fast determination of the thermal resistance on the wafer level, as well as of lateral diamond thermal conductivity, are presented.Copyright