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Imec reveals novel thermal modelling tool

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Case studies with GaN HEMTs and InP HBTs show benefits of thermal optimisations in next-gen RF devices for 5G and 6G

At the 2022 International Electron Devices Meeting (IEEE IEDM 2022), Imec will presents a Monte Carlo Boltzmann modelling framework that uses microscopic heat carrier distributions to predict 3D thermal transport in advanced GaN and InP RF devices intended for 5G and 6G wireless communication for the first time.

Their case studies with GaN HEMTs and InP heterojunction bipolar transistors (HBTs) revealed peak temperature rises that are up to three times larger than conventional predictions with bulk material properties. Imec says its new tool will be very useful in guiding optimisations of next-gen RF devices toward thermally improved designs.

GaN- and InP-based devices have emerged as interesting candidates for 5G mm-wave and 6G sub-THz mobile front-end applications, respectively, due to their high output power and efficiency. To optimise these devices for RF applications and make them cost-effective, much attention is paid to upscaling the III/V technologies to a Si platform and making them CMOS compatible. However, with shrinking feature sizes and rising power levels, self-heating has become a major reliability concern, potentially limiting further RF device scaling.

Nadine Collaert, program director of advanced RF at Imec said: “Tuning the design of GaN- and InP-based devices for optimal electrical performance often worsens thermal performance at high operating frequencies. For GaN-on-Si devices, for example, we recently achieved tremendous progress in electrical performance, bringing the power-added efficiencies and output power for the first time on par with that of GaN-on-SiC.

"But further enlarging device operating frequency will require downsizing the existing architectures. In these confined multilayer structures, however, thermal transport is no longer diffusive, challenging accurate self-heating predictions. Our novel simulation framework, yielding good matches with our GaN-on-Si thermal measurements, revealed peak temperature rises up to three times larger than previously predicted. It will provide guidance in optimising these RF device layouts early in the development phase to ensure the right trade-off between electrical and thermal performance.”

Such guidance also proves very valuable for the novel InP HBTs, where Imec’s modeling framework highlights the substantial impact non-diffusive transport has on self-heating in complex scaled architectures. For these devices, nanoridge engineering (NRE) is an interesting heterogeneous integration approach from an electrical performance point of view.

“While the tapered ridge bottoms enable low defect density within the III-V materials, they, however, induce a thermal bottleneck for heat removal towards the substrate,” explains Bjorn Vermeersch, principal member of technical staff in the thermal modeling and characterisation team at Imec. “Our 3D Monte Carlo simulations of NRE InP HBTs indicate that the ridge topology raises the thermal resistance by over 20 percent compared to a hypothetical monolithic mesa of the same height. Our analyses furthermore highlight the direct impact of the ridge material (e.g., InP vs. InGaAs) on self-heating, providing an additional knob to improve the designs thermally.”

These results are presented in two invited papers at the 2022 IEDM, by Bjorn Vermeersch, on thermal modeling, and by Nadine Collaert, on GaN and InP technologies for next-generation high-capacity wireless communication, respectively.

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