Simulation framework predicts thermal transport in 5G and 6G RF devices

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imec has presented a Monte Carlo Boltzmann modelling framework that uses microscopic heat carrier distributions to predict 3D thermal transport in advanced RF devices intended for 5G and 6G.

At this year’s International Electron Devices Meeting (IEEE IEDM 2022), imec presented case studies with GaN high-electron-mobility (HEMTs) and InP heterojunction bipolar transistors (HBTs) that revealed peak temperature rises that are up to three times larger than conventional predictions with bulk material properties.

According to imec, this new tool will be useful in guiding optimisations of next-gen RF devices toward thermally improved designs.

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

Nadine Collaert, programme director of advanced RF at imec explained. “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-silicon carbide (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.”

This guidance will prove valuable for the novel InP HBTs, where imec’s modelling framework highlights the substantial impact non-diffusive transport has on self-heating in complex scaled architectures. For these devices, nano-ridge engineering (NRE) is a 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 induce a thermal bottleneck for heat removal towards the substrate,” said Bjorn Vermeersch, principal member of technical staff in the thermal modelling 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.”