December 6, 2022
At the 68th annual IEEE International Electron Devices Meeting (IEDM 2022) in San Francisco (3-7 December), the imec nanoelectronics research center in Leuven, Belgium, presented a Monte Carlo Boltzmann modeling framework which, for the first time uses microscopic heat-carrier distributions to predict 3D heat transport in advanced RF devices for 5G and 6G wireless communication.
The results were presented in two guest papers, by Bjorn Vermeersch on thermal modeling and by Nadine Collaert on gallium nitride (GaN) and indium phosphide (InP) technologies for next-generation high-capacity wireless communication , respectively. [papers 11.5 and 15.3].
Case studies with GaN high electron mobility transistors (HEMTs) and InP heterojunction bipolar transistors (HBTs) have revealed peak temperature increases up to three times greater than conventional predictions with bulk material properties . Imec believes the new tool will be useful in guiding optimizations of next-generation RF devices towards thermally-enhanced designs.
Figure 1. Measured and predicted thermal resistance as a function of finger width of two-finger GaN-on-Si HEMTs.
GaN and InP-based devices have emerged as attractive candidates for 5G millimeter-wave (mm-wave) and 6G sub-THz mobile front-end applications, respectively, due to their high output power and efficiency. In order to optimize these devices for RF applications and make them cost effective, much attention is paid to scaling III/V technologies to a silicon platform and making them CMOS compatible. However, with decreasing feature size and increasing power levels, self-heating has become a major reliability issue, potentially limiting further scaling of RF devices.
“Tuning the design of GaN and InP-based devices for optimal electrical performance often deteriorates thermal performance at high operating frequencies,” notes Nadine Collaert, director of the advanced RF program at imec. “For GaN-on-Si devices, for example, we have recently made huge strides in electrical performance, bringing added efficiency and power output to par with GaN-on-Si for the first time. – silicon carbide (SiC). But further increasing the operating frequency of the devices will require reducing the existing architectures. In these confined multilayer structures, however, thermal transport is no longer diffusive, which challenges accurate self-heating predictions,” she adds. “Our new simulation framework, which agrees well with our GaN-on-Si thermal measurements, revealed maximum temperature increases up to three times larger than expected. It will provide guidance for optimizing these RF device layouts early in the development phase to ensure the right trade-off between electrical and thermal performance.
Figure 2. Geometry of the HBT InP nanoridge used in the 3D simulation.
Figure 3. Impact of non-diffusive thermal transport effects (as captured by imec’s Monte Carlo simulation) in InP HBT nanoridges.
Such guidance also proves invaluable for new InP HBTs, where imec’s modeling framework highlights the substantial impact that non-diffusive transport has on self-heating in complex-scale architectures. . For these devices, nanoridge engineering (NRE) is an attractive heterogeneous integration approach from an electrical performance perspective. “While tapered ridge bottoms allow for low defect density in III-V materials, they do however induce a thermal bottleneck for heat removal to the substrate,” explains Bjorn Vermeersch, Senior Staff Member technique of the modeling and thermal characterization team at imec. “Our 3D Monte Carlo simulations of NRE InP HBT indicate that the ridge topology increases thermal resistance by more than 20% compared to a hypothetical monolithic mesa of the same height,” he adds. “Our analyzes further highlight the direct impact of ridge material (e.g., InP versus InGaAs) on self-heating, providing an additional button to improve designs thermally.”
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