Understanding nanoscale thermal transport is of substantial importance for designing contemporary semiconductor technologies. Heat removal from small sources is well established to be severely impeded compared to diffusive predictions due to the ballistic nature of the dominant heat carriers. Experimental observations are commonly interpreted through a reduction of effective thermal conductivity, even though most measurements only probe a single aggregate thermal metric. Here, we employ thermoreflectance thermal imaging to directly visualise the 2D temperature field produced by localised heat sources on InGaAs with characteristic widths down to 100 nm. Besides displaying effective thermal performance reductions up to 50% at the active junctions in agreement with prior studies, our steady-state thermal images reveal that, remarkably, 1–3 μm adjacent to submicron devices the crosstalk is actually reduced by up to fourfold. Submicrosecond transient imaging additionally shows responses to be faster than conventionally predicted. A possible explanation based on hydrodynamic heat transport, and some open questions, are discussed.

The recent developments in nanoscale solid-state cooling are reviewed. This includes both theoretical and experimental studies of different physical concepts, as well as nanostructured material design and device configurations. We primarily focus on thermoelectric, thermionic and thermo-magnetic coolers. Particular emphasis is given to the concepts based on metal–semiconductor superlattices, graded materials, non-equilibrium thermoelectric devices, Thomson coolers, and photon assisted Peltier coolers as promising methods for efficient solid-state cooling. Thermomagnetic effects such as magneto–Peltier and Nernst–Ettingshausen cooling are briefly described and recent advances and future trends in these areas are reviewed. The ongoing progress in solid-state cooling concepts such as spin-calorimetrics, electrocalorics, non-equilibrium/nonlinear Peltier devices, superconducting junctions and two-dimensional materials are also elucidated and practical achievements are reviewed. We explain the thermoreflectance thermal imaging microscopy and the transient Harman method as two unique techniques developed for characterization of thermoelectric microrefrigerators. The future prospects for solid-state cooling are briefly summarized.

High-temperature and temperature nonuniformity in high-performance integrated circuits (ICs) can significantly degrade chip performance and reliability. Thus, accurate temperature information is a critical factor in chip design and verification. Conventional volume grid-based techniques, such as finite-difference and finite-element methods (FEMs), are computationally expensive. In an effort to reduce the computation time, we have developed a new method, called power blurring (PB), for calculating temperature distributions using a matrix convolution technique in analogy with image blurring. The PB method considers the finite size and boundaries of the chip as well as 3-D heat spreading in the heat sink. PB is applicable to both static and transient thermal simulations. Comparative studies with a commercial FEM tool show that the PB method is accurate within 2%, with orders of magnitude speedup compared with FEM methods. PB can be applied to very fine power maps with a grid size as small as 10 μm for a fully packaged IC or submicrometer heat sources in power electronic transistor arrays. In comparison with architecture-level thermal simulators, such as HotSpot, PB provides much more accurate temperature profiles with reduced computation time.

Thermoreflectance imaging with high spatial resolution is used to inspect self-heating distribution in active high power (4A) metal-oxide-semiconductor field-effect transistor transistor arrays designed for high-frequency (MHz) operation. Peak temperature change and self-heating distribution is analyzed for both low- and high-dc bias cases and for different ambient die temperatures (296-373 K). Thermoreflectance images reveal temperature nonuniformity greater than a factor of two over the full area of the transistor arrays. Thermal nonuniformity is revealed to be strongly dependent on both bias level and ambient die temperature. Verification based on the fine grain power dissipation in the transistor array was performed using the R3D method for electrical simulation and power blurring for thermal simulation. Results demonstrate thermoreflectance imaging as an effective tool for fast submicrometer noncontact thermal characterization of active power devices.