Until about a decade ago, thermal energy was considered almost entirely as an analog, bulk phenomena. Regardless of whether the heat flux was conducted or radiated, the flux was related to the temperature difference between a hot object and a colder environment. Convective heat transfer was simply a physical result that many things become less dense when heated at constant pressure; therefore in liquid or gas systems, due to gravity, a bulk material flow could be established by conducting heat into a cooler fluid thereby making it buoyant. Not that heat transfer problems are simple to solve in the real world, but the underlying physics has been understood for a long time. In many cases complex systems can be adequately analyzed using lumped models of thermal resistances.

Things began to change when heat flux was successfully described as the movement of phonons. In 2006 researchers at the University of California, Berkeley, demonstrated a form of thermal diode, in which heat flux in one direction could be controlled to be higher than the other direction along a carbon nanotube. Recently, researchers at the Charles Fabry Laboratory in France and the Carl von Ossietzky University in Germany showed a thermal transistor that can operate using radiated thermal energy (photons instead of phonons). It has been suggested previously that such devices could eventually work as logic gates in MEMS (Micro electro-mechanical systems) devices.

These non-linear thermal systems are an exciting area of development, but most of what we are concerned about in electronic systems, whether analog, digital, or mixed signal, is what to do with the heat generated the old-fashioned way by various dissipative processes in the chips and passive components that make up useful electronic systems.

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