Thermoelectric devices convert heat into electricity or electricity into cooling, and are typically constructed of alternating thermoelectric ‘legs’ made of n- and p-type semiconductors (with the legs connected electrically in series and thermally in parallel via conductive electrodes). The devices can be divided into two broad classes: macro-thermoelectric devices and micro-thermoelectric devices. Macro-thermoelectric devices, which include thermoelectric generators and thermoelectric coolers, have already been commercialized and are of use in applications ranging from the thermal management of batteries to power generators for space missions. Micro-thermoelectric devices, which have a smaller leg cross-section and height than their macroscopic counterparts, are at an earlier stage of development. But their capabilities have advanced quickly in recent years and the devices are of potential use in a range of emerging applications.

A scanning electron microscopy image of an integrated micro-thermoelectric device1. Credit: IFW Dresden.

In a Review Article in this issue of Nature Electronics, Heiko Reith, Kornelius Nielsch and colleagues explore the development of micro-thermoelectric devices. The researchers — who are based at IFW Dresden and the Technical University of Dresden — first consider the various device designs available and how these can be optimized. They then explore the different integration technologies that have been developed, and provide a comprehensive analysis of the state-of-the-art performance of micro-thermoelectric devices in terms of both cooling and power generation.

The potential applications of the devices can be split into three categories — thermal management, power generation and sensing — and Nielsch and colleagues highlight key areas in all of these. These areas include the on-chip thermal management of integrated circuits, power sources for devices in the Internet of Things (IoT), and sensors for electronic skins and health monitoring. Finally, the researchers address the challenges that need to be tackled in order to seamlessly integrate micro-thermoelectric devices into electronics and achieve the full commercial potential of the technology.

Thermal management of electronics, which is required to keep device temperatures within a reliable operating range, is a targeted application for micro-thermoelectric devices. Other technologies have, of course, been wrestling with the issue for years — and it is an issue that is becoming increasingly problematic. Heat spreaders, which are made from a material with a high thermal conductivity and placed in contact with the heat-generating device, are often used. But these struggle to reach shadowed regions beneath devices. In an Article elsewhere in this issue, Nenad Miljkovic and colleagues report the development of a heat-spreading technology in which copper is monolithically integrated directly on electronic devices, allowing the heat-spreading metal to reach confined regions beneath devices on circuit boards and systems.

The researchers — who are based at the University of Illinois at Urbana-Champaign, the University of California, Berkeley and Kyushu University — first coat the devices with an insulating layer of a polymer known as parylene C, which has a high dielectric strength, using chemical vapour deposition. A monolithic layer of copper is then grown on the polymer using a series of deposition steps: thermal evaporation, electroless plating and finally electroplating. To explore the capabilities of the technique, Miljkovic and colleagues integrate the copper heat spreaders directly onto gallium nitride (GaN) power transistors, illustrating that it can be used with systems operating up to 600 V. They also show that the approach offers superior performance over established copper heat sinks and copper-plane heat spreaders. The compactness of the technique — and the fact that it avoids the need for large heat sinks — means that it could be used to build compact and power-dense electronics.