Gallium nitride (GaN) is a compound semiconductor that has tremendous potential to facilitate economic growth in a semiconductor industry that is silicon-based and currently faced with diminishing returns of performance versus cost of investment. At a material level, its high electric field strength and electron mobility have already shown tremendous potential for high frequency communications and photonic applications. Advances in growth on commercially viable large area substrates are now at the point where power conversion applications of GaN are at the cusp of commercialisation. The future for building on the work described here in ways driven by specific challenges emerging from entirely new markets and applications is very exciting. This collection of GaN technology developments is therefore not itself a road map but a valuable collection of global state-of-the-art GaN research that will inform the next phase of the technology as market driven requirements evolve. First generation production devices are igniting large new markets and applications that can only be achieved using the advantages of higher speed, low specific resistivity and low saturation switching transistors. Major investments are being made by industrial companies in a wide variety of markets exploring the use of the technology in new circuit topologies, packaging solutions and system architectures that are required to achieve and optimise the system advantages offered by GaN transistors. It is this momentum that will drive priorities for the next stages of device research gathered here.

We present a "nanoparticle-in-alloy" material approach with silicide and germanide fillers leading to a potential 5-fold increase in the thermoelectric figure of merit of SiGe alloys at room temperature and 2.5 times increase at 900 K. Strong reductions in computed thermal conductivity are obtained for 17 different types of silicide nanoparticles. We predict the existence of an optimal nanoparticle size that minimizes the nanocomposite's thermal conductivity. This thermal conductivity reduction is much stronger and strikingly less sensitive to nanoparticle size for an alloy matrix than for a single crystal one. At the same time, nanoparticles do not negatively affect the electronic conduction properties of the alloy. The proposed material can be monolithically integrated into Si technology, enabling an unprecedented potential for micro refrigeration on a chip. High figure-of-merit at high temperatures (ZT approximately 1.7 at 900 K) opens up new opportunities for thermoelectric power generation and waste heat recovery at large scale.

The ability to precisely control the thermal conductivity (kappa) of a material is fundamental in the development of on-chip heat management or energy conversion applications. Nanostructuring permits a marked reduction of kappa of single-crystalline materials, as recently demonstrated for silicon nanowires. However, silicon-based nanostructured materials with extremely low kappa are not limited to nanowires. By engineering a set of individual phonon-scattering nanodot barriers we have accurately tailored the thermal conductivity of a single-crystalline SiGe material in spatially defined regions as short as approximately 15 nm. Single-barrier thermal resistances between 2 and 4 x 10(-9) m(2) K W(-1) were attained, resulting in a room-temperature kappa down to about 0.9 W m(-1) K(-1), in multilayered structures with as little as five barriers. Such low thermal conductivity is compatible with a totally diffuse mismatch model for the barriers, and it is well below the amorphous limit. The results are in agreement with atomistic Green's function simulations.

Autonomous devices that are self-powered over a full lifetime, by extracting their energy from the environment, are crucial for applications such as ambient intelligence, active security in smart cards or monitoring. As the energy availability and power dissipation are not constant over time, energy management becomes a key function and determines the potential for information processing. All these challenging constraints have been taken into account to develop an autonomous system enabling thermal energy harvesting and power storage in the microwatt range.The microsystem architecture, illustrated in Fig. 3.1.1, is comprised of two power sources, RF and thermoelectric, a microbattery used as a storage unit and integrated circuits to transform and manage the harvested energy and interface the microbattery. Both sources are managed by the ICs: the microbattery being charged either using thermal energy harvested by the thermogenerator associated with the DC/DC converter or using external RF power converted by the RF converter. The state of charge of the storage device is monitored periodically.Thermoelectric power generators have three main advantages: no human intervention is required throughout their lifetime, they are highly reliable and quiet since there are no moving mechanical parts and the materials used are environment friendly. Micro and nanotechnologies enable production of the small power generators required to match the decreasing dimensions of standard wireless sensor modules. The thermal micro-generator illustrated in Fig. 3.1.2 has an output power of 4µW/cm 2 per degree C, a 90Ω series resistance and generates 1V for a temperature difference of 60°C.A micropower up-converter switching power supply is used to convert the available power from the thermogenerator into a regulated power supply ( Fig. 3.1.3). The difference between one part of the boost filter output voltage (α*Vout) and a voltage setpoint (SP) is amplified, then modulated into pulse density information for control of the MOS power switch. A sub-1V bandgap voltage reference [1] is used as the voltage reference (570mV). The error amplifier is comprised of a 50dB gain OTA and a buffer. The innovative pulse density modulation (PDM) is based on an asynchronous passive ∆Σ modulator instead of the traditional PWM controller for simplicity of implementation (2 RC filters and 3 inverters), very low power consumption (1µW) and spectral spread of the switch noise. A low-voltage, high-performance charge pump, composed of one clock booster and two stages of voltage doublers [2], is used to increase the PDM signal voltage four-fold. This allows a large decrease in the equivalent Ron of the MOS switch.The RF converter is composed of a limiter, a rectifier and a control loop to provide a stabilised DC output. In standard 13.56MHz RFID applications, RF power and conversion efficiency depend on the distance between the RF source and the IC; the input RF power is much greater than the needed power and the superfluous current is diverted through ballast MOS...

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In major applications, optimal power will be achieved when thermoelectric films are at least 100 lm thick. In this paper we demonstrate that screenprinting is an ideal method to deposit around 100 lm of (Bi,Sb) 2 (Te,Se) 3 -based films on a rigid or flexible substrate with high Seebeck coefficient value (90 lV K À1 to 160 lV K À1 ) using a low-temperature process. Conductive films have been obtained after laser annealing and led to acceptable thermoelectric performance with a power factor of 0.06 lW K À2 cm À1 . While these initial material properties are not at the level of bulk materials, the complete manufacturing process is cost-effective, compatible with large surfaces, and affords a mass-production technique.

Buffer liquids currently used in biotechnology contain surfactants. These surfactants modify the wetting properties of the liquid and affect the behavior of droplets in electrowetting on dielectric (EWOD) microsystems. An analysis of the effect of surfactants in EWOD-based microsystems, at equilibrium and in a non-equilibrium state, is given here. First, a reference for the value of the surface tension and its time variation is obtained by the use of the pendant drop method, showing that the time variation of the surface tension follows a decreasing exponential law. Next, equilibrium experiments have been performed on a specially designed EWOD experimental microsystem. It has been observed that the Lippmann–Young theoretical equation for electrowetting is satisfied at equilibrium, even in the presence of surfactants. However, at the CMC, equilibrium surface tension values are different from those obtained by the pendant drop method. This difference is explained by an internal convective motion that remixes the surfactants. This internal motion is observed by following the motion of fluorophores. Consequently, surfactant concentrations about ten times the CMC are required to obtain the same values of the surface tension in the pendant drop method and by EWOD measurements. In a second section, it is shown that the behavior of surfactant-loaded droplets in EWOD-based microsystems can be explained by considering that the surface tension relaxes with time according to the exponential law found in the first section.

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