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Gallium nitride for energy efficient voltage converters

About 40% of the total consumed energy worldide is provided in the form of electrical energy today. It is expected that this share will rise to about 60% by 2040. These enormous amounts of electrical energy have to be not only produced in a sustainable and environmentally friendly way, but also need to be efficiently distributed and used. Power electronical components, circuits and systems are required to produce different voltage and frequency profiles according to the application at hand. It is necessary to minimize the energy losses and production costs of these systems. Untapped energy savings potentials of 20% to 35% can be made accessible by the introduction of more efficient power electronics into renewable energies and the automotive sector. Novel high power transistors made from Gallium Nitride (GaN) enable substantial improvements regarding the basic electrical properties of voltage converters: the specific on-resistance, the switching frequency and in consequence the efficiency of the voltage conversion.

The consideration of the basic physical properties indicates that GaN-based transistors can obtain an on-resistance ten times lower than comparable silicon transistors. Additionally, the switching frequency can be increased, further boosting efficiency. Simulations indicate that the next generation of GaN transistors will exhibit a power efficiency 33% higher than state of the art silicon solutions. With its outstanding physical properties, the product of on-resistance and specific gate capacity being ten times higher than silicon, GaN will revolutionize highly integrated circuits for energy efficient voltage conversion.

In order to increase robustness and lifetime of devices under high voltages AlGaN/GaN layer systems of extremely high quality capable of withstanding electric field strengths of up to 2.5 MV/cm have to be investigated. Current projects are working with single crystalline GaN as substrate material for AlGaN/GaN layer systems. Transistors built from these materials are then evaluated with regard to their electrical properties.

AlGaN/GaN-based high frequency power transistor

AlGaN/GaN-based high power transistor for high frequencies (Photo: Fraunhofer IAF)

ScAlN for microacoustic devices

Within a short time, our communication systems have evolved from one-way radio systems to complete mobile solutions. We can speak and hear at once and use GPS, Internet, mobile radio and Bluetooth at the same time. Each communication requires its own frequency band, which has to be filtered out of the user's frequency spectrum by a power-efficient and compact bandpass filter. Today there are already up to 12 bandpass filters based on surface or bulk wave components using piezoelectric excitation in every smartphone.

Mechanical motion into electrical signals is used in electroacoustic filters. Due to its good compatibility with silicon technologies, high thermal stability and high sound velocity, piezoelectric aluminum nitride (AlN) dominates the active layers in surface or volume wave based frequency filters. However, the relatively low electromechanical coupling and the low piezoelectric coefficients are limiting factors for future applications at the high operating frequencies required in the future.

The decisive motivation for this project is a recently published Japanese publication which shows that ScAlN with scandium concentrations of up to 40 % has significantly higher piezoelectric coefficients and a higher electromechanical coupling compared to AlN. For this reason, manufacturers of bulk wave and surface wave devices have a great interest in replacing AlN with the ScAlN developed in "COMET" in the next generation of high frequency filter applications.

Aluminum finger contact (cutout) of a surface acoustic wave device

Aluminium finger contacts (cutout) of a surface acoustic wave device

Diamond power electronics

Satellite communication has become a ubiquitous service in our days, bringing telephone service to remote places on our earth and and internet connection on board transcontinental flights. This development necessitates a decrease in size and weight of the satellites, because of cost and time savings during the development phase, the launch of the sattelite and a higher flexibility in ground coverage. These smaller satellites need smaller and more efficient electronics without compromising the capability of receiving and transmitting high amounts of data in the harsh conditions in space.

Synthetic diamond offers advantages to a new generation of fast and efficient amplifiers for transmitters. It combines physical properties of low mass density, high breakthrough voltage, high charge carrier mobility, high heat conductivity and resistance against radiation and is thus extremely well suited for application in satellites. Successful fabrication of isolating and conducting, monocrystalline diamond layers using plasma enhanced chemical vapor deposition has opened up an interesting perspective to develop electronic components for high voltages, currents and frequencies. Researcher at the chair of power electronics evaluate the structural and electrical properties of synthetic diamond with regard to processing it into high power diodes and transistors for applications in communication technologies.

Cut and polished synthetic diamond

Cut and polished synthetic diamond gem