Topic: Power Electronics

wind / solar plant
Power converters are core devices in wind power stations and solar systems (© Jiss/Dreamstime.com)

GaN power electronics for energy-efficient technological solutions

Energy efficiency is one of the most important issues of the future world-wide. Consequently, there is a strong demand for technical solutions consuming less energy or utilizing it more efficiently. A broad range of approaches therefore deals with this topic in multiple technical disciplines. Electric power converters, for example, are integrated in practically every electric and electronic system. They can reduce primary energy consumption by converting either DC or AC current particularly efficient into system-relevant electricity levels. Switching elements with increased power density may further reduce system size and weight and thus save energy additionally. Highly efficient power converters are also indispensable for energy conversion in solar systems, wind power stations and modern electric vehicles as well as for power supplies in mobile base stations and computer systems. Due to the proven high radiation hardness of gallium nitride (GaN) devices, they are becoming increasingly attractive for space applications, in particular, for satellite-based solar energy converters.

Hence, GaN-based semiconductors are particularly important for the technical realization of such systems. Power converters utilizing the specific GaN material properties are able to switch high power levels at high switching speeds, typically in the MHz range. The resultant adjustments in system design lead to small and light-weighted systems. At a given operation voltage, the on-state resistance of GaN devices outperforms competing silicon (Si) devices by more than one order of magnitude. This potential has been repeatedly verified on laboratory scale at FBH and world-wide – the technology is on the verge of industrial maturity. GaN transistors from the FBH have already passed the high quality requirements for space applications and will soon be implemented on board of the geostationary Alphasat communication satellite.

Key devices for these and further applications in power electronics are normally-off power transistors. They are switched on at positive gate bias and automatically switched off when the gate bias declines back to zero voltage. Therefore, the devices are inherently secure and suitable for energy converters requiring specifically high system reliability. Due to the long-term research activities in this field, FBH achieves state-of-the-art results. The portfolio of normally-off power transistors realized so far ranges from devices with high current capability of 150 A optimized for 250 V bias to power transistors with 1200 V blocking voltage and 5 A maximum current.

Since GaN semiconductor epitaxial layers have already been demonstrated on up to 200 mm Si substrates, the excellent electronic properties of GaN devices can now be combined with stable and cost-efficient production processes of the silicon industry. It is therefore to be expected that the costs for GaN power transistors, in the medium run, can compete with those for sophisticated Si transistors.

GaN high-voltage transistors for power electronics

Normally-off power transistor
Normally-off 250 V/150 A power transistor with lead-tin bumps for flip-chip mounting
Output characteristics
Output characteristics of a normally-off GaN power transistor (250 V operating voltage, 85 mOhm on-state resistance)

Gallium nitride (GaN) is characterized by its excellent dielectric breakdown strength of 3.3 MV/cm – this value is approximately ten times higher than that of silicon (Si). Thus, with GaN-based power transistors significantly higher power densities and switching frequencies as compared to Si-based transistors can be achieved. High Electron Mobility Transistors (HEMTs) combine high electron mobility with high saturation velocity and are therefore well-suited for high frequencies and very fast switching applications. They consist of layers of various semiconductor materials with differently sized band gaps. In the case of GaN-based HEMTs, an AlGaN/GaN heterojunction induces a very conductive electron layer at the interface forming the channel in the transistor, the so called two-dimensional electron gas (2DEG). Consequently, unipolar devices with a particularly high proportion of breakdown strength to specific on-state resistance can be realized.

Converter systems with GaN transistors benefit from high voltage operation, high current levels, and high switching frequencies up to the MHz range. GaN HEMTs are inherently normally-on due to their basic principle of operation. However, they can get transformed into normally-off transistors, which is due to safety reasons the preferred transistor type in power electronic

FBH’s key R&D activities in this field:

  • Normally-off GaN transistors
    For normally-off GaN power transistors, the FBH focuses on the p-GaN gate technology. By using this technology, an intrinsic potential distribution close to the gate is generated such that the devices can only be switched on at positive control voltage. A threshold voltage of around +1.5 V and a gate dynamic range in the region of +5 V are characteristic. Switching dynamics, in other words the difference between switched-on- and switched-off voltage, is at least six orders of magnitude.

  • GaN transistors with 1000 V electric strength
    By incorporating a back barrier into the GaN semiconductor layers, electrons can be concentrated within the channel even at high operation voltages. An electric breakdown strength above 150 V per µm gate drain distance is achieved. The specific on-state resistance of the 1000 V devices is reduced to <1 mWcm2.

  • High-current transistors up to 150 A
    A cellular transistor layout developed at FBH enables a simple two-dimensional scaling of the device size towards higher current levels. 150 A/250 V transistors have been realized for flip-chip mounting on a ceramic sub-mount with integrated high-current wiring.

The combination of these properties qualifies FBH transistors for power applications in automotive electronics, terrestrial and space-based solar converter technology and others.

Publications:

J. Würfl, E. Bahat-Treidel, F. Brunner, M. Cho, O. Hilt, M. Weyers, R. Zhytnytska, "High voltage normally-off transistors and Schottky diodes based on GaN technology", ECS Trans., vol. 41, no. 8, pp. 127-138 (2011).

J. Würfl, E. Bahat-Treidel, F. Brunner, E. Cho, O. Hilt, P. Ivo, A. Knauer, P. Kurpas, R. Lossy, M. Schulz, S. Singwald, M. Weyers, R. Zhytnytska, "Reliability issues of GaN based high voltage power devices", Microelectron. Reliab., vol. 51, no. 9-11, pp. 1710-1716 (2011).

O. Hilt, E. Bahat-Treidel, R. Zhytnytska, P. Kotara, and J. Würfl, "Bauteile aus GaN - Sicht auf die Halbleitertechnologie", ETG-Fachbericht 128, "Bauelemente der Leistungselektronik und ihre Anwendungen", pp. 47-56 (2011).

Packaging of GaN power transistors

GaN high-voltage chip
50 A GaN high-voltage power transistor chip with lead-tin bumps flip-chip mounted on thermally matched sub-mount

GaN power transistors from the FBH are characterized by a fully lateral design. As all connecting pads are located at the front side of the chips, rather simple flip-chip mounting techniques can be applied. Compared to more conventional chip mounting methods, this technique leads to considerably smaller parasitic inductances, which is one of the prerequisites for fast and efficient switching.

FBH has developed a technology for optimized heat dissipation from the active chip regions through the bumps to the heat-sink by automatically placing small metallic spheres (bump contacts) on the chips. A sophisticated annealing process is converting them into solder bumps. The present technology is optimized for lead-tin bumps, copper bumps ensuring a much better thermal conductivity are under development. As a matter of course, the power transistor process also allows conventional wire bonding techniques if requested. In addition to the lateral device concepts, FBH is currently developing GaN power devices utilizing a quasi vertical device architecture. This concept is ideally suited for heat dissipation to both sides of the chip and more efficiently utilizes active chip area. GaN high-voltage power transistors on silicon carbide substrates will be commercially available via FBH’s spin-off company BeMiTec in 2012.

GaN-based Schottky diodes for efficient converter topologies

GaN Schottky diodes
Packaged GaN Schottky diodes for 600 V, 1 A operation
Characteristics Schottky diodes
Typical characteristics of Schottky diodes with carbon-doped GaN buffer structure

Wide band gap semiconductor material combinations like GaN/AlGaN-based heterojunction structures are of great interest for fast power switching diodes due to their properties such as very high breakdown strength at off-state and high conductance at on-state conditions. However, one of their main drawbacks is the high onset forward voltage – it increases the losses at on-state conditions. In this respect, Schottky barrier diodes with low onset forward bias combined with fast recovery time are favorable. The absence of a body diode in GaN HEMT high-power switching transistors calls for the development of high-performance diodes that may be used as free wheeling diodes in modern converter topologies. The electrical properties of these diodes such as blocking voltage and on-state resistance as well as the switching properties should be compatible to novel GaN-based high-power switching transistors in order to enable inherently efficient converter topologies. To meet these requirements, Schottky diodes with lateral device topology have been developed at FBH. These devices fully utilize the high channel conductivity provided by the 2-dimensional electron gas (2DEG) at the GaN/AlGaN heterojunction interface. They are characterized by a very low onset forward bias of 0.5 V combined with high blocking capability (VBR > 1000 V), high switching speed and a very fast recovery time.

Publication:

E. Bahat-Treidel, O. Hilt, R. Zhytnytska, E. Cho, J. Würfl and G. Tränkle, "AlGaN/GaN/GaN:C back-barrier Schottky diodes for power switching", 35rd Workshop on Compound Semiconductor Devices and Integrated Circuits (WOCSDICE), Catania, Italy, May 29 - Jun 1, ISBN 978-88-8080-123-8, pp. 165-166 (2011).

p-GaN gate technology for normally-off GaN transistors

Conduction band distribution p-GaN gate
Conduction band distribution underneath a normally-off p-GaN gate (blue line, assembly: inset above) compared to a Schottky gate (red line, assembly: inset below)

GaN HEMTs utilize a thin conductive layer at the interface between GaN buffer and AlGaN barrier as transistor channel. With increasing negative voltage at the metallic control electrode (gate) the channel depletes underneath the gate, the transistor starts to block the current. In p-GaN gate transistors, the gates of GaN HEMTs are replaced by p-type GaN semiconductor gates. Thus, a negatively charged depletion region arises which suppresses electrons in the transistor channel underneath the gate. This effect even appears without applying an external voltage (normally-off transistor). At a positive threshold voltage of around 1.5 V, the transistor channel starts to switch on and is fully unblocked (on-state) at approximately 5 V. When the transistor is switched on, electrons flow around 10 - 20 nm below the p-doped gate region without getting in contact with the acceptors. Consequently, full electron mobility within the channel can be preserved.

Publications:

O. Hilt, F. Brunner, E. Cho, A. Knauer, E. Bahat-Treidel and J. Würfl, "Normally-off High-Voltage p-GaN Gate GaN HFET with Carbon-Doped Buffer", Proc. Int. Symp. on Power Semiconductor Devices & IC's (ISPSD), San Diego, CA, May 23-26, pp. 239-242 (2011).

O. Hilt, A. Knauer, F. Brunner, E. Bahat-Treidel and J. Würfl, "Normally-off AlGaN/GaN HFET with p-type GaN Gate and AlGaN Buffer", Int. Symp. on Power Semiconductor Devices & IC's (ISPSD 2010), Hiroshima, Japan, Jun. 6-10, pp. 347-350 (2010).

High-voltage concepts for GaN power transistors

Simulation electron concentration
Simulation of electron concentration distribution in pinched-off GaN-HEMT device (a) with GaN buffer layer showing punch-through and (b) with back barrier layer with suppressed punch-through

The off-state breakdown voltage of GaN‑based HEMTs is mainly determined by a sudden increase of the sub‑threshold drain current. At high drain bias and under off-state conditions, electrons might leave the transistor channel bypassing the region controlled by the gate. This phenomenon is called "punch-through effect". To prevent electrons from drifting into the GaN buffer, a back barrier is mandatory. Due to polarization-induced p-type doping, an AlGaN buffer acts as back barrier and prevents electrons from punching through the buffer. Carbon-doping of the GaN buffer additionally creates acceptor-like trap states that scavenge any injected electrons. Transistors withstanding 190 V/µm field strength and blocking more than 1000 V have been realized at FBH with a carbon-doped GaN buffer as back barrier.

Publications:

E. Bahat-Treidel, F. Brunner, O. Hilt, E. Cho, J. Würfl, and G. Tränkle, "AlGaN/GaN/GaN:C Back-Barrier HFETs With Breakdown Voltage of Over 1 kV and Low RON × A", IEEE Trans. Electron Devices, vol. 57, no. 11, pp. 3050-3058 (2010).

E. Bahat-Treidel, O. Hilt, F. Brunner, J. Würfl, and G. Tränkle, "Punchthrough-Voltage Enhancement of AlGaN/GaN HEMTs Using AlGaN Double-Heterojunction Confinement", IEEE Trans. Electron Devices, vol. 55, no. 12, pp. 3354-3359 (2008).