Our modern society is facing an ever increasing penetration of communication and computing devices and systems in everyday life. This process will even intensify and significantly influence areas like mobility and industrial production. The challenge is to combine internet of things with a low carbon dioxide consuming society. Novel approaches for energy-efficient conversion of electrical energy are needed to power modern digital society.
Power converters based on efficient semiconductor power switches are playing the crucial role in this context. These devices often go unnoticed by the general public but are taking over the important task of converting electricity. Electrical energy delivered from the grid or from batteries – like in electric cars – needs to be transformed to various systems, requiring different forms of electricity, different voltage and power levels. More than 50% of the world-wide generated electricity has to be converted by semiconductor power converters. Thus, an increase in efficiency by a few present only can save enormous amounts of primary energy. Actually, increasing energy efficiency in power converters is the prerequisite to efficiently operate our energy-hungry digital infrastructure and electrically driven transportation systems – and last but not least to cope with their expected growth rate.
Wide bandgap semiconductor devices – transistors and diodes – as explored at FBH are playing a key role in these kinds of developments. They have the potential to reduce the power converter volume by 50% and raise efficiency levels up to 99%. This corresponds to reducing the size of a laptop power supply to the scale of a mobile phone charger. Semiconductors like gallium nitride (GaN), aluminium nitride (AlN) or gallium oxide (Ga2O3) all feature a very high breakdown strength. Hence they enable power switching devices with unprecedented power density and low switching losses – the most important conditions for being efficient. For 2025, the Oakridge National Laboratory has estimated the energy saving potential for the United States to pile up to a value between 35 and 74 terawatt hours per year when consequently using wide bandgap semiconductors. As a result, the electrical power delivered by six to eight average coal-fired power plants could be saved – avoiding the emission of billions of tons of carbon dioxide. Extrapolating these values to the whole world would add up to a truly impressive figure.
It is evident that demands requested by modern societies can only be realized when power electronics gets more efficient. The Ferdinand-Braun-Institut is making its contribution with innovative developments, ranging from lateral and vertical GaN power transistors to ß-Ga2O3 power switching devices and AlN-based electronics in various implementations. FBH is closely cooperating with industry and academic partners, thus working on power electronic systems, circuit design and power electronic measurement techniques in short development cycles.
Further Information on FBH's power electronics department
Novel semiconductor materials such as freestanding gallium nitride wafers and gallium oxide substrates are not always available in diameters larger than 1 inch. For a competitive device development, however, it is necessary to realize extremely precise submicron lithographic dimensions with alignment accuracies between different lithography levels of better than 200 nm. Therefore, FBH has developed a special chuck technology based on laser structuring of silicon (Si) wafers and Si-on-Si wafer bonding. This method allows precise processing of 1 x 1 cm² and 1-inch substrates in an i-line wafer stepper environment. The small samples are aligned to a definite mechanical stopper integrated into the chuck. The chuck itself mimics a 3-inch wafer, thus enabling wafer stepper pre-alignment and exposure alignment. This allows realizing sub-micron structures with the required precision. Thus, devices fabricated with this technique can be easily compared to other types of devices having similar dimensions.
The electrical properties of gate insulators have a crucial influence on the GaN MISFET’s gate modulation properties and are a key for proper transistor functionality. Therefore, the FBH has been systematically investigating and optimizing the gate insulator deposition process. To this end, different Al2O3 films were atomic layer deposited (ALD) by two methods, thermal ALD (ThALD) and remote plasma-enhanced ALD (PEALD). Subsequently, the films were electrically analyzed by capacitance-voltage and electrical breakdown strength measurements. The deposition was performed in an ALD system (SENTECH SI PEALD) and combined with an in situ ammonia plasma surface pre-treatment. While the PEALD films show high breakdown robustness, the ThALD films offer a lower capacitance-voltage hysteresis. Additionally, an alternating layer utilizing both methods was deposited for the first time, combining the advantages of each approach. It exhibits superior electrical properties with less hysteresis, indicating reduced oxide film charging, lower flat-band forward voltage shift and high electrical breakdown robustness.
Increasing the power density of switch-mode power-electronic converters requires higher operation frequencies and thus transistors with high switching speed. GaN-based 600 V switching transistors feature a particularly low gate charge and a low output capacitance which may result in switching transients of up to 200 V/ns slew rate. This is why GaN transistors demonstrate clear advantages in terms of switching speed over Si-based superjunction metal-oxide-semiconductor field-effect transistors (MOSFET) and even over SiC MOSFETs. Essential for gaining benefit from high-speed switching in a power converter system is the realization of a low-inductance environment for both the gate and the power loop. However, typical parasitic inductances from packaging and package inter-connects in the nH-range already generate ringing of switching transients in the 10 ns timescale. Increased switching losses and device over-voltage stress are the consequence, resulting in reduced converter efficiency.
Low-inductance designs with particularly small current loops are required for the circuit board layout and for transistor packaging to take full benefit of the GaN transistor’s inherently high switching speed. To optimize GaN-based converter switching cells, FBH uses its own established 600 V normally-off GaN technology that is based on a p-GaN gate module. As a hybrid integration approach FBH developed an AlN-based 600 V, 10 A two-layer platform. It enables a very compact assembly of the gate loop and the power commutation loop for a GaN-based half bridge (shown on cover, bottom). Two stacked metal layers allow for particularly small vertical power loops. The platform supports die bonding and wire bonding as well as SMD soldering of gate driver integrated circuits (ICs) and capacitors. The AlN substrate serves as electrically insulating heat sink with low thermal impedance. For further power commutation loop reduction FBH combined two 600 V, 170 mW GaN transistors on one chip as monolithically integrated half bridge (Fig. 1). Indeed, the 6 A, 300 V switching transients (Fig. 2) of the integrated half bridge show faster turn-on, faster turn-off and less ringing as compared to a setup with traditional PCB-mounted discrete transistors from the same GaN wafer.
Unlike for Si-based vertical MOSFETs of the 650 V class or SiC FETs, the GaN heterojunction field-effect transistor (HFET) lateral design makes source, gate and drain accessible from the chip top-side. Moreover, it offers the opportunity to laterally integrate different device functionalities on one die and to realize very fast GaN-based ICs. A 60 V, 1 A half bridge chip with integrated power switching transistors and integrated gate drivers (Fig. 3) was designed to explore power conversion at switching frequencies as high as 100 MHz. A power loop efficiency of 87% was achieved for 14 W power conversion from 30 V to 20 V.
New wide bandgap materials combined with innovative device concepts have the potential to further increase power density and to overcome challenges of lateral gallium nitride (GaN) technologies. In this connection, FBH has started new initiatives targeting novel power switching devices. True vertical GaN power transistors fabricated on free-standing GaN substrates and devices relying on thermally highly conductive but electrically insulating aluminum nitride (AlN) as buffer material are among FBH’s R&D foci. Another emphasis is on the new material gallium oxide.
Vertical GaN-based field effect transistors for high voltage power switching applications have the potential to outperform Si- and SiC-based competitors in terms of power density and switching speed. FBH’s vertical GaN MISFET technology currently focusses on fast pulsed laser driving applications with maximum voltages < 100 V. Drivers for pulsed lasers are required to deliver very high currents up to 250 A with pulse lengths as short as 3 - 10 ns. Vertical GaN MISFETs are particularly suited for realizing very steep current slopes due to their low output capacitance and gate charge figure-of-merits. Further, the vertical GaN transistor topology enables a compact assembly to achieve ultimately small current loop inductances for fast laser pulsing. The enhancement-mode trench MISFET concept is chosen since it allows a normally-off gate drive characteristic, enables aggressive device downscaling and high current densities per unit area. The vertical GaN transistors have been successfully integrated with a GaAs-based diode laser using chip-on-chip mounting schemes. This arrangement demonstrated 4 W laser pulses at 905 nm wavelength with 3.5 ns pulse duration.
Lateral GaN-based transistors (HFETs) from the FBH have recently demonstrated superior performance as high-speed switches in small and light-weighted power converters up to 600 V. This current technology requires compensation doping with iron or carbon to keep the channel electrons well confined at high drain voltages and to suppress off-state leakage currents. Dopant-related trap states are, however, the root cause for dispersion effects, frequently seen in GaN transistors.
The FBH now targets new aluminum nitride (AlN) based devices that promise to surmount performance limitations inherent to GaN HFETs. Using the ultra-wide bandgap material AlN as buffer material does not require compensation doping and allows for excellent transistor channel confinement. Dispersion effects thus should reduce. FBH now confirmed the principal benefits of AlN buffer layers and demonstrated AlGaN/GaN HFETs with higher breakdown strength and with less dispersion as compared to AlGaN/GaN HFETs with conventional GaN buffer. The new AlN-buffer devices combine a superior breakdown voltage scaling of 140 V/mm gate-drain separation with low increase in dynamic on-state resistance of only 18%, when pulsing for 0.2 µs from 65 V off-state drain bias into device on-state. In comparison, different GaN-buffer epi designs feature either a low dynamic RON – with poor breakdown strength – or a high breakdown strength – but with a high dynamic RON.
The ultra-wide bandgap semiconductor gallium oxide (β-Ga2O3) has received great attention in recent years due to its unique material properties. It is considered as an attractive alternative to conventionally used materials such as SiC or GaN for future power electronic applications. The estimated dielectric strength of 8 MV/cm offers the possibility to drastically reduce the gate-to-drain distance, thus allowing to fabricate more compact and efficient transistor devices with reduced switching and conduction losses. This is emphasized by Baliga’s Figure of Merit (BFOM), describing the basic suitability of a semiconductor material for power switching applications – the BFOM of β-Ga2O3 is a factor of 3,000 higher than that of silicon.
In collaboration with the Leibniz Institute for Crystal Growth, FBH has been developing lateral β-Ga2O3-based power transistor devices on 10 mm x 10 mm substrates. The partners combine improved layer growth of n-type epitaxial layers, exhibiting low defect densities as well as enhanced electron mobility, with an optimized high-resolution process technology using projection lithography. As a result, first transistors were fabricated featuring high average breakdown field strengths of around 2 MV/cm at off-state along with low on-state resistance. These values already outperform the results of more established wide bandgap device technologies like SiC and GaN and demonstrate the high potential of this promising new material.
K. Tetzner et al., “Lateral 1.8 kV β-Ga2O3 MOSFET with 155 MW/cm2 Power Figure of Merit”, Electron Device Lett. (2019). doi: 10.1109/LED.2019.2930189
E. Bahat Treidel, O. Hilt, M. Wolf, L. Schellhase, A. Thies, J. Würfl "Vertical GaN n-channel MISFETs on ammonothermal GaN substrate: Temperature dependent dynamic switching characteristics" Mater. Sci. Semicond. Process., vol. 91, pp. 146-150 (2019).
O. Hilt, E. Bahat Treidel, M. Wolf, C. Kuring, K. Tetzner, H. Yazdani, A. Wentzel, J. Würfl "Lateral and vertical power transistors in GaN and Ga2O3" IET Power Electron., vol. 12, no. 15, pp. 3919-3927 (2019).
- research news: 13.09.2019