SiC chipmakers from all over the world have been recently releasing SiC products, including high-current diodes and high-voltage MOSFETs and JFETs. Many of these devices, and some of the modules that were built with them, were unveiled at the fourth International SiC Power Electronics Applications Workshop. Enrique Lamoureux reports from this event.

When work begins on a new type of compound semiconductor, conferences on this theme tend to focus on material issues. Following progress, devices are then discussed as much as epilayers and substrates, enabling the community to reach an even greater level of maturity. At this point, the number of conferences in this field multiplies, with some focusing on commercial products and manufacturing processes.

This evolution from materials to devices and products has occurred in the SiC community. In this field, the number of conferences is on the up, and at International SiC Power Electronics Applications Workshop that was held in Stockholm in late May, chipmakers from all around the world gathered to unveil a rafter of new products.


The fourth international SiC power electronics applications workship was held at the Kista Science Tower, Stockholm, on 29-30 May

One of the most established manufacturers in this industry is the US chip and substrate manufacturer Cree. At this meeting the company’s Director of Marketing for Power Products, Paul Kierstad, presented two recently released products in the 1700 V range: A 40 mΩ MOSFET and a 50 A Schottky diode. In partnership APE Inc., Cree used this pair of products to develop a 40 kW inverter with an efficiency in excess of 98.5 percent.

An attractive feature of the SiC MOSFET is its ruggedness – it outperforms other types of switch, especially when it comes to operation in the avalanche regime. According to Kierstad, Cree has demonstrated that its MOSFET can withstand repetitive avalanche with an energy of 1.5 J, which is one-to-two orders of magnitude higher than that for a silicon MOSFET.

Another strength of the MOSFET is that its resistance (Rdson) varies less with changes in temperature than that of other switches. Mineo Miura from Rohm highlighted this benefit in a talk that also revealed that the Japanese firm will release two SiC devices this year: A Schottky barrier diode and a MOSFET. Both have been used in the construction of a 120 A SiC module that features five switches placed in parallel.

The commercial availability of SiC Schottky diodes goes back to 2001, when Infineon brought the first product to market. In Stockholm, an application engineer from the company, Uwe Jansen, unveiled Infineon’s fifth generation of Schottky diode. Compared to second-generation devices, its attributes include better performance (defined as the product of lower forward voltage and junction capacitance) for a lower cost. Jansen also announced that the company is releasing a 1200 V, 100 mW SiC JFET that features an internal body diode. The operation of this normally on device is simplified with a dedicated IC driver that enables normally off operation. To do this, a low-voltage silicon PMOS transistor is placed in series with the JFET, a configuration that ensures a safe off state when the device is turned on. When it is in normal operation, the PMOS is not switched and it is kept in the on-state. The driver IC directly controls the JFET, leading to more efficient, better gate driving than a cascode approach. This superiority stems from the lower capacitance of JFET gates compared to a MOSFET.



Paul Kierstad (Cree), Uwe Jansen (Infineon) Joachim Wuerfl (Braun Institute) and Philippe Roussel (Yole Développement) during a panel discussion on the merits of GaN and SiC

Efforts at developing a buried-grid, SiC junction-barrier-Schottky-diode were described by engineer Mietek Bakowski from the Swedish research institute Acreo. He explained that advances in epitaxy enabled the fabrication of a doped buried-grid architecture using SiC. The merits of this diode include an improved ‘trade-off’ between on-state performance and reverse characteristics. Strengths when operating in the latter regime include an extremely low leakage current for the diode when it is operated in a blocking state.

Bipolar power modules

Delegates in Stockholm also witnessed multi-national chipmaker Fairchild Semiconductor discuss its SiC products in development. This firm will soon release 1200 V-15 A (57 mΩ) and 1200 V-50 A (19 mΩ) switches built from SiC bipolar transistors. Both variants will be released in two different packages: A plastic TO-247 for high-efficiency operation and a TO-258 for high-temperature operations. These products can deliver a gain of 100 at 25 °C, falling to 60 at 150 °C. According to the company, this reduction in gain as temperature increases is relatively small for bipolar technology, which has traditionally struggled in this area. To help to spur the integration of these devices into commercial products, Fairchild has also developed a dedicated base driver and a simulation model for the LTspice simulator.

 

Figure 1.Researchers at the Royal Institute of Technology in Stockholm have built a NOR gate based on SiC bipolar emittercontrolled logic technology

The firm has also created a 1200 V, 300 A power module in partnership with Kiel University and Danfoss, a manufacturer of power modules and cooling solutions for power electronics. This power module is based on SiC BJTs from Fairchild and Cree’s SiC diodes. In this project the 24 die – 12 transistors and 12 diodes – were sintered in a transfer-moulded module, rather than a framed module. Higher efficiency results, thanks to lower parasitics that stem from a more compact design and reduced wire bonding. Greater device reliability under temperature cycling and a higher operating temperature are other benefits of the sintering process.

During the design of the module, great care was given to select the most suitable leadframe dimensions, so that the product could handle large currents (a conductor cross section greater than 4mm2) . Air gaps and creeping distances – both pin-to-pin and pin-to-cooler – were taken into account when choosing the mechanical dimensions for the assembly. When this module is pressed onto a water-cooling device with a mounting bracket, the die-to-water thermal resistance is just 0.8K/W.

Aside from its prowess at handling high currents and voltages, SiC is often championed as a tremendous material for operating at high temperatures. To take full advantage of this attribute, Liam Mills, design and development lead engineer at Semelab, UK, showcased a new module based on a SiN substrate. This offers direct interfacing (no base plate), a hermetic package and multilayer possibilities, hence reducing inductance. Mills claimed that this module delivers excellent results, in terms of its thermal resistance and robustness to temperature cycling.

This effort by Semelab highlights the progress made to increase the operating temperatures that can be accommodated by packing technology. However, there is still a substantial difference between the temperature SiC can handle and the temperature that traditional driving electronics can withstand. One company trying to shrink this gap is the Belgium-based fabless outfit  Cissoid. At the gathering in Stockholm this company detailed its high-temperature, half-bridge isolated-gate-driver reference design, HADES. According to Cissoid, this unit can be placed as close as possible to the power devices and can drive a wide range of devices. This includes traditional silicon MOSFETs and IGBTs, plus the latest generation of wide bandgap enhancement unipolar semiconductor devices, such as bipolar and normally on JFETs. The HADES design is compatible with a 1200 V supply, and it can operate at more than 200 kHz. Polyimide is used to make the demonstration board, and all components, transformer included, can operate up to 225 °C (175 °C ambient).

A more radical approach is needed to reach far higher temperatures, such as 400 °C or more. Carl Michael Zetterling’s group from the Royal Institute of Technology in Stockholm are pursuing this goal and trying to produce driver ICs that combine higher operating temperatures with the ability to withstand high levels of radiation. Zetterling told the delegates gathered in Stockholm that their IC development had taken many directions, including characterising transistors, investigating radiation hardness and developing high-temperature metallization processes that could lead to the design and the construction of emitter-coupled logic OR and NOR gates made in SiC bipolar technology.

Battling with GaN

One of the biggest differences between this conference and its forerunners was the inclusion of a session dedicated to a rival wide bandgap semiconductor, GaN. In Stockholm, several representatives from this community were invited to present their latest developments.This included Joachim Wuerfl from the Ferdinand Braun Institute of Germany, who is part of a team developing GaN transistors and diodes for the European Space Agency. Efforts have led to the fabrication of a GaN HEMT that delivers a maximum output of 200 V and 50 A, and combines a threshold voltage of 1.2 V with a resistance of 85 mΩ. Switching to an under-bump package creates a superior version, which delivers an output of 250 V and 75 A and has 75 mΩ resistance.

Engineers at Panasonic have also used silicon as the foundation for making several devices. Company spokesman Nobuyuki Otsuka described an epitaxial process on 6-inch silicon, which involves the deposition of a GaN and AlN superlattice to reduce stress and prevent wafer cracking. Efforts at device fabrication have focused on a gate injection transistor, which combines normally off operation and low on-resistance. This formed a building block for a monolithic three-phase inverter and a bidirectional switch for AC supplies equipped with two gates. Otsuka also mentioned a 9.4 kV diode.

One of the advantages of these GaN devices over their SiC rivals is that their geometry is lateral, rather than vertical. This enables the fabrication of a monolithic inverter featuring six transistors on a single die. Monolithic inverters combine very high integration with very low parasitic inductances.

However, despite constant development and important investment in the GaN domain, this market is still dwarfed by that of SiC. According to analyst Philippe Roussel from Yole Développement, France, it will take two years for GaN to catch up with SiC if sales of GaN-on-silicon products take off – and far longer if they don’t. The supremacy of SiC for products operating at over 1kV is beyond question, but GaN could be a tough competitor at 600 V and 900 V.

The SiC industry will fight very hard to hold on to this sub-kV market. “Cree will not let go the 900 V market,” said Paul Kierstad, who remarked that the cost-per-device for SiC has plummeted since these products first appeared on the market. Further competition between suppliers, including entrants from China, is likely to drive prices even lower.

Today’s SiC products are produced primarily on 3-inch and 4-inch substrates. Moving to larger substrate sizes will help to trim manufacturing costs, although such a move has to be weighed against initial capital expenses associated with putting together lines for handling larger wafers. In 2010, Cree demonstrated 150 mm SiC substrates, and it will soon release a version of this. Is there sufficient demand for a substrate of this size? Probably. The power device market is tipped to grow at a compound annual growth rate of 8 percent through to 2020 – by which time it will be worth $35 billion – and SiC devices promise to gain market share from the incumbent, silicon. In short, SiC devices should have a strong commercial future.

Industrial applications

One of the highlights of the fourth ISiCPEAW conference was the presentation of several applications using SiC components. This included an account by Carl Ho from ABB Switzerland of the benefits of using SiC devices in a photovoltaic inverter. Replacing silicon diodes with SiC equivalents in the pre-regulator of 2.5kW photovoltaic inverter helped to improve efficiency by 1 percentage point. This gain might seem very small, but it leads to lower thermal dissipation and a smaller cooling system. It is then possible to slash the pre-regulator size by almost 50 percent, and trim its manufacturing costs. Ultimately, the user benefits from more compact, more efficient equipment that potentially has a lower price tag, while the engineer designing the equipment delights in the high breakdown voltages of the SiC switches, as well as their high operating frequency, which allows simplification of the inverter topology. It is even possible to lower the component count. Meanwhile, Shashank Krishnamurthy from the research centre of United Technologies, CT, showcased a high-frequency, SiC-based 5.5 kW inverter for aerospace applications. This unit, which was built from SiC MOSFETs and SiC Schottky diodes, had a pulse-width modulation frequency of 400 kHz. This enabled the construction of a very compact transformer and lighter passive filters.