How SiC FETs Are Changing the Semiconductor Landscape

Article By : Anup Bhalla, UnitedSiC

The current generation of leading SiC devices has changed the face of the semiconductor industry; what sectors will be the drivers for the future?

The third generation of silicon carbide (SiC) semiconductor devices has delivered remarkable performance with practical benefits in a growing number of applications. But with the pace of innovation rapidly increasing in sectors such as electric vehicles (EV), renewable energy, and 5G, engineers are increasingly looking for new solutions and demanding more from power switch technology to meet consumer and industry demand.

Silicon carbide’s constituents, carbon and silicon, are respectively the fourth and eighth most abundant elements in the galaxy. Despite this, it rarely appears naturally on earth with just minute traces found in meteorites and some rock deposits. It can be produced synthetically quite easily though, and has been used as an abrasive (carborundum), for more than a century. Even in electronics it was used as a detector in early radios, and the first LED effect was produced in 1907 with a SiC crystal.

In power electronics, we now know SiC as a wide bandgap (WBG) semiconductor which has revolutionized power conversion performance, yielding efficiency figures previously unattainable at high frequencies, with further knock-on benefits of smaller associated passive components, particularly magnetics. That all comes with cost, weight, and size savings.

SiC FET cascodes lead the WBG pack
Now in their third generation, SiC FETs, as a cascode arrangement of a Si-MOSFET and SiC JFET, are at WBG technology’s leading edge. They have the best figures of merit for normalized on resistance with die area, RDSONA, and normalized on-resistance with turn-off energy RDSONEOSS, key indicators for low conduction and switching losses.

In absolute terms, SiC FETs are achieving less than 7 milliohms on-resistance for 650V devices and less than 10 milliohms at 1200V rating, while matching Si pricing. Paralleled parts in module packaging can do even better with 2 milliohm, 1200V performance demonstrated in a SOT-227 format by UnitedSiC (Figure 1).

Six SiC FETs in a SOT-227 package rated 1200V 2 milliohms
Figure 1. Six SiC FETs in a SOT-227 package rated 1200V 2 milliohms.

A major application for SiC FETs is as drop-in replacements for Si-MOSFETs and IGBTs, facilitated by the easy, compatible gate drive and popular TO-247 packaging. Existing applications, particularly with IGBTs, might have low switching frequencies but new designs can take advantage of the high frequency and edge rate capability of SiC FETs in the newly-available DFN8x8 package. This has much reduced inductance making it ideal for hard and soft switching applications such as LLC and phase-shift full bridge converters. The inherent reverse conduction through the SiC FET channel, acting as a low-loss, fast-recovery body diode also helps in this respect.

Where we find SiC FETs today
As direct replacements for IGBTs and Si-MOSFETs, SiC FETs are used to upgrade motor drives, UPS inverters, welders, high-powered AC-DC and DC-DC converters, and more. In the motor drive application, efficiency can be instantly improved without changing switching frequency, with a reduction in static and dynamic losses in the channel but also in the gate drive circuit, which can dissipate significant power in IGBTs and larger Si-MOSFETs.

Typically, gate drive components will be adjusted with simple changes to tame the switching speed of the SiC FET. Other benefits can be considered, such as reducing the size of snubbers and even deleting commutating diodes, which are necessary in IGBT drives but may be effectively replaced by the SiC FET body diode effect. In the EV motor drive inverter application, there are efficiency gains to be had, and if frequency is increased, compared with IGBT solutions, the EV motors can run more efficiently and smoothly. In industrial and automotive drives, the efficiency improvements address the pressing needs for smaller size and longer range respectively.

EV battery chargers, both on-board and static, also use SiC FETs to advantage. Here, the low-loss, high frequency operation allows much smaller magnetics in output filtering — which saves weight, size, and cost — again helping with EV range for an OBC. Roadside rapid chargers using SiC FETs operating at the 100kW+ level with 400V or 800V DC outputs also see the benefit, with efficiency savings over IGBTs. Discrete SiC FET devices, paralleled as necessary are often practical and lower cost, as replacements for expensive IGBT modules. Overall, costs and wasted energy to the environment can be saved.

New designs in all power conversion areas, including high-power AC-DC and DC-DC converters are increasingly using SiC FETs. With ground-up designs, the full potential of the devices can be exploited; totem-pole power factor correction followed by resonant conversion stages either LLC or phase-shift full bridge with synchronous rectification, all using SiC FETs switching at high frequency give very high efficiency. Consequent savings then occur in cooling hardware, magnetics for filtering and energy storage, capacitors, snubbers, housings, and more — all reducing total system cost while reducing carbon footprint.

The future of SiC FETs
Performance of SiC FETS is impressive but designers always want more, fueled by pressure to save energy and cost while increasing functionality. Markets that are expanding rapidly are 5G infrastructure, EV/HEVs, renewable energy generation and data centers, and in all cases, next-generation SiC FET technology has a role to play in achieving

Headline SiC FET characteristics and their proportional change and direction of evolution. blue is today, orange is a potential future scenario
Figure 2. Headline SiC FET characteristics and their proportional change and direction of evolution. blue is today, orange is a potential future scenario.

even better performance.

There are many device parameters that have a roadmap for improvement, some which trade off. Figure 2 shows the direction of travel for some, and potential proportional gains, in a future scenario. All these gains are theoretically achievable and can be expected to appear as development continues. Improving parameters are not all related to loss reduction, important though this is. Ruggedness is also set to improve with better short circuit withstand rating, higher breakdown voltages and lower package thermal resistance for easier cooling and better reliability. There is known scope for improvement in package and SiC FET cell design, which will yield the expected reduction in RDSON and die area. Happily, this also reduces die capacitances which in turn reduce dynamic losses.

Applications for JFETs in SiC are also expanding; they have distinct benefits as solid-state circuit breakers and current limiters where their normally-on characteristic is actually an advantage. SiC technology allows extreme tolerance to high peak junction temperatures and gives low on-resistance with well-defined saturation current and fast switching. As circuit breakers, SiC JFETs can switch thousands of times faster than traditional mechanical types with low insertion loss.

Even linear operation is improved in circuits such as electronic loads with SiC JFETs; compared with Si-MOSFETs, the SiC parts do not suffer from current crowding within the cell structure, as the individual cell gate threshold voltages are insensitive to temperature. Si-MOSFETs, on the other hand, have a strong negative temperature coefficient for VGTH which can lead to local hotspots and thermal runaway.

Packaging will develop as well
SiC FETs are already opening up new applications at higher power and higher switching frequencies – this is from a standing start only a few years ago. Compared with the long track record of silicon device development, SiC is only at the beginning of a long road with exciting performance milestones already in view.

As potential applications for SiC FETS widen, package options will expand as well. The TO-247 package in three-and four-lead format currently provides a drop-in replacement for many current IGBTs and Si-MOSFETs, but TO220-3L devices are also available. In surface mount styles, D2PAK-3L and -7L are popular and the low-profile DFN8x8 from UnitedSiC suits very high frequency operation with its low package inductance. More SMD options will become available and silver sintering will be increasingly used for die attach for improved thermal performance. Modules using SiC FET die will become common with versions rated at 6000V or higher using a stacked “Supercascode” arrangement. These will find applications in MV-XFC fast chargers, traction, renewable energy generation, solid-state transformers, and high voltage direct current (HVDC).

Generations to come
SiC FETs are already opening up new applications at higher power and higher switching frequencies – this is from a standing start only a few years ago. Compared with the long track record of silicon device development, SiC is only at the beginning of a long road with exciting performance milestones already in view.

— Anup Bhalla is vice president of engineering, UnitedSiC.

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