How robust testing forms part of the SiC supply chain

Silicon carbide (SiC) is not new. It was first identified as the rare mineral moissanite in a meteorite in Arizona in 1893 and synthesized material has been used as gemstones and in electronics as diode detectors from as early as 1907.

Today, it’s a wonder-material to replace silicon in semiconductors that switch faster and have lower losses. Applications that benefit the most from SiC include the three very much in focus today: electric vehicle (EV) charging and powertrains, solar, and energy storage systems.

Along with its own end-to-end SiC supply chain, onsemi has also developed robust testing and manufacturing controls to deliver reliable SiC solutions.

Focus applications: EVs, solar and energy storage

An important application for SiC semiconductors is in roadside fast DC chargers and on-board chargers, where better efficiency combined with higher switching frequency make for smaller, cheaper and lighter converters. Lower losses mean reduced energy costs, less environmental impact and shorter charge time for a given load on the grid. For on-board converters, the smaller size is welcomed in the crowded under-hood space and the lower weight means a little more range per charge. In EV powertrains, although SiC MOSFETs are making inroads, their low switching losses are not so much of an advantage, as the switching frequency is kept low to suit the traction motors. Conduction losses are more of an issue in traction inverters and it can be expected that as SiC on-resistance improves, their uptake will increase, replacing IGBTs whose saturation voltage will remain near-constant despite other incremental performance improvements.

Smart home application

A smart home shown with interconnected EV, solar and energy storage. (Source: Avnet)

In solar installations, the improved efficiency with SiC makes the most of an energy source that is intermittent and as such directly affects the revenue obtained from an installation and its capital expense payback time – an important metric for any solar user, from domestic to grid-scale. SiC MOSFETs are ideal in the boost converter stages that generate a high voltage bus from the solar cell DC and they also excel in the inverter stages that generate grid AC. This is particularly true for string inverters which operate at tens to hundreds of kW levels. Centralized inverters at higher power and into the megawatt range might still use IGBT technology. As in EV powertrains, SiC MOSFETs will move up the power scale as their on-resistance reduces.

With the trend toward a smart and distributed power grid, Energy Storage Systems (ESS) —particularly battery versions — have become essential building blocks to provide back-up to renewable energy sources and a way to smooth demand by peak shaving. In this function, they are often seen coupled with large ultra-fast EV charging stations, where demand can vary from sometimes zero overnight to megawatts during a busy day. A battery storage system can be residential at less than 10 kW or at a utility scale greater than 150 kW. At high power, the bidirectional power conversion stages employed are often modular in format for scalability and fault tolerance, so all ranges of installations suit SiC MOSFET solutions.

A wide variety of SiC devices apply in the focus applications

Common building blocks exist across applications: AC-DC converters with power factor correction (PFC), isolated DC-DC converters and single- and three-phase inverters. In each case, the converter could be unidirectional or bidirectional.

In the PFC stage, the simplest arrangement of a boost converter can use a SiC MOSFET and SiC flywheel diode. onsemi has a range to suit DC link bus voltages up to 1100 V, with devices rated at 1200 V and 650 V. For higher power, interleaved simple boost converters can be used as can the Totem Pole PFC stage, which can be fully synchronous, capable of operating bidirectionally and at high efficiency.

The circuit eliminates the lossy AC rectification bridge but has been impractical with silicon MOSFETs as it is a hard-switched topology when operating in continuous conduction mode, necessary at high power. In hard switching, the switch body diode conducts but SiC has a diode with sufficiently low losses. The switches see the full DC link voltage so, again, the onsemi 1200 V devices are ideal for common rails, either as a supply to a non-isolated inverter stage or for an isolated DC-DC stage. At higher power still, active front ends such as the Vienna rectifier can be used which, although unidirectional in its basic form, stresses the switches at only half of the DC link voltage so 650 V devices can typically be used with their relatively lower losses and cost.

Isolated DC-DC stages used to create a system or battery charge voltage come in a variety of flavors depending on power level, efficiency and cost targets. In the applications we are discussing, the simplest might be an LLC converter, which is a resonant type operating with variable frequency. In the LLC, transistor voltage stress is clamped to the DC input voltage, so 1200 V onsemi devices can safely be used with around 1000 V rails. 650 V devices are well suited to nominal 370/400 V buses with a good margin. At higher power, soft or hard-switched bridge converters may be used and SiC MOSFETs again are a good choice – switching frequency can be pushed up with dynamic losses minimized, because of low values of device capacitances. Even hard-switched arrangements are viable due to the fast, low-loss body diode and low turn-on and off-state energy of a SiC MOSFET.

In all cases, performance can be enhanced by paralleling devices. IGBTs can be made to share current but total dissipation is about the same, whereas MOSFETs reduce total dissipation in proportion to the number of devices. It’s actually better than that, as the lower shared currents produce lower junction temperature rises and consequently lower on-resistance rise than a single device. This becomes an increasingly attractive approach as device costs decline.

SiC semiconductor manufacturing helps secure supply chain

onsemi’s end-to-end supply chain process

onsemi owns its end-to-end supply chain and performs comprehensive verification tests on all products. (Source: onsemi)

SiC devices are clearly the future for power conversion at increasingly high-power levels. To compete with IGBTs and Si-MOSFETs, they must have a robust supply chain and a manufacturing process that guarantees quality and consistency.

Like all new technologies, SiC MOSFETs had to be verified in design and there were initial difficulties and surprises when first developed in the 1980s and early 1990s. Lattice defects were observed that caused high leakage, breakdown and gate threshold instability. Even today, SiC MOSFET manufacture requires careful screening.

To ensure automotive-grade quality, onsemi's approach to SiC manufacturing vertically integrates all processes with the tightest control maintained at all stages, from wafer to design-in support.

Alongside strict quality control systems to automotive AEC-Q standards, onsemi performs comprehensive verification and validation tests on new SiC products, including reliability testing at 100% of rated voltage and 175°C. The critical metric of gate oxide reliability is evaluated, again at 175°C at high field stress, and higher than rated values. Concerns over gate threshold stability during bias stress are addressed with tests that show no significant variation in parameters over extended periods. Tests are at high temperatures and with full positive and negative rated voltages applied.

Gate threshold testing

onsemi tests confirm no significant variation in their target device SiC MOSFET gate threshold under stress. (Source: onsemi)

Tests are also performed in real application circuits representing worst-case use, such as in a hard-switching H-bridge in continuous conduction mode with an inductive load. Test duration is typically 168 hours with multiple units from different lots and checks are made for any channel or body diode degradation. Of course, the full suite of tests required for AECQ-101 qualification are also performed which include electrical, mechanical and environmental stresses.

During the manufacturing process, wafers are scanned for defects both before and after epitaxy and all die are 100% tested for avalanche performance and burnt in at high temperature, to detect and remove any extrinsic gate oxide failures.

A wide range of SiC devices available from onsemi

A range of package styles available from onsemi suit the typical applications for SiC diodes and MOSFETs, from the latest leadless TOLL package for high-speed switching with its low inductance and Kelvin connections, through traditional three- and four-leaded TO-247 devices to modules in 2/4/6-pack styles with parts specifically designed for half and full-bridge legs and Vienna rectifiers. Voltage ratings are up to 1700 V and current in discrete devices is up to 163 A. Modules are available rated to 1200 V and with die on-resistances as low as 10 mohm at 1200 V and 2.2 mohm at 900 V.

With world-class design, technology and manufacturing, onsemi SiC semiconductors meet or exceed competitor performance as demonstrated by multiple comparative figures of merit (FoMs) showing the excellent combinations of conduction and dynamic losses achieved in different application circuits. As a further benefit, onsemi is renowned for its deep application know-how in all areas, particularly automotive and industrial and with application support available globally from centers in EMEA, U.S. and Asia.