WBG SEMICONDUCTORS How can power system quality control help to improve EV sales?

Related Vendors

Diotec Semiconductor AG

ROHM Semiconductor GmbH

CISSOID S.A.

Wide Bandgap Semiconductor devices’ high power density and high power capacity make them attractive for EV designers – but they must be subjected to rigorous QA procedures to ensure their long term reliability.

Electric vehicles (EVs) have enjoyed significant growth over recent years, due both to government green initiatives worldwide, and consumers’ enthusiasm for driving quieter, cleaner and more environmentally – friendly vehicles.

Yet reports from research organizations like Goldman Sachs show that sales momentum for EVs is slowing globally. There are three well-understood contributors to this; range anxiety, lack of charging infrastructure, and high capital costs of EVs compared to conventional types.

However, reliability issues can also be a factor. Consumers intuitively believe that EVs will be more reliable, as they have many fewer moving parts than cars that have to detonate small quantities of hydrocarbon fuel thousands of times a minute. But reality does not bear this idea out; a report from Consumer Reports has found that EVs have 79 % more reliability problems than fossil fuel cars.

Most of the problems come from electric motors, the vehicle’s charging system, and battery issues. Another big factor is that EVs are in their infancy as mainstream vehicles, so manufacturers, to some extent, are still working out the kinks.

As designers come under pressure to meet modern vehicles’ ever-increasing power level and power density demands, they are naturally turning to wideband semiconductor devices – typically gallium nitride (GaN) and silicon carbide (SiC) – as these can deliver the power performance so critically needed. They are currently the preferred Wide Bandgap (WBG) semiconductors for fast charging, reducing conduction losses and enabling faster switching speeds.

E-BOOK

Semiconductors: A Comprehensive Guide

GaN devices can operate at higher switching frequencies, reducing size while increasing power density. Conversely, SiC excels in higher-power applications, such as power transmission, due to its suitability for handling higher power levels and efficiency in demanding environments.

The shift from 400V to 800V battery systems in EVs is one factor driving their adoption of WBG semiconductors. They enable faster charging times, improved power retention, and longer range for the vehicles. They also contribute to smaller, lighter power converters and higher conversion efficiencies, enhancing overall EV performance.

Additionally, they are used in EV main traction inverters to convert DC battery power to AC power for the electric motor, enabling efficient power delivery. They also provide faster charging and higher efficiency in onboard chargers (OBCs). Within DC-DC converters, they regulate EV system voltage levels, and help to optimize power flow between different subsystems.

GaN and SiC WBG semiconductors improve EV reliability in several ways. Losses are reduced, with efficiency gains from 95 % to 99 % being achievable. With higher breakdown voltages and enhanced operating temperatures, WBG semiconductors provide more robust performance and improved reliability even in demanding conditions. They also enable faster switching, reducing energy losses during transitions.

However, as WBG devices find their way into all these EV applications, their long-term reliability must be absolutely assured; and this calls for developing and applying appropriate quality control strategies throughout the devices’ development and manufacturing processes. Below, we look at four key stages: device characterization and validation, reliability testing, dielectric testing and partial discharge prevention, and product inspection during manufacturing.

Device characterization and validation

Tektronix offers methodologies for dynamic characterization of WBGs; their Keithley instrument range can accurately measure all switching parameters and improve performance. Essential measurements for characterizing WBG devices include testing for current versus voltage, breakdown voltage, and leakage current. In fact, I-V characterization is a key technique for comprehending the current-voltage relationship of silicon, SiC and GaN, as well as their essential characteristics.

The double-pulse test (DPT) is the established procedure for quantifying switching parameters and assessing the dynamic characteristics of Si, SiC and GaN MOSFETs and IGBTs. DPT is employed to quantify the energy dissipation during the activation and deactivation of a device, as well as to determine the reverse-recovery characteristics.

A free Tektronix whitepaper titled “Learn how to do ‘Double Pulse Testing’ with an Oscilloscope” is available through Power & Beyond, and is available for downloading.

In spite of the insights provided by DPT, engineers increasingly want to screen their devices more upstream, directly on the silicon. Keithley’s products can do much quality control process monitoring like IV characterization directly on the wafer with a probing system, but other tests like RDS(on), rise times and fall times are more challenging.

Validation of WBG devices: The capacity to examine power loss and enhance power supply efficiency has become crucial in power applications. Tektronix simplifies the process of doing switching loss measurements using mixed signal oscilloscopes and automated power analysis software.

Measuring floating differential quantities, such as high-side Vgs, is challenging or perhaps impossible due to the rapid switching on and off at high frequencies and the existence of large common-mode voltages, such as Vds. This is because oscilloscope probes lack the necessary ability to reject common-mode signals at high bandwidths.

The inadequate common-mode rejection results in the measurement being overwhelmed by the common-mode error rather than accurately capturing the differential signal. To address these problems, Tektronix IsoVu isolated probes can be used. By maintaining their performance regardless of common-mode voltage when used with GaN and SiC devices, they enable precise differential measurements to be made. Using IsoVu probes, engineers can accurately measure and validate conduction losses, deadtime losses and switching losses.

expert THE FUTURE OF POWER ELECTRONICS

Power device development trends – from Silicon to Wide Bandgap?

Reliability testing

An application brief prepared by Keithley Instruments Inc. describes how a variety of QC tests should be performed both by manufacturers and end-user designers to minimize early defect rates and continuously improve the overall reliability and lifetime of power semiconductors

Many of these tests are outlined in JEDEC Standards such as JESD22-A108D “Temperature, Bias, and Operating Life,” JESD22-A110D “Highly Accelerated Temperature and Humidity Stress Test (HAST),” or JESD236 “Reliability Qualification of Power Amplifier Modules”.

Typical reliability tests involve stressing a batch or batches of sample devices for hundreds or thousands of hours with bias voltages that are greater than or equal to their normal operating voltages while subjecting them to temperatures that are well beyond normal operating conditions. During this stress, a variety of key operating parameters are measured at specific time intervals.

Some of the more popular reliability tests for power semiconductors are HTOL (High Temperature Operating Life), ELFR (Early Life Failure Rate), HTFB (High Temperature Forward Bias), HTRB (High Temperature Reverse Bias), and HAST (Highly Accelerated Temperature & Humidity Stress Test). These tests will either use a continuous bias or cycled bias. A continuous bias can be a fixed voltage or a staircase ramp. A cycled bias will typically vary the duty cycle and/or frequency of the bias voltage. In both cases, key device parameters will be tested continuously or at specific time intervals.

Reliability testing of today’s WBG power semiconductors presents several key challenges for engineers and test system designers. Most importantly, since most of these devices are being targeted for energy-efficiency applications, they have much lower leakage and on-resistance specifications compared to traditional silicon. The test instrumentation must therefore be capable of providing the necessary accuracy, resolution, and stability to meet the electrical requirements of these devices.

In addition, since WBG devices exhibit failure mechanisms that are different from silicon, effective reliability testing per JEDEC standards requires larger sample sizes and longer stress durations to adequately predict important reliability parameters. This requires test instrumentation that is capable of supplying enough power to test many devices in parallel, while maintaining the accuracy and resolution mentioned above. Finally, the test instrumentation must be able to respond to the high speed behaviors associated with these devices and produce the masses of data associated with testing devices in parallel. Each instrument in the system must be fast, and all units must operate in a highly synchronized manner.

Keighley offers reliability test solutions based on their Series 2600B and 2650A System SourceMeter SMU Instruments. These modular, independent, isolated SMUs provide up to 200W per channel with measurement resolutions to sub-pA levels.

Dielectric Testing

While WBG semiconductors offer power level and density advantages as described, increased voltage blocking capability and a trend towards more compact packaging can enhance the local electric field to levels large enough to create partial discharges (PDs) within the module. High PD activity damages the insulating silicone gel, leads to electrical insulation failure and reduces the module’s reliability. Electrical insulators can be eroded more quickly than those within conventional power technologies.

To prevent partial discharge in wide bandgap semiconductors, manufacturers can use methods such as encapsulating the module with a soft dielectric like silicone gel to protect against electrical discharges in air, as well as humidity, dirt, and vibration. Additionally, applying an a-Si:H coating layer has been found to satisfy partial discharge requirements based on IEC 61287-1 by not exceeding 10 pC up to a voltage of 10 kV. These methods help ensure the semiconductors’ reliability and performance.

sponsored MANAGEMENT STATEMENT

"The next generation in power semiconductors will be driven by Silicon Carbide technology"

However, research into this vital area continues. In July 2023, the US Office of Naval Research awarded a five-year research grant to Chanyeop Park, assistant professor, electrical engineering at University of Wisconsin. The project is to develop thin film inorganic electrets to prevent partial discharge within WBG devices. The electret films will be designed to survive in the harsh electrical and thermal operating conditions of WBG power semiconductors.

Inspection is important too

The demanding applications being tackled by modern WBG semiconductors means that novel methods of manufacturing and inspection of production devices are important complements to electrical characterization and reliability testing.

For example, WBG semiconductors are being adopted into EV battery switching systems because of the very high power densities involved. The 800V EV batteries can deliver hundreds of amps, which means that hundreds of kilowatts of power must be switched in spaces often sized no larger than a mobile phone. Up until very recently these devices were bonded to a substrate using conductive epoxies or solders, but there has been a shift away from this traditional method of die attach to Silver Sinter die attach methods to handle the high power density requirements. Silver sintered joints are capable of operating at near 1000°C temperatures.

Custom Interconnect Limited is an Andover, UK – based electronics manufacturer involved in designing and manufacturing both SiC and GaN assemblies for such applications. These assemblies comprise devices mounted on substrates and interconnected through copper conductors - which must be very heavy gauge - bonded to manage the required power levels. The associated die bonding process can produce voids; these can compromise thermal properties and result in early failures.

However, inspection for voids, which may previously have been performed using X-ray technology, has become challenging due to the copper’s thickness; X-rays cannot penetrate to the required depth within these conductors.

The solution is to use c-mode scanning acoustic microscope (C-SAM) technology, as acoustic imaging can reach the required depths within the copper conductors. In fact, C-SAM is the final piece of the jigsaw. The silver sintering method used is a very exacting process, requiring precision at every stage. It can only be properly controlled using three complementary inspection methods – X-ray, C-SAM and CT-Scan; CIL accordingly has all three.

Are You Involved with inverters, Motors, or Drive Analysis?

Learn how to Make Measurements on 3-Phase Motor Drives

For example, acoustic imaging works by collecting reflected sound waves. Air gaps and voids reveal themselves clearly by reflecting the signal strongly. By contrast, X-ray images are created by shadow imaging instead of reflection. All material features are shown at once. Rounded objects that would scatter acoustic waves can be imaged in detail.

CIL is implementing this three-part inspection strategy using Nordson technology supplied by UK inspection and test specialist Cupio. This comprises a Nordson Dage Quadra 5 X-ray and CT-scan machine, and a Nordson SONOSCAN Gen7 C-SAM acoustic microscope.

Substantial projected growth and enhanced quality control

Projections for 2024 have indicated substantial growth in the WBG Power Devices market, presenting promising opportunities for innovation and application. The global adoption of WBG Power Devices is anticipated to witness a noteworthy upswing, signaling a pivotal moment in their market penetration.

Advances in crystal growth and wafer processing are facilitating cost reductions, with China’s robust market entry potentially expediting this decline. The maturation of WBG technology is evident as packaging challenges are tackled through innovations in substrates, cooling solutions, and device design. Concurrently, reliability concerns are being addressed through refined manufacturing processes and enhanced quality control measures, collectively contributing to the transformative potential of WBGs in the technological landscape.

And the story doesn’t end there. Ultrawide-bandgap (UWBG) semiconductor technology - including aluminum gallium nitride alloys (AlxGa1–xN), boron nitride (BN), diamond, β-phase gallium oxide (β-Ga2O3), and a number of other UWBG binary and ternary oxides - is presently going through a renaissance exemplified by advances in material-level understanding, extensions of known concepts to new materials, novel device concepts, and new applications.

(ID:50114696)