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Power Electronics

Test Equipment Must Keep Pace with New High Power Semiconductor Developments

Today, the power semiconductor industry’s focus is on increasing energy efficiency, reliability, and performance, while lowering the cost of high power devices. To date, manufacturers have traditionally relied on silicon technology for devices that control motors, regulate voltages, convert power, etc. Now, newer semiconductor technologies can operate at higher voltage, current, power, and frequency, and test requirements are complicated by multiple versions of the same technology.

Now emerging are power semiconductors employing silicon carbide (SiC) and gallium nitride (GaN) transistors. These semiconductors have higher power density, smaller size, better high temperature performance, higher frequency response, and lower ON resistance than silicon. In turn, devices based on SiC and GaN have lower leakage than silicon, so there is a need for sourcing higher test voltages, as well as appropriate current measurement sensitivity. For SiC, leakages are more than two orders of magnitude lower than similarly rated silicon devices, so current measurements in the single micro-amps are required.

Dynamic testing of devices switching kilowatts in micro-seconds will require more advanced driver circuits and very well designed interconnect and layout of tester electronics as well as device-under-test sockets and contactors. Tight design for creepage and clearances must be considered to ensure safe and reliable results. Very fast, high power transition times will require attention to ground noise and EMI.

Among the new characterization challenges for these new power devices is testing at both low and high current levels. It may be necessary to use special triaxial test cables to provide sufficient noise immunity at the low currents. In addition, the existing GaN and SiC semiconductors will require unique test fixtures because they are sometimes housed in different packages. Another complication is that the International Rectifier and Transphorm GaNs are basically depletion mode devices that are normally on so they include an integrated driver circuit. In contrast, EPC’s GaN is enhancement mode that is normally off so it doesn’t need an integrated driver. Cree’s SiC devices are packages in industry standard packages. However, as the portfolio grows, packages optimized for high-speed, high-power switching will potentially need an entirely different type of test fixture.

These new high power semiconductors have become increasingly demanding, requiring test instrumentation capable of characterizing significantly higher rated voltages and peak currents than ever before (Table 1). Also, breakdown and leakage tests are typically performed at two-to-three times the level of the rated or operating voltage. Devices in the ON state have to pass through tens or hundreds of amps with minimal loss. In the OFF state, they may have to block thousands of volts with very low leakage currents.

From a processing standpoint, SiC and GaN are more difficult to work with and more difficult to control than traditional silicon. These technologies are simply not as mature as silicon technology, which creates challenges for the engineers responsible for designing and characterizing these devices, as well as for those involved in quality assurance (QA), failure analysis (FA), and process monitoring. The higher cost of dealing with these challenges, as well as the cost of the materials themselves, also means that the prices of these devices are generally higher than their silicon counterparts. This puts pressure on the cost of test, especially final test, which these manufacturers can’t afford to short-change due to reliability requirements.

New Test Equipment Required

Therefore, there is a critical need for new approaches to power semiconductor test instrumentation. When silicon devices were the predominant technology, measurement ranges were not nearly as challenging as they are today. The relatively slow rate of change in the power semiconductor industry for a number of years meant that existing equipment types largely met power semiconductor manufacturers’ test requirements, so instrument manufacturers had little motivation to innovate and develop new, more capable solutions. Manufacturers of some of these test solutions allowed them to become obsolete due to a fall-off in demand, rising support costs, and reduced profitability.

The recent worldwide thirst for more energy-efficient, environmentally conscious, and “green” products is breathing new life into the power semiconductor industry. Innovation is surging again as power semiconductor manufacturers push to improve the efficiency and reliability of their products by squeezing performance from new devices, whether they’re based on silicon, SiC, or GaN. Unfortunately, after returning to growth after years of stagnation, power semiconductor manufacturers now realize the test equipment they’ve been using for so long can’t meet the requirements of their new product development initiatives. Moreover, these older test solutions simply can’t provide the low current measurement capability or accuracy required to characterize next-generation devices and materials. In many cases, they lack the necessary power to support today’s operating or characterization levels. As a result, the test industry is being forced to play catch-up to serve these test needs.

Although some new parameter analyzer products have entered the market to serve certain niche applications (such as R&D characterization), their prices are often beyond the means of many of today’s cost-conscious equipment buyers. Moreover, they don’t address the broader application requirements for production test and quality assurance/failure analysis (QA/FA). Similarly, integrating a system that combines high power capability with low current measurements is a big technical challenge and such custom-designed systems typically require large test engineering teams to develop and maintain them, so they’re practical only for production test applications. Traditional curve tracers, although suited for some lower power R&D and QA/FA applications, lack low current measurement capability and can only be found in the used equipment marketplace.

The newer instrumentation models still lack the necessary low current capabilities, precision, power levels, and affordable price points that today’s power semiconductor manufacturer customers demand. Although commercial ATE systems have always been used for power semiconductor production test applications, their cost, size, and lack of characterization and low current measurement capabilities make them impractical for R&D and QA/FA applications.

As single-quadrant devices, power supplies cannot sink power; therefore, they require several seconds for the capacitance charge to bleed off after testing, slowing the test process, which is problematic in production test applications.

Safety Concerns

Safety is another area of concern when configuring high voltage test systems, which require safety interlocks, double grounds, and other safety features to protect operators and sensitive system instrumentation. For example, in Europe, the Low Voltage Directive (LVD) requires that test and measurement equipment demonstrate its safety properties, by complying with EN61010-1. Internationally, the safety agency requirement is IEC61010-1, while in the USA it is UL61010-1.

Because of the introduction of higher power semiconductors, manufacturers have been looking for a new approach to high power semiconductor testing. In response, Keithley Instruments started applying the integrated sourcing and measurement capabilities of SMU (source measurement unit) instruments for this need. Essentially, SMUs are fast-response, read-back voltage and current sources with high accuracy measurement capabilities, all tightly integrated in a single enclosure.

Initially, the shortcoming of SMUs was their limited range – they couldn’t deliver the power levels required for accurate characterization of high power semiconductors. To circumvent these limitations, Keithley introduced the Model 2651A High Power System SourceMeter® instrument in 2011, an instrument specifically designed for high current characterization of high power semiconductors.

The Model 2651A combines the capabilities of a semiconductor parametric analyzer, precision power supply, true current source, DMM, low-frequency ARB, pulse generator, electronic load, and trigger controller – all in one full-rack, four-quadrant instrument. It also has a wide current range, with 2000W ([email protected]) of pulsed power capability and 200W of continuous DC power (Fig. 1). The instrument can source and measure currents from 1pA to 50A. And, by linking two of these units together, users can expand the top end of the current pulsing range to 100A.

SourceMeter Instruments

In response to the need for higher voltage semiconductor testing, Keithley developed a companion product optimized for applications that demand a combination of high voltage sourcing, fast response, and precise voltage and current measurements. The Model 2657A High Power System SourceMeter instrument (Fig. 2) meets the needs of power semiconductor designers and manufacturers and researchers working with new high power semiconductors. Together, the Model 2651A and Model 2657A address the needs of many of today’s power semiconductor applications and can be used across multiple departments within a single organization.

The Model 2657A High Power System SourceMeter® Instrument can source or sink up to 180W of DC power (±3,[email protected], ±[email protected]). It also offers 1fA resolution, allowing it to make fast, accurate sub-picoamp measurements, even when sourcing up to 3000V.

Optimized for testing power semiconductors that include diodes, FETs, and IGBTs, the Model 2657A can also characterize GaN and SiC semiconductors. Plus, it is useful for characterizing high-speed transients and performing breakdown and leakage tests on various electronic devices at up to 3,000V. The Model 2657A has the speed necessary to source high voltage pulses quickly, like a 3000V pulse in less than 15 mS or a 500V pulse in less than 2 mS (Fig. 3).

This instrument offers a highly flexible, four-quadrant voltage and current source/load coupled with precision voltage and current meters. It combines the functionality of multiple instruments in a single full-rack enclosure: semiconductor characterization instrument, precision power supply, true current source, 6-1/2-digit DMM, arbitrary waveform generator, voltage or current pulse generator, electronic load, and trigger controller, and is fully expandable into a multi-channel, tightly synchronized system via Keithley’s TSP-Link® technology.

Choice Of Measurement Modes

There is a choice of digitizing or integrating measurement modes for characterizing both transient and steady state behavior, including rapidly changing thermal effects. Each mode is defined by two independent analog-to-digital (A/D) converters—one for current and the other for voltage—that run simultaneously to ensure accurate source read back without sacrificing test throughput.

Digitizing measurement employs 18-bit A/D converters that support 1µS-per point sampling, so users can capture voltage and current transients simultaneously.

Integrating measurement employs 22-bit A/D converters, which optimizes operation for applications that demand the highest measurement accuracy and resolution. This ensures extremely precise measurements of the very low currents and high voltages common in GaN and SiC semiconductors.

No Programming Needed

You can perform basic device characterization with no need for software installation or programming with TSP Express, an LXI-based I-V test software utility. Users can connect a PC to the LXI LAN port and access TSP Express with any Java-enabled web browser. Test results can be viewed in either graphical or tabular format and then exported to a file for use with spreadsheet applications. There are two additional tools for creating test sequences:

Test Script Builder application (for creating, modifying, debugging, running, and managing TSP scripts

IVI-based LabVIEW® driver (to simplify integrating the Model 2657A into LabVIEW test sequences).

ACS Basic Edition software is an option for component characterization. It includes measurement libraries to support both DC and pulse mode operation of the high voltage Model 2657A and high current Model 2651A, as well as the rest of the 26xxA line of SourceMeter Instruments. These libraries address a variety of power devices, with tests that include input, output, and transfer characteristics on most devices. A special “Trace Mode” provides real-time control over an instrument’s voltage or current output using a simple slider.

The Model 2657A can be connected to other instruments in a test system with standard safe high voltage (SHV) coaxial cable connections compatible with existing high voltage test applications. However, for applications that depend on getting the most from the instrument’s low current measurement performance, there is a special HV triaxial (guarded) connections to optimize the Model 2657A’s measurement accuracy.

As an option, there is the Model 8010 High Power Device Test Fixture that provides connections for testing packaged high power devices at up to 3000V or 100A. This makes it safer and simpler to configure a device test system that includes the high voltage Model 2657A, one or two high current Model 2651A instruments, and up to three low power SMU instruments (other Series 2600A instruments or the Model 4200-SCS semiconductor characterization system). Besides standard banana jumpers, the Model 8010 has rear-panel oscilloscope and thermal probe ports to simplify system integration for further DUT characterization.

Controlling Transients Key in GaN Testing

Carl Blake, Transphorm

We use some Keithley equipment combined with our internally built test equipment.

We put more emphasis on fast transients, the real problem with existing equipment. You cannot have a standardized rack that will work for all device testing. I do not believe a standard rack could ever be made that would respond fast enough. This is not linked to GaN only, but with the DirectFET silicon devices an issue eight years ago. But now, with GaN, you add high voltage to the previous impossible requirements.

Whether its depletion mode or enhancement mode for GaN does not matter. It is the combination of high voltage, high current and extremely fast switching times, such as up to 400 volts per nanosecond transitions coupled with tens of amperes of current that must be done without inductive induced spikes.

A Device Maker’s View on GaN Testing

Alex Lidow, Efficient Power Conversion

In producing GaN transistors, we use a combination of designing our own test equipment and buying commercial types. For typical datasheet testing, we use a suite of commercial instruments. These often have to be configured in a non-conventional way to allow for testing of GaN devices. Also, we usually have to combine several instruments to extend the range of a single instrument. Overall, the testing process requires a lot of custom test fixture design, as well as custom software and analysis tools.

For example, in testing device capacitance, we use a commercial LCR instrument as the main workhorse, but it lacks the necessary voltage range (sometimes up to 600V) to fully characterize the GaN devices. Therefore, we have to use an external programmable voltage source in conjunction with a custom test fixture to prevent large voltages from damaging the LCR meter. These two instruments have to be synchronized in software, requiring custom programming and data analysis tools.

Another example is in testing device output curves. We use one instrument to characterize the low voltage/high current behavior, and a separate set of instruments to characterize the high voltage/low current side. Each regime requires its own test fixtures, and the process of marrying the data sets together is often cumbersome. The new Keithley units address this problem nicely.

Much of our testing, such as for reliability, in-circuit performance or fundamental physics characterization, requires completely custom test equipment design. For reliability testing, the need for continuous monitoring of a large number of parts in parallel is outside of the scope of existing commercial hardware. In these cases, the test capability is built from the ground up.

We continually run into issues testing our parts at high power yet maintaining desired low current accuracy. Our testing requires a dynamic range in current sensitivity from 1 μA up to 100A, 8 orders of magnitude! We have not found a commercial instrument that can cover this range. Safety and shielding of high voltage (also mentioned in the article) are also big concerns in test fixturing.

Our suggestions for the characteristics desired in test instruments include:

  • Voltage range from 0 to 1 kV
  • Current range up to 100A, with sensitivity at the μA level.
  • Pulse capability, with the ability to adjust pulse duration from 50 ns up to DC.
  • Max instantaneous power of 1 kW.
  • Multi-pulse capability. For example, ability to take multiple measurements in time, with the time interval between measurements less than 1 μs.

Device Heating, Stability

A few main issues we encounter that also need to be addressed include:

a) Device stability in the chosen test fixture. For GaN devices, with their high gain and switching speeds, improperly designed test fixtures can readily lead to undesired device oscillations, which at best, corrupt the measurement, and at worst, destroy the device. We spend a lot of engineering effort battling oscillation. Commercial instruments that target GaN need to pay close attention to bandwidth limiting techniques near the device itself.

b) Device heating: The high instantaneous power capability of our parts (up to 1 kW) requires careful control of the pulse duration to avoid melting the part. Most commercially available testers do not allow for sufficiently short pulses. For instance, the unit we use has a 250 μs pulse duration. This pulse duration is too long, and can lead to variable device heating. This could result in the data measurements being corrupted, as not all measurements are taken at the same device temperature.

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