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Posted on 06 May 2019

600V GaN-on-Si Device’s Extended Lifetime Testing Show High Levels of Reliability Results

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Developing the next generation power semiconductors for efficient electricity conversion is an important task for a sustainable economy. The adoption of any new semiconductor device is based upon the confidence that the device will improve system performance and not degrade during the life of the targeted application.

By Carl Blake, Kurt Smith and YiFeng Wu of Transphorm, Inc.

Until now, the results for 600V GaN devices in Extended Lifetime testing have not been available. Customer confidence has been based upon the performance of the early samples, followed by beta samples that had passed most qualification tests and, finally, by JEDEC qualified samples which have been produced only by Transphorm Inc. With the availability of qualified devices, it has become possible to perform meaningful extended lifetime testing to predict lifetime for these new devices.

Power devices using GaN are still in their early adoption phase, so questions remain regarding the reliability of these devices. Unfortunately, there is also a great deal of misinformation circulating about GaN devices, mostly due to failures of devices that have never passed even a basic JEDEC-type qualification. This has raised the question: Are the failures due to the material or the device design?

To address these questions we have taken examples of high-voltage GaN devices that have passed the typical 1,000 hour stress tests and have subjected them to long-term elevated stress testing. This article reports the results beyond the first 1,000 hours of stress testing. It also provides insight into the quality and intrinsic reliability of these first generation, high-voltage GaN power devices.

Building a Strong Foundation

Before any meaningful extended life-testing could commence, it was necessary to establish a baseline stable process that could successfully pass a series of tests, such as those defined by JEDEC, to determine if silicon devices are suitable for use in commercial applications. The goal of these tests are shown in Table 1.

JEDEC qualification tests and goals

The graph shown in Figure 1 illustrates the traditional bathtub curve used to describe the failure distribution over time. During the early development of Transphorm’s EZ-GaNTM product family, a high early failure rate showed problems in both the design and manufacturing. By 2010 those problems had been reduced sufficiently so that the formal qualification process could begin.

Bathtub curve

    Figure 1: Bathtub curve

Early results of the tests (Table 1) showed that the tests related to packaging produced results similar to the existing silicon type devices, and it became clear that the most difficult test to pass would be high temperature reverse bias (HTRB) test. This was reasonable to expect since the number one issue that needed to be solved with the GaN power devices was “Trapping” -- as has been previously reported by multiple groups attempting to develop such devices. An early indication of trapping was current collapse of dynamic Ron, both of which Transphorm solved in 2009.

As we learned during the development of the EZ-GaN™ product family, the testing, mentioned previously, was very effective in identifying weaknesses that required changes in the device design in order to eliminate the failure modes that were identified. In 2012, Transphorm’s GaN-on-silicon carbide (SiC) products successfully passed these basic tests, which was followed in 2013 with the successful qualification of its 600V GaN-on-Silicon (Si) products [1].

Effect of increasing Activation Energy EA on end of life activity

The Extended Life testing program began with the accelerated electric field testing at multiple voltages to determine the electric field acceleration factors. The predicted effect of a higher activation energy for GaN (Figure 2) shows longer life at similar operating temperatures because GaN has more margin at these temperatures. The voltage test was selected because it is more critical than temperature and because long-term high-field stress had been the most difficult test to pass during the development phase of the 600V devices. This test is still in progress, having generated failures at the 1150V and 1100V while the testing at 1050V continued. These preliminary test results can be used to predict an expected lifetime, but the third data point is needed to confirm the predicted life and the validity of the testing. Testing at further higher voltages was not possible because it introduced additional failure modes that would not be encountered when the device operated at its targeted 600 volts.

Results

In parallel with the tests to determine the activation energy for the 600V HEMT (high electron mobility transistors) devices, the HTRB continued until either 50% of the devices failed or the testing reached 10,000 hours and the high temperature operating life (HTOL) was also extended over a longer period.

In March 2014 the results for extended HTRB, HTOL and highly accelerated voltage and temperature tests, which are ongoing, had the following results:

HTRB testing has now passed 8,000 hours and shows approximatly 2% failures (Figure 3).

HTRB results

HTOL has passed 3,000 hours at 175°C and shows no measureable degradation in either leakage current or on resistance.

These results are very significant because even for matured Si devices, it is note worthy to achieve such a high level of robustness. It also illustrates the fact that the design metrics used for the initial devices was conservative enough to overcome any weakness in the new device technology, and the performance now matches the original target of the design team.

Predicted life results

The results of the voltage acceleration testing can be used to predict the lifetime of the product family. Using the inverse power law model, the predicted lifetime, as shown on the right side of Figure 4, is greater than 100,000,000 hours. While this result is consistent with the predicted lifetime of GaN RF HEMT devices, it is an excellent confirmation that the GaN 600V device’s stresses have been managed as planned and that the buffer layer is performing as designed. This allows the EZ-GaN™ product family to lead the way in commercializing the use of GaN HEMT devices in power conversion applications.

Conclusions and Future Work

Extended lifetime testing beyond the JEDEC qualification standards has strengthened the overall understanding of reliability in GaN 600V power devices. The tests described in this article are continuing to provide data, while customer adoption is ramping up. Additional testing at multiple temperatures to determine the acceleration factor, including tests above 300oC, will be performed to project the lifetime based upon temperature acceleration. These tests will be used to establish the recommended operating temperature of the GaN devices, since it is known to be a higher temperature material than silicon. But for that application, the packaging will become the most significant issue to overcome.

Also, additional testing at high currents and various temperatures will continue to determine at what point electromigration becomes an issue.

References
Y.-F. Wu, J. Gritters, L. Shen, R.P. Smith, J. McKay, R. Barr and R. Birkhahn, “Performance and Robustness of First Generation 600V GaN-on-Si Power Transistors”, Proceeding, 1st IEEE Workshop on Wide Bandgap Devices and Applications, S1-002, Ohio, Oct 27-19, 2013.

 

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