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GaN HEMT Reliability: Why the Industry Can’t Agree on One Testing Standard

September 22, 2020 by Antonio Anzaldua Jr.

If standardized testing methodology for GaN-based HEMT reliability is far on the horizon, what are manufacturers doing to help deliver quality GaN devices in the meantime? 

GaN high electron mobility transistors (HEMTs) are gaining traction in the semiconductor industry because of their exceptionally high-temperature tolerance and high power densities. As such, these devices can exist in harsher conditions for aerospace and military applications. But these industries require stringent quality and reliability standards for any power devices—silicon or GaN—and that's where GaN HEMTs run into problems.

 

A comparison of the basic electrical characteristics of a gallium nitride HEMT vs. a silicon MOSFET.

A comparison of the basic electrical characteristics of a gallium nitride HEMT vs. a silicon MOSFET. Image used courtesy of Fujitsu
 

While there are over 50 years of standardized quality assurance testing methods for silicon power devices, there are no such standardized tests for comparable gallium nitride (GaN) power devices.

Why can't the same testing methods for silicon be reused for GaN to ensure its reliability? To answer this question, we might first look at two councils focused on high-quality reliability testing: JEDEC and AEC. 

 

Current Testing Standards: JEDEC and AEC

The Joint Electron Device Engineering Council (JEDEC) is responsible for establishing an extensive set of memory standards for semiconductor circuits and storage devices. For more than five decades, JEDEC has created open standards for microelectronics ranging from main memory like DDR4 to wideband gap semiconductors.

While GaN devices are included in the umbrella of wide bandgap semiconductors, and JEDEC provided one document for GaN power conversion devices in February of this year, it only establishes testing methods for the switching reliability of these devices

 

The switching characteristics of silicon MOSFETs vs. gallium nitride HEMTs

The switching characteristics of silicon MOSFETs vs. gallium nitride HEMTs. Image used courtesy of Fujitsu
 

The Automotive Electronics Council (AEC) was formed in the 1990s by Chrysler, Ford, and General Motors to establish common part quality for standardized automotive systems. Manufacturers heavily rely on AEC’s guidelines because of its explicit reliability standards for transistors, diodes, and other discrete semiconductors. Perhaps AEC is best known by electronic designers for its AEC-Q100 standard, which provides guidelines to test for failure mechanisms in integrated circuits.

Although AEC provides guidelines for automotive, defense, and aerospace applications, it fails to address developing technologies that are increasingly switching to GaN power devices, such as communication base stations.

 

What are GaN HEMTs?

GaN HEMTs are field-effect transistors (FETs) that can switch faster than silicon power transistors. This feature, combined with GaN HEMTs' small footprint, allows the devices to be more energy-efficient while creating more space for external components. These devices can also operate in higher voltages.

 

Simplified GaN HEMT structure

Simplified GaN HEMT structure. Screenshot used courtesy of NovaTCAD
 

But because there isn’t a clear path to test the reliability of GaN devices, it is difficult to determine device lifetime, failure rates, and application relevance.

Professor Roberto Menozzi at the University of Parma in Italy has discussed the lack of reliability testing for GaN-based HEMT devices. “If one looks at the scientific literature, the knowledge database on GaN-based HEMT reliability seems to be characterized by a few features indicating that the maturity goal is still somewhat far ahead.”

If standardized testing methodology for GaN-based HEMT reliability is far on the horizon, what are manufacturers doing to help deliver quality GaN devices in the meantime? 

 

Companies Create In-House GaN HEMT Reliability Tests

To ensure the reliability of GaN HEMTs, many semiconductor manufacturers have penned their own in-house testing procedures. Here's a glimpse at what some of those company-specific standards look like.

 

Texas Instruments

Sandeep R. Bahl, Texas Instruments' devices and modeling manager for GaN reliability, shares a brief breakdown of TI's methodology to qualify GaN products.

First, a stress test accelerates temperature over a certain period of time on the device. For silicon, the calculated acceleration factor is approximately 0.7eV activation energy, however, for GaN devices, the factor can range from 1.05eV to 2.5eV. This range can lead to unpredictable environments, which make it harder to determine accurate lifetimes and exact failure modes.

 

Diagram of a test vehicle for an inductive switching application test.

Diagram of a test vehicle for an inductive switching application test. Image used courtesy of Texas Instruments
 

TI reports few issues testing GaN devices thanks to the wide bandgap, which allows the device to operate at higher temperatures than silicon. While TI adheres to JEDEC and AEC for reliability testing that involves conducting electrostatic discharge (ESD), package effects, and device reactions in actual-use environments, Sandeep comments that for GaN, "It is left to the manufacturer to define the testing."

 

GaN Systems

Like TI, GaN Systems also sees the value in simulating severe conditions for stress tests in order to qualify GaN products and find failure mechanisms. GaN Systems has found ways to modify the tests once used on silicon power devices to determine GaN reliability. GaN Systems’ key elements for testing are identifying failure modes, ensuring long-term wear out, and simulating real-world applications.

In order to test for minimal to no failures, GaN Systems performs robust failure mode analysis, which tests at high-risk failure modes and also conducts a collaborative feedback session with trusted partners. During the analysis, devices are put under accelerated voltage, temperature, and humidity for an extended period of time. 

With JEDEC and AEC-Q101 being a foundation for their testing methods, GaN Systems is able to identify extrinsic and intrinsic failures.

 

MACOM

Since 2006, MACOM has performed testing on GaN-on-Si technology for military and defense applications for wireless base station infrastructures. Testing revolves around operating in extreme temperature, humidity, and voltage variances.

MACOM’s first test is called a highly accelerated stress test (HAST), which simulates a 20-year system lifetime in 96 hours of intensive stress testing utilizing JEDEC procedures. 

 

MACOM’s Automated Accelerated Reliability Test System can perform up to 60 devices.

MACOM’s Automated Accelerated Reliability Test System can perform up to 60 devices. Image (screenshot) used courtesy of MACOM

 

The next step in the testing phase is the high temperature operating life (HTOL) exam, which requires zero failures while operating at the hypothesized temperatures off the datasheet specifications. The same plan goes into the “off” state to see the breakdown behavior of the device.

The final reliability test is the accelerated life (ALT) stress examination. This test observes the degradation and failure of the device to determine its life expectancy.

 

Infineon

Infineon Technologies has developed a four-part process that qualifies its CoolGaN 600V technology and products.

The first phase requires developers to create an application profile for the GaN device in line with JEDEC and AEC-Q101 guidelines. This profile includes a list of potential stressors that the device may encounter during operation. 

The second phase is to establish a quality requirement profile, which assesses an end customer's goal for the device: its target lifespan, maximum allowed failure rate, ESD rating limits, and operating humidity requirements. 

The third phase critically conducts a reliability investigation to look for intrinsic and extrinsic failure modes. If a failure occurs when the device wears out or certain materials deteriorate, the test yields an intrinsic failure.

 

Infineon’s reliability bathtub curve allows for observation of failure mechanisms over time

Infineon’s reliability bathtub curve allows for observation of failure mechanisms over time. Image used courtesy of Infineon Technologies

 

The final phase involves creating degradation models of key failures in GaN devices, such as the "reliability bathtub curve." This curve showcases three failure regimes: early life, the constant, and wear-out region. 

 

Why Do Manufacturers Have Their Own Testing Methods?

The answer is simple: to keep a competitive edge. The desire to stay ahead of the game is the main reason the industry hasn't collectively created a standardized reliability and qualification test for GaN HEMTs. Each company has its own specialized GaN IP and crafts reliability tests for its own devices. 

To create an industry-wide standard would require multiple manufacturers to combine their knowledge and agree on a test across the board for decades to come.