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Conference Report

Magazine Feature
This article was originally featured in the edition:
Volume 30 Issue 5

IRPS: Gaining greater insight into the GaN HEMT


A more comprehensive understanding and evaluation of the GaN HEMT comes from a rigorous model of how it is deployed and new approaches to scrutinise this device.


When deployed in power electronics, the GaN HEMT delivers a great deal of bang per buck. Its strong performance stems from a number of attractive attributes, including a capability to withstand very high electric fields, an excellent electron mobility, and a high operating frequency. These strengths are realised alongside a competitive cost, thanks in part to the growth of GaN on large-diameter silicon wafers that can be processed in established lines.

The compelling performance of the GaN HEMT is driving its uptake in ever more applications. It’s first taste of significant success came from its incorporation in fast-chargers for mobile devices. And over the coming years, shipments will continue to rise as it wins sales in more markets, with deployment forecast in telecommunication and data centre infrastructure, motor drives and inverters for solar panels.

Figure 1. The set-up used by the University of Padova, in partnership with BelGaN, for on-wafer measurements of threshold voltage and on-resistance. This set-up includes an arbitrary waveform generator, two amplifiers that connect the arbitrary waveform generator to the device under test, an oscilloscope to acquire all the signals, and a clamp circuit that is needed to measure the drain voltage, in order to extract the on-resistance of the device.

However, there is still much to do to ensure that the GaN HEMT has long-term, growing success in these markets. As it will be deployed in more demanding applications and undergo a range of stresses, including those that come from hard switching and ringing, it is vital to understand the level of reliability it offers in these situations. In addition, the GaN HEMT is still a work in progress, so it’s important to gain a deeper understanding of its weaknesses – they include a trapping of carriers, which can lead to an increase in on-resistance and a shift in threshold voltage. In some cases, measurements are needed on production devices, ideally undertaken on-wafer, as this saves time and money.

The good news is that progress is being made on all these fronts, with gains reported at this year’s International Reliability Physics Symposium (IRPS), which took place on 14-18 April in Dallas, Texas. At this conference Sandeep Bahl from Texas Instruments outlined a more robust approach to assessing GaN reliability in various power-supply applications; on-wafer approaches to measure on-resistance, threshold voltage and degradation that have been pioneered by the University of Padova, STMicroelectronics and BelGaN, were described by two spokesmen from the university; and Yu-Shan Lin from TSMC detailed the impact of different processes on trapping in field plates.

Figure 2. There are relatively small changes in the on-resistance (RON) and the threshold voltage (VTH) during soft-switching.

Reliability in the field

To help ensure that power electronics offers sufficient reliability in a range of applications, standards exist for assessing device performance. This is not a new innovation, having been established for silicon technologies several decades ago. There are documents from the mid-1990s specifying the methodology to evaluate this device, consider electrostatic discharge, and to put this package through its paces. Such standards are accompanied by documentation for accelerated test-to-failure, used to calculate lifetimes from relevant failure mechanisms.

More recently, standards have expanded to GaN HEMTs. The guideline JEP 180, published in 2020, offers guidance for calculating the switching lifetime from accelerated lifetime testing. Bahl highlighted this documentation when describing what is claimed to be the first generalised approach for determining the hard-switching lifetime of a GaN transistor.

“We first use a test-vehicle circuit appropriate for accelerating the desired switching-stress type, which was hard switching,” remarked Bahl. “We then ran accelerated lifetime testing on a TI GaN part, generated a model and calculated the switching lifetime.”