Study develops physics model for GaN reliability
The report, the 12th phase of reliability reporting from EPC on its GaN devices, details a test to fail methodology that identifies intrinsic failure mechanisms and can accurately project the safe operating life of a product over a more general set of operating conditions.
This reliability and prediction is key for adoption of GaN devices in power designs across the automotive market, from lidar drivers to DC-DC converters and traction inverters, as well as power supplies for telecoms and data centre applications.
“With Phase 12 we didn’t just build observational models that fit the data but created a physics-based model of the crystal to construct the lifetime model of the devices. Now we can derive the behaviour, both electrical and lifetime, from the physics,” said Dr Alex Lidow, CEO and founder of EPC, talking to eeNews Power.
“The bottom line is that the GaN devices out in the field for ten years are far more reliable than any silicon device ever made. The statistics are adding up really quickly, we have the data now. We take the devices and make them fail, look at how they failed, characterise them and create a model to give you an idea of how it will work in other applications.
The model helps to identify the failure modes and, interestingly, the areas of higher performance.
“One of the insights is that temperature is a negative accelerant,” said Lidow. “If you go to higher temperature the lifetime gets better. We are educating people on that. Voltage is an accelerant but more nuanced that take into account impact ionisation and mean length of a hot carrier in the lattice with phonon interactions,” he said. “It shows GaN is becoming a mature technology.”
The physics-based lifetime model includes supporting evidence to project the lifetime of an eGaN device under gate stress over all voltages and temperature ranges, not just those devices that have been tested over the last four years and 216bn hours in vehicles and telecoms systems.
Next: GaN reliability model
The first-principles mathematical model describes the dynamic RDS(on) effect in eGaN FETs from the basic physics of hot carrier scattering into surface traps has been developed. This model is most useful for predicting lifetimes over all voltages and temperatures in more complex performance profiles.
EPC developed a custom system to assess eGaN reliability over long-term ultra-high dv/dt and di/dt pulse stress conditions such as might be encountered in automotive lidar systems. As of the report date, devices have passed thirteen trillion pulses (about triple a typical automotive lifetime) without failure or significant parametric drift.
The study includes assessment of the thermo-mechanical stress under temperature cycling and intermittent operating life ie power cycling was conducted to experimentally generate lifetime predictions and guidelines for the selection of underfill based on key material properties.
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