Effect of HTRB lifetest on AlGaN/GaN HEMTs under different voltages and temperatures stresses
Introduction
Power GaN transistors have demonstrated to be excellent devices for application in power electronics. Gallium nitride has a wide band gap (3.4 eV), that allows operation in high temperature. Moreover, GaN transistors can remain functional up to 500 °C [1]. This could reduce the size and the weight of the cooling systems. GaN has a breakdown field of 3.3 MV·cm−1, which is ten times higher than silicon. For the same breakdown voltage, GaN transistors can be ten times smaller than silicon ones. In addition, the two-dimensional electron gas presents a high density (>1013 cm−2) and a high mobility (>2000 cm2/Vs [2]). These characteristics enable components to be operated at high frequencies and allow the weight reduction of passive components in critical block such as converters.
However, these components need yet to meet the high reliability standards demanded by the automotive and aerospace industry. The reliability of normally-on GaN HEMTs was addressed in several researches [[3], [4], [5], [6]]. Recently, some good work has been done in order to understand the failure mechanisms of normally-off GaN HEMTs with p-gate [7,8]. However, much more work is still needed to understand the failure mechanisms especially for normally-off HEMTs.
In this work normally-off industrial COTS HEMTs have been submitted to High Temperature Reverse Bias (HTRB). Different temperatures and voltages have been applied to distinguish the effect of each stressor and have a better understanding of the activated failure mechanisms.
Section snippets
Device under test characteristics
The device under test (DUT) is a 200 V enhancement mode power AlGaN/GaN HEMT. This device has a typical RDSon of 50 mΩ and a maximum continuous drain current of 8.5 A. The gate of this component is composed of a P-GaN layer (55 nm) under a TiN layer (90 nm). The gate length is 0.48 μm and the distance between the drain and the source is 6.8 μm.
In order to carry out the high temperature aging tests, these devices were soldered using SAC305 on high TG FR4 PCB or polyimide (see Fig. 1).
Aging and characterization protocol
In this
HTRB step stress
According to the precedent work, the HTRB-T and HTRB-V step stresses allowed us to make the following statements:
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Both of the step stresses affected the RDSon (measured at VGS = 5 V & ID = 7A). In fact, a 25% rise of the RDSon was observed at the end of both step stresses.
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HTRB-V had no effect on the VGSth (measured at VDS = 1.4 V & ID = 1.5 mA).
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An increase of VGSth was observed during the HTRB-T step stress when the aging temperature exceeded 150 °C (see Fig. 3).
These results put in evidence the
Conclusion
In this work, HTRB lifetest stress was performed on normally-off GaN HEMTs. In prevision of this lifetest, temperature and voltage step stress tests were done in order to distinguish the effect of each stressor. This work showed that there are two different degradation mechanisms for the HTRB step stresses occurring each at a different temperature ranges.
In particular, temperatures higher than 150 °C induce damages in surface states leading to a shift in the threshold voltage. This kind of
Acknowledgment
This work is supported by the IRT Saint-Exupery Robustesse Electronique project (CDP-E-008-006) sponsored by Airbus Operations, Airbus Group Innovations, Continental Automotive France, Hirex Engineering, Nexio, Safran Electrical & Power, Thales Alenia Space France, Thales Avionics and the French National Research Agency (ANR).
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