technology  GaN HEMTs


imec is renowned for being equipped with state-of- the-art


processing tools


Researchers at imec have found that nitride HEMTs exhibit an


increase in gate leakage below the critical voltage, which leads to performance degradation and eventual device breakdown


Getting warmer We have also studied the effects of the operating temperature on device reliability, by comparing the gate degradation of three sets of 16 devices at 298 K, at 398 K and at 473 K. In each case, the gate voltage was fixed to –55 V. We adopted the same methodology used to study our devices at room temperature and found that the η-values extracted from each set of HEMTs follow the Arrhenius law with an activation energy as small as 0.12 eV (Figure 4a). Our conclusion: gate degradation has weak temperature dependence. One key consequence of this finding is that any temperature- accelerated test, including the widely used three- temperature life test, can fail to highlight gate degradation phenomenon. That’s because temperature, rather than voltage, is used as the accelerator factor. Our work shows that a high-temperature stress that leads to no gate failures after thousands of hours cannot guarantee that there will be no gate degradation within 20 years. A standard temperature-accelerated test only shows that the applied voltage stress is not high enough to observe gate degradation within the stress time.


We have repeated our time-to-breakdown experiments at an ambient temperature of 200 °C with gate stress voltages of –60 V, –55 V and –50 V. Again, when performing lifetime extraction, we found that the power law model provides a good fit to our experimental data (Figure 4b). In this case, an operating voltage of 22 V that corresponds to 31 percent of VCRITICAL


guarantees a 20


year lifetime at 200 °C with 1 percent failure level. This lifetime applies to a device with a 0.7 µm gate to drain distance. Since breakdown voltage scales with this distance, increasing the separation of gate and drain to 4 µm guarantees a failure rate of just 1 percent for


FURTHER READING D. Marcon et al. Proc of IEEE International Electron Devices Meeting 472 (2010) R. Degraeve et al. Microelectronics Reliability 39 1445 (1999) G. Meneghesso et al. Int. J. Microwave Wireless Technology 2 39 (2010) J. A. del Alamo et al. Microelectronics reliability 49 1200 (2009)


18 www.compoundsemiconductor.net June 2011


HEMTs operating at 70 V and 200 °C for 20 years. Interestingly, lifetime extrapolation curves at room temperature are not parallel to those at 200 °C, implying that gate degradation exhibits different activation energies at each bias point. In other words, it is not possible to extract the device lifetime for any arbitrary temperature by just using the Arrhenius equation and the lifetime extraction from time-to-breakdown measurements performed at fixed temperature. The major consequence of this finding is that in order to avoid extrapolation errors, time-to-breakdown experiments should be performed at the targeted temperature. By taking this approach, which is conventionally used in the CMOS industry, temperature effects are intrinsically included in the failure data. Although the nature of gate degradation is unclear, it is possible that the root cause is the well-studied and documented inverse piezoelectric effect. In this scenario, additional strain induced in the AlGaN layer during application of the bias condition would determine the rate of defect generation. Nevertheless, it is possible that an alternative and not yet known phenomena might be behind the time depended gate degradation. Further investigation is required.


What is abundantly clear is that the increase in gate leakage below the critical voltage, which leads to performance degradation and eventual device breakdown, is a very important reliability issue for AlGaN/GaN HEMTs. It is not exposed by conventional reliability approaches based on temperature-accelerated tests, but the pace that it proceeds does depend on the applied bias. So in our opinion a comprehensive reliability evaluation must include voltage-accelerated, time- dependent breakdown experiments at the targeted temperature to correctly assess gate degradation phenomenon and ultimately provide a reliable prediction of HEMT lifetimes.


© 2011 Angel Business Communications. Permission required.


GaN reliability studies were undertaken at imec, a world-leading research institute in nanoelectronics and nanotechnology with more than 1,850 staff


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