Non-homogenous gamma process: Application to SiC MOSFET threshold voltage instability
Introduction
The reliability of power electronic devices is assessed, most of the time, by using the method called Accelerated Lifetime Test (ALT) [2]. This approach aims to collect the failure and censoring times of devices under stress conditions in order to estimate their lifetime under nominal conditions. However, this type of test could be costly and time consuming when applied to highly reliable products which cannot fail during the course of the ageing experiments or if long mission profiles have to be simulated.
An alternative method to estimate the reliability of such electronic components consists in monitoring the changes in a performance related parameters during the course of the ageing test. Once this parameter reaches a pre-specified threshold level, the device could be considered as failed. This approach, known as an Accelerated Degradation Test (ADT) [3], is especially suited for devices experiencing a “soft” failure which is not supposed to generate a complete failure of the device but only a degradation of its performance to a non-acceptable level.
In this study, we propose to apply an ADT procedure to a commercial SiC MOSFET aged by a High Temperature Gate Bias test. The threshold voltage (Vth) of the devices collected during these tests will be considered as a degradation indicator and a Non-Homogeneous Gamma process is proposed in order to model the degradation phenomenon. In the last part of this article, a criterion for the test termination will be applied to the collected data set in order to justify the relevance of the test duration.
Section snippets
Context
The recent years have seen the emergence of the More Electric Aircraft concept [4]. From the power supply architecture perspective, this new approach consists in replacing, whenever it is possible, the hydraulic or pneumatic actuators by electrical ones. The recent developments of All Electric Aircrafts, such as the E-Fan or Solar Impulse have kept the trend even further. Nevertheless, the traditional silicon based power electronic devices do not seem relevant to overcome the challenge of
Experimental results
The High Temperature Gate Bias test procedure is a reliability test widely used in the semiconductor industry [10]. The objective of this test is to identify the evolution of some electric parameters of the devices including the threshold voltage due to the gate oxide ageing. The HTGB procedure consists in a continuous bias on the gate pad while the drain and source are grounded. All the devices under test are kept at an elevated constant temperature by putting them within a regulated oven.
Optimization of the ageing tests duration
The achievement of accelerated ageing tests provides the opportunity to gather data on the product's lifetime within a reasonable amount of time. This approach enables the equipment manufacturers to provide reliability data (MTTF, 100p-th percentile of the lifetime distribution…) to their customers. Nevertheless, the duration of acceleration needed to provide significant data could also be optimize when the degradation data are used. The question of termination time for the ageing test is quite
Conclusion
The instability of the threshold voltage is well known for being a key limitation of the commercial SiC MOSFETs. This phenomenon could be observed and accelerated through a High Temperature Gate Bias test. For the purpose of this study, this test has been carried out on 10 devices with VGS = 30 V and Tc = 100 °C during 300 h. This failure mechanism is characterized by a gradual increase of the VTH value to a level considered as a failure, which is set to a 20% increase. The impact of such drift for
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