(Invited) Reliability Considerations for Al-Rich Aluminum Gallium Nitride Power Switching Transistors
Authors: Baca Albert G. , Klein Brianna , Stephenson Chad A , Armstrong Andrew M. , Allerman Andrew A. , Douglas Erica A. , Fortune Torben R. , Colon Albert 
Published: February 2020

High breakdown voltage and low on-state resistance are important factors that have enabled improved performance and efficiency of power switching transistors. Based on wide-bandgap (WBG) semiconductors, these transistors have recently entered commercial applications. The next generation of power transistors utilize a new class of semiconductor materials with bandgaps (Eg
) greater than that of GaN (3.4 eV), the “ultra” wide-bandgap (UWBG) semiconductors, Eg
= 3.4 – 6.2 eV [1-3]. In particular, Al-rich AlGaN alloys and b-Ga2O3 represent two UWBG semiconductors with considerable recent interest [2,3]. The large bandgap of an UWBG semiconductor is an important factor in determining the critical electric field (EC
, scaling as approximately as Eg

2.5 [2]), which not only affects breakdown, but also confers operability advantages in harsh environments embodied by 500°C temperatures [4].

We will discuss the progress and challenges for maturing the Al-rich AlGaN transistors (the percent of Al approaches 100%, e.g. AlN) to the point where they represent viable technology choices for extreme environments, and perhaps more broadly. After presenting the highlights of the Al-rich transistor research, attention will be given to reliability considerations, which are motivated by the ability to influence early technology decisions. Thus, the purpose of reliability research into emerging technologies is to provide early data that impacts material and process choices for the Al-rich transistors and not to attempt reliability determinations, per se.

Step-stress experiments are one of the main tools available for undertaking early reliability analysis. Examples of both enhancement-mode [5] and depletion-mode HEMT step-stress experiments will be described and analyzed. Such experiments allow quick examination of multiple types of stress, including, those absent of drain current and those with varying concentrations of hot electrons, all while sampling progressively increasing magnitude of gate- or drain-bias derived electric fields. In this manner, one discovers parametric boundaries affecting degradation and may identify the most relevant follow-up experiments. Step-stress experiments may involve both reversible parametric effects and permanent ones. The reversible effects are those that heal or re-occur with abrupt changes in bias conditions and are associated with carrier capture and emission processes. They have the potential to confound the reliability analysis because considerable parametric changes can be observed. They must be accounted for so that proper interpretation of potential degradation is made. They also have the potential to reveal important details of how the capture and emission processes affect device operation. Although many examples of these experiments will highlight parallels with conventional AlGaN/GaN transistors, some notable differences unique to Al-rich semiconductors will be described as well.

[1] J.Y. Tsao et al., Advanced Electronic Materials, 4, 1600501 (2018).

[2] R. J. Kaplar et al., ECS J. Solid State Science and Technology, 6, Q3061 (2017).

[3] M. Higashiwaki and G. H. Jessen, Appl. Phys. Lett.
112, 060401 (2018).

[4] P. H. Carey et al., J. Electron Devices Society, 7, 444 (2019).

[5] A.G. Baca et al.,
Int. Reliability Physics Symp., P.WB.2 (2019).

This work was supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. The views expressed in the presentation do not necessarily represent the views of the U.S. Department of Energy or the United States Government.