Electrochemical Degradation Mechanisms in AlGaN/GaN HEMTs

Feng Gao, Carl. V. Thompson, Jesús del Alamo and Tomás Palacios Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA 2Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA, email: [email protected] The last twenty years have witnessed numerous developments in the design and performance of GaN-based high electron mobility transistors (HEMTs). The unique combination of the high critical electric field of wide band gap materials and the existence of a high mobility two-dimensional electron gas (2DEG) allows AlGaN/GaN transistors to be one of the most promising candidates for high power and high frequency applications [1]. Nowadays, the most of the focus of the GaN-based device development has shifted from breaking performance records or designing new device structures to achieving a higher level of device reliability. Various degradation modes have been found to limit the device performance of AlGaN/GaN HEMTs. Unfortunately, there has not been been a consensus on the physical mechanisms nor a clear solution yet on how to overcome the device degradation without de-rating the devices [2][3][4]. The main goal of this talk is therefore to provide physical understanding of some of main degradation mechanisms and provide new approached to solve them. We have carried out systematic studies on the origin of permanent structural and electrical degradation in AlGaN/GaN HEMTs. Hydroxyl groups (OH) from the environment and/or adsorbed water on the III-N surface are found to play an important role in the formation of surface pits during OFF-state electrical stress. The mechanism of this water-related structural degradation is explained by an electrochemical cell formed at the gate edge where gate metal, the III-N surface and the passivation layer meet (Fig. 1). In this electrochemical cell, the gate metal acts as the cathode which provides electrons to the water at the interface between the passivation layer and the AlGaN layer when the gate-to-drain diode is reversed biased. The AlGaN layer acts as the anode and is decomposed and subsequently anodically oxidized in the presence of holes and hydroxyl ions (OH). The complete balanced electrochemical reaction is in fact a reduction-oxidation (redox) reaction between AlGaN and water: