Traps and Defects in Pre- and Post-Aged AlGaN-GaN High Electron Mobility Transistors
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Traps and Defects in Pre- and Post-Aged AlGaN-GaN High Electron Mobility Transistors Yongkun Sin, Brendan Foran, Nathan Presser, Stephen LaLumondiere, William Lotshaw, and Steven C. Moss Electronics and Photonics Laboratory, The Aerospace Corporation El Segundo, CA 90245-4691 ABSTRACT High electron mobility transistors (HEMTs) based on AlGaN/GaN hetero-structures are promising for both commercial and military applications that require high power, high voltage, and high temperature operation. Reliability and radiation effects of AlGaN-GaN HEMTs need to be thoroughly studied before they are successfully deployed in potential satellite systems. A few AlGaN HEMT manufacturers have recently reported encouraging reliability, but long-term reliability of these devices under high electric field operation and extreme space environments still remains a major concern. A large number of traps and defects are present in the bulk as well as at the surface, leading to undesirable characteristics including current collapse. The present study is part of our investigation to study traps and defects in the AlGaN HEMT devices using micro-analytical techniques before and after they are life-tested. INTRODUCTION In order for AlGaN-GaN HEMTs to be successfully deployed in high reliability applications including potential satellite systems, their reliability and degradation mechanisms need to be fully understood. A few AlGaN HEMT manufacturers have in recent years reported encouraging reliability data [1, 2] and a few research groups have proposed degradation mechanisms including hot carrier effects [3], hot phonon effects [4], and inverse piezoelectric effects [5]. However, long-term reliability of these devices under high electric field operation still remains a major concern, let alone their long-term reliability under extreme space environments. The present paper is to study the effects of stresses on electrical characteristics of AlGaN HEMTs and to use micro-analytical techniques to investigate the root causes of degradation in AlGaN HEMT devices. EXPERIMENTAL METHODS AND RESULTS Devices under Study We studied MOCVD-grown AlGaN HEMTs on semi-insulating (SI) SiC substrates developed for X-band power amplifiers. Our HEMTs consisting of a GaN cap, Al0.28Ga0.72N/AlN barrier, and 2 μm GaN buffer layers had a Schottky gate length of 0.25 μm, a total gate width of 200 − 400μm periphery (Sample A: 4×100μm, Sample B: 2×100μm), and SiNx passivation. Figure 1 shows an SEM image of Sample A. Our HEMT structures consisting of undoped AlGaN barrier and GaN buffer layers grown on an AlN nucleation layer show a charge sheet density of ~1013/cm2 and a Hall mobility of ~1500cm2/V⋅sec.
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Figure 1. SEM image of an AlGaN-GaN HEMT (Sample A). Stress Tests We performed accelerated life-tests and short-term step stress tests under DC conditions. Accelerated life-tests performed under Vds=40V, Ids=250mA/mm, and a channel temperature of ~355°C typically yielded a gradual decrease in drain current, whereas short-term step stress tes
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