P. Ivo^a, U. Zeimer^a, P. Kotara^a, E. Cho^a, L. Schellhase^a, E. Bahat-Treidel^a, J. Würfl^a, G. Tränkle^a, A. Glowacki^b, and C. Boit^b

Published in:

Reliability Of Compound Semiconductors Workshop (ROCS 2011), Palm Springs, USA, May 16 (2011).

Abstract:

AlGaN/GaN HEMTs with different GaN buffer thicknesses on n-type SiC substrates have been analyzed with respect to their degradation mode during drain voltage step stressing tests. The analysis was accompanied by electrical measurement of DC and pulsed device characteristics and complemented by electroluminescence (EL) characterization to localize defective device regions before and after degradation tests. It has been recognized that devices manufactured on wafers with different buffer thicknesses (1.7 µm vs. 2.4 µm) show very different degradation modes. It is apparent that devices fabricated on thicker buffer structures tend to degrade in close vicinity of the gate by opening a gate leakage path through the AlGaN barrier [1]. In contrast, devices on thin buffer layer are more prone to show a drain current increase during step stressing whereas gate leakage is not affected. This mode of degradation is attributed to electrons bypassing the gate region via defect-assisted conductivity through the buffer layer underneath the gate and the drain side channel region. At the same time the total available device current drops significantly and large EL signals can be seen even at fully opened channel conditions. At these conditions the EL intensity is rather low for non-degraded devices or for devices made on thicker buffer structures even after degradation in the gate AlGaN barrier region. The appearance of the EL signal at ON-state conditions together with a significantly reduced drain current indicates that permanent trapping states are created during degradation. These traps are believed to negatively charge up during device operation and are therefore significantly reducing the available drain current.

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