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(Color online) EL images overlaid on white-light images of an AlGaN/GaN HEMT (device 1) stressed at VDS = 25 V, 16 V for: (a) 30 min and (b) 80 h. Circles indicate the location and spatial resolution of subsequent 244 nm excitation PL measurements.
(Color online) EL spectrum of a typical AlGaN/GaN HEMT device at VDS = 25 V in off state (16 V, solid black line). For comparison an EL spectrum recorded in the on state is also shown (VDS = 25 V, VGS = 0 V, dashed red line). The ripples visible in the spectra are due to interference of light caused by internal reflections within the device (Bragg interference). Prior to the measurements the device was stressed for 17 h at VDS = 25 V and VGS up to −16 V. The inset shows hot-electron EL (red circles, 1.9-2.2 eV) and defect-related EL (blue squares, 2.8-3.1 eV) as a function of stress time, together with leakage current (black diamonds).
(Color online) PL spectra from three regions of device 1: (a) without hotspots, (b), (c) with hotspots. The inset shows a cross-section of a typical HEMT and indicates schematically the areas where EL occurs and PL was recorded.
(Color online) AlGaN to GaN PL intensity ratio (integrated peak area) recorded at selected locations along the gate edge of degraded devices, in regions with and without hotspots. EL images of the recorded areas are depicted. Inset shows the EL intensity in the 2.8–3.0 eV spectral range as a function of device temperature.
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