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Record power performance at 4 GHz has been obtained using field-plated AlGaN/GaN HEMTs on sapphire substrate. High power density (12 W/mm) as well as high efficiency (58%) have been measured. A comparison between devices with and without field plate on the same sample showed a significant reduction in knee-voltage walk-out for the field-plated device, thus enabling high power and efficiency operation.
Introduction: AlGaN=GaN HEMTs represent the most promising devices for microwave and millimetre-wave power applications [1]. However, device performance is known to be often limited by 'DC-to-RF dispersion'. Surface traps, acting as a 'virtual gate' in the gatedrain access region, prevent proper device operation, reducing the available current swing as well as degrading the knee-voltage [2]. The introduction of a field-plated gate structure proved an effective method for reducing dispersion. This method was successfully applied by Asano et al. [3] on AlGaAs=GaAs HFETs. Recently, Ando et al. [4] also applied this technique to AlGaN=GaN HFETs, showing a significant improvement in device performance. In this Letter we compare performance of devices with and without an overlapping field-plate structure. Field-plated devices yielded power densities as high as 12 W=mm, more than twice (5.2 W=mm) those obtained for devices without the field plate.
Device fabrication: Fig. 1 shows a cross-section of the fabricated HEMT. AlGaN=GaN heterostructures were formed using a 290 Å - thick Al0.22Ga0.78N barrier layer on sapphire substrate. Details on device processing can be found in [5]....