Growth and characterization of AlGaN/GaN/AlGaN field effect transistors

Field effect transistors (FETs) on both GaN and AlGaN buffers were grown and compared. Development of high quality AlGaN is the key to obtain high performance AlGaN buffer FETs. An optimized Al_0.04Ga_0.96N layer on SiC substrate had (002) and (201) X-ray rocking curves with full wide at half maximum (FWHM) of 140 and 450 arc sec, respectively, indicates the comparable crystal quality of AlGaN to that of a typical GaN layer. X-ray reciprocal space mapping and ω -2θ scans showed that the AlGaN barriers grown on GaN and Al_0.04Ga_0.96N buffers had different Al compositions and thickness, which was attributed to the compositional pulling effect. At room temperature, the AlGaN buffer FET had a sheet carrier concentration and electron mobility of 8.3×10¹² cm^-2 and 1750 cm²/Vs, respectively. AlGaN/GaN/AlGaN FETs demonstrated a status-of-the-art power added efficiency (PAE) of 53.5%, and a status-of-the-art power gain of 13.4 dB at a drain bias of 20 V at 30 GHz. This device also exhibited lower output conductance and better pinch-off due to the improved electron confinement resulting from the increase in the effective buffer energy height. The device results imply that AlGaN buffer FETs are promising devices for highly scaled transistors in RF applications in the Ka- band and beyond.