Cascode connected AlGaN/GaN HEMTs on SiC substrates

Abstract

We report on the fabrication and characteristics of cascode-connected AlGaN/GaN HEMTs. The HEMTs were realized using Al/sub 0.3/Ga/sub 0.7/N/GaN heterostructures grown on 6-N semi-insulating SIC substrates. The circuit reported here employs a common source device having a gate length of 0.25 μm cascode connected to a 0.35 μm common gate device. The gate width of each device is 250 μm. The fabricated circuit exhibited a current density of 800 mA/mm and yielded an f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> and f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> of 24.5 and 56 (extrapolated) GHz, respectively. Large signal measurements taken at 4 GHz produced 4 W/mm saturated output power at 36% power-added efficiency Comparisons to the performance of a 250×0.35 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> common source device taken from the same wafer show that the cascode configuration has 7 dB more linear gain and 3 db more compressed gain than the common source device at 4 GHz.