AlGaN/GaN-based millimeter-wave high electron mobility transistors
Gallium Nitride (GaN) offers unique material characteristics to enable the fabrication of field effect transistors with high output powers at millimeter wave frequencies. At the start of this work GaN-amplifiers operating at K-band frequencies were available. However, an increasing demand exists for power amplifiers beyond 50 GHz such as radar applications or RF-broadcasting systems. In this work a scaling approach is studied to develop a transistor technology which achieves a high gain as well as a high output power at W-band frequencies and can be applied in the existing fabrication process for monolithic microwave integrated circuits (MMIC). Following the theoretical scaling rules for field effect transistors lateral and vertical critical dimensions of 100 nm and 10 nm must be achieved, respectively. Therefore various new fabrication pro-cesses were developed in this work to enable the new critical dimensions with a sufficient production yield for MMIC fabrication. Transistors fabricated with these methods were evaluated regarding the influence of the scaled geometries on the device characteristics using S-parameter as well as DC-measurements. As a result a transistor technology could be established which achieves a transconductance above 600 mS/mm this is one of the highest reported values for GaN-based HEMTs so far. Furthermore, a very low parasitic capacitance of 0.3 pF/mm was achieved. As a consequence, these transistors feature a current-gain cut-off frequency of more than 110 GHz. Besides the high frequency characteristics short channel effects and their influence on the device characteristics were also evaluated. From these studies the following results were obtained: The scaled transistors are dominated by a drain induced barrier lowering (DIBL) which is mainly a function of the aspect ratio of gate length to barrier thickness. It was also found that a critical aspect ratio of approximately 14 is necessary to suppress the DIBL-effect.
Zugl.: Freiburg/Brsg., Univ., Diss., 2011