Trade-offs and challenges of short channel design on millimetre-wave power performance of GaN HFETs

For the application of undoped AlGaN/GaN HFETs to Ka-band millimetre(mm)-wave high frequency power performance, the maximum frequency of oscillation, f_max, was found to be seriously limited by gate resistance and output conductance with the gate length down to 0.1 μm. This makes it difficult for devices to achieve both high f_T and f_max at the same time. However, the technology of field-plate gate, to increase device breakdown voltage, will add extra gate capacitance. It makes the optimum gate structure design more important. The influence of gate metal thickness and gate length on f_max based on the lumped small signal circuit model analysis and the possibility to obtain high f_T and f_max simultaneously for the GaN material structure is discussed for application to the Ka-band mm-wave operating system.