Influence of the access resistance in the rf performance of mm-wave AlGaN/GaN HEMTs

After demonstrating impressive performance in the X-band frequencies, one of the next goals for GaN-based power transistors is to increase the frequency of operation to the mm-wave range. Monte Carlo simulations predict that the saturation electron velocity in AlGaN/GaN high electron mobility transistors (HEMTs) should be around 2.5×10⁷ cm/s (M. Singh and J. Singh, J. Appl. Phys. vol. 94, p. 2498, 2003). The study of f_T vs I_DS from pinch-off to saturation is a very useful tool to understand the maximum frequency performance of mm-wave HEMTs. In this work, we have studied the effect of parasitic resistances in the shape of the f_T vs I_DS curve. R_s increases with the current density in the channel. The extraction of the effect of the differential access resistance out of f_T greatly changes the shape of the curve. The access resistances play an important role in the rf behavior of AlGaN/GaN transistors. Not only the absolute values, but also their increase with current must be controlled in order to get the maximum performance out of the GaN-based HEMTs.