Characterizing and suppressing DC-to-RF dispersion in aluminum gallium nitride/gallium nitride high electron mobility transistors

In the last 10 years, significant progress has been achieved in AlGaN/GaN high electron mobility transistors (HEMTs) for high power microwave applications. Improvements in material growth and device design have produced state-of-the-art AlGaN/GaN HEMTs with almost an order of magnitude larger power densities (∼20 W/mm) than is available from GaAs-based transistors. Despite the impressive state-of-the-art performance, many AlGaN/GaN HEMTs fall short of their predicted power performance. The reason is their dynamic I(V) characteristics are different from their DC characteristics. The discrepancy between the dynamic I(V) curves and the DC I(V) curves is known as dispersion. In order for the commercialization of AlGaN/GaN HEMTs to become a reality, dispersion must be controlled.; This work is focused on characterizing and suppressing dispersion in AlGaN/GaN HEMTs. The time scale at which dispersion occurs varies significantly (<1 ns–1 ms), resulting in the need for several different characterization techniques. Pulsed I(V) is used for characterizing dispersion on time scales greater than 100 ns (the limit of most pulsed I(V) systems). In order to determine the amount of dispersion in the sub-nanosecond regime, a technique known as RF I(V) is developed. If the equipment necessary for RF I(V) are not available, it is also shown how to use load pull power measurements to measure dispersion in the sub-nanosecond regime.; A semi-quantitative model on how surface-states cause dispersion in AlGaN/GaN HEMTs is developed. Using this model, methods for suppressing dispersion are discussed. Three different device designs that should be less sensitive to surface potential fluctuations are investigated. The first device investigated uses selective area regrowth in the gate-drain region for dispersion control. The second device is a GaN junction field effect transistor (JFET). Finally, a recessed gate device that uses a p-GaN screening layer in the gate-drain region for dispersion control is investigated. Limitations for all three devices are discussed.