The impact of surface states on the DC and RF characteristics of AlGaN/GaN HFETs

GaN based HFETs are of tremendous interest in applications requiring high power at microwave frequencies. Although excellent current-voltage (I-V) characteristics and record high output power densities at microwave frequencies have been achieved, the origin of the 2DEG and the factors limiting the output power and reliability of the devices under high power operation remain uncertain. Drain current collapse has been the major obstacle in the development of reliable high power devices. We show that the cause of current collapse is a charging up of a second virtual gate, physically located in the gate drain access region. Due to the large bias voltages present on the device during a microwave power measurement, surface states in the vicinity of the gate trap electrons, thus acting as a negatively charged virtual gate. The maximum current available from a device during a microwave power measurement is limited by the discharging of this virtual gate. Passivated devices located adjacent to unpassivated devices on the same wafer show almost no current collapse, thus demonstrating that proper surface passivation prevents the formation of the virtual gate. The possible mechanisms by which a surface passivant reduces current collapse and the factors affecting reliability and stability of such a passivant are discussed.