At-Bias Extraction of Access Parasitic Resistances in AlGaN/GaN HEMTs: Impact on Device Linearity and Channel Electron Velocity

AlGaN/GaN high-electron mobility transistor "hot" parasitic source and drain resistances R_S,D are determined under operating biases through wideband S-parameter measurements, without the use of "ColdFET" biasing conditions. Both R_S and R_D are found to increase dramatically over ColdFET values, both for biases approaching threshold and for open-channel conditions. Parasitic resistance values have a significant effect on the extracted small-signal equivalent circuit model elements, as well as on the apparent device linearity. The bias dependence of access resistances modifies the understanding of the transistor physical operation: A revised delay time analysis accounting for the bias dependence of parasitic resistances shows that the effective average electron velocity in the AlGaN/GaN two-dimensional electron-gas channel is approximately equal to 1.9 times 10⁷ cm/s. This new value of channel velocity is also consistent with the C_GS/g_MO ratio obtained when the bias dependence of R_S and R_D is accounted for during the extraction of the transistor small-signal equivalent circuit model