Surface-related drain current dispersion effects in AlGaN-GaN HEMTs

Drain current dispersion effects are investigated in AlGaN-GaN HEMTs by means of pulsed, transient, and small-signal measurements. Gate- and drain-lag effects characterized by time constants in the order of 10^-5-10^-4 s cause dispersion between dc and pulsed output characteristics when the gate or the drain voltage are pulsed. An activation energy of 0.3 eV is extracted from temperature-dependent gate-lag measurements. We show that two-dimensional numerical device simulations accounting only for polarization charges and donor-like traps at the ungated AlGaN surface can quantitatively reproduce all dispersion effects observed experimentally in the different pulsing modes, provided that the measured activation energy is adopted as the energetic distance of surface traps from the valence-band edge. Within this hypothesis, simulations show that surface traps behave as hole traps during transients, interacting with holes attracted at the AlGaN surface by the negative polarization charge.