Effects of hydrogen and nitrogen electron cyclotron resonance (ECR) plasma surface treatments on the current transport of two-dimensional electron gas (2DEG) at AlGaN/GaN interface were investigated by using a gateless heterostructure field-effect transistors (HFETs) test device. Current-voltage (I-V) characteristics of the gateless device exhibited linear and saturation current behavior similar to that of a gated device, indicating presence of strong Fermi level pinning. As compared with the air-exposed surface. the current reduced significantly with appearance of hysteresis after H-2-plasma treatment, whereas it slightly increased after N-2-plasma treatment. After experiencing a positive pulse voltage, current in the H-2-plasma-treated device dropped and then showed a fast exponential recovery followed by highly non-exponential slow recovery. Temperature dependence of initial recovery gave an activation energy of 0.37 eV. Observed current transients could be reproduced by numerical simulation assuming presence of dominant discrete near-surface deep donors and a U-shaped surface state continuum. X-ray photoelectron spectroscopy (XPS) analysis indicated near-surface reduction of N atoms after the H-2-plasma treatment, indicating that the discrete states are related to N-vacancies. Since generation of N-vacancies is likely to occur in growth and processing of GaN-related materials, the present observation explains surface-related instabilities such as current collapse in AlGaN/GaN HFETs. For oxide removal, the N-2-plasma treatment is much better than H-2-plasma treatment, effectively removing oxides without producing defects. (C) 2003 Elsevier Science B.V. All rights reserved.