This paper reports the observation of defect-related anomalous low temperature drain current-voltage characteristics in AlGaN/GaN modulation-doped field effect transistor (MODFETs). The study compares devices that have a relatively large number of lattice defects, generally referred to as nanopipes or nanotubes, in the vicinity of their active area to defect free devices. At low temperatures (T <210 K), the measured I-ds-V-ds characteristics of the devices with defects exhibit anomalous "kinks". Such anomalous behaviour is not found in defect free devices even at low temperatures. We attribute the observed behaviour to trapping and thermally activated detrapping of charge injected into the AlGaN layer from the 2DEG via real space transfer (RST). The existence of these mechanisms indicates that device self-heating induced channel mobility degradation is not necessarily the sole mechanism for the negative differential resistance at high electric fields in AlGaN/GaN MODFETs. (C) 2000 Elsevier Science Ltd. All rights reserved.