The results of microcathodoluminescence (MCL) and deep levels transient spectroscopy (DLTS) studies of AlGaN/ GaN modulation doped field effect transistor (MODFET) structures and modulation doped and undoped AlGaN/GaN superlattices (SLs) grown by metalorganic chemical vapor deposition are presented. A strong blue shift in the energy position of the band-edge MCL peak in modulation doped SLs compared to undoped SLs is observed and attributed to screening of the built-in piezoelectric field in AlGaN/GaN SLs by the high density of two-dimensional electron gas in the GaN quantum wells. The dominant traps in AlGaN/GaN MODFETs and doped SLs with Al composition in the barrier close to 28% were found to be the electron traps with activation energy 0.9 eV located near the AlGaN/GaN interface. In undoped SLs similar traps were observed and their energy position was found to strongly depend on the Al composition in the barrier and on the strength of the electric field in space charge region of the Schottky diode used for DLTS measurements. (C) 2002 Elsevier Science Ltd. All rights reserved.