Electrical and luminescent properties of normal p-side-up and inverted n-side-up light emitting diodes (p-LEDs and n-LEDs) and the effect of Mn incorporation into the upper n-type contact layer structure of GaN-based multiquantum-well MQW LEDs were studied. The latter structure is considered as promising for spintronic applications (spin-LED). It is shown that n-LEDs have more pronounced tunneling compared to p-LEDs and that the active region of the n-LEDs shows domain-like nonuniformities that are absent in p-LEDs. These nonuniformities seem to be related to local segregation of In in the GaN/InGaN MQWs and result in the emergency of double peaked electroluminescence (EL) spectra in n-LEDs. Introduction of Mn leads to strong increase of the resistivity of the GaMnN layer which gives rise to increased threshold voltage for observing the EL and decreased EL intensity. (C) 2003 Elsevier Science Ltd. All rights reserved.