Structural and optical properties of AlGaN-based ultraviolet (UV) light-emitting diodes (LEDs) with a thin undoped GaN layer and thick n-type AlGaN/GaN superlattices have been investigated. Some V-defects were observed in the n-type superlattice layers. The V-defect density decreases enormously with increasing the undoped GaN thickness. Simultaneously, the output power of the UV-LEDs increases remarkably as the undoped GaN thickness increases. Since the threading dislocation density is the same for all the LEDs, the improved LED performance could be attributed to the largely reduced V-defect density. (C) 2004 Elsevier B.V. All rights reserved.