Pure ultraviolet (UV) light emitting diodes (LEDs) using n-ZnO nanowires as an active layer were fabricated with an insulating MgO dielectric layer as a carrier control layer, where all depositions were continuously performed by metalorganic chemical vapor deposition. The current-voltage curve of the LEDs showed obvious rectifying characteristics, with a threshold voltage of about 7 V in the sample with 4 nm i-MgO. Under the forward bias of the samples with proper MgO thickness, a sharp UV electroluminescence, located at around 380 nm, was emitted from the active ZnO nanowires, while weak visible emission of around 450-700 nm were observed. The pure UV emission from the ZnO nanowires in the n-ZnO/i-MgO/p(+)-Si heterostructures was attributed to the electron accumulation in the ZnO by asymmetric band offset and preemptive hole tunneling from Si to ZnO by i-MgO. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.029202jes] All rights reserved.