  UV sensing in pure ZnO is due to oxygen adsorption/desorption process from
ZnO surface. Vanadium doping improves UV sensitivity of ZnO. Enhancement in UV
sensitivity in doped ZnO is attributed to trapping and de-trapping of electrons
at V4+ & V5+-related defect states. An extra electron in the V4+ state is
excited under UV illumination while in absence of the same a trapping happens
at the V5+ state. An insight to the mechanism is obtained by an analytic study
of the response phenomenon.
