Efficient, high-frequency quantum light sources are a prerequisite for advanced quantum information processing. Here, we report the observation of a Purcell enhancement in the radiative decay rate of a single quantum dot, embedded in a microcavity light-emitting diode structure. An annulus of low-refractive-index aluminium oxide, formed by wet oxidation, is used to simultaneously achieve lateral confinement of both the optical mode and the current through the device. This technique reduces the active area of the device without impeding the electrical properties of the p-i-n diode. We measure a photon collection efficiency of 14 ± 1% and demonstrate single photon electroluminescence at repetition rates up to 0.5 GHz.