THe memristor is a key memory element for neuromorphic electronics and new generation flash memories. One of the most promising materials for memristor technology is silicon oxide SiOx, which is compatible with silicon-based technology. In this paper, the electronic structure and charge transport mechanism in a forming-free SiOx-based memristor fabricated with the plasma enhanced chemical vapor deposition method is investigated. The experimental current-voltage characteristics measured at different temperatures in high-resistance, low-resistance and intermediate states are compared with various charge transport theories. The charge transport in all states is limited by the space charge-limited current model. The trap parameters, responsible for the charge transport in a SiOx-based memristor in different states, are determined.