Persisent photoconductivity (PPC) has been investigated in a Zn0.02Cd0.98Te semiconductor alloy. A transition from a photoconductivity phase to a PPC phase has been observed and a coexistence curve that separates these two phases has been obtained. We find that such a transition can be controlled by the bias voltage and excitation photon dose. The relaxation of PPC is found to follow a power law, IPPC(t)t-α. The decay parameter α is obtained as a function of the bias voltage, which shows that the carrier decay rate decreases almost linearly with increasing bias voltage in the PPC phase. A possible mechanism is the presence of random local-potential fluctuations in the sample, which strongly influence the carrier transport properties. The fluctuations could be induced either by alloy disorder or impurity compensation.