A semiconductor structure includes a transistor over a substrate, the transistor comprising a gate and a contact region, which is adjacent to the gate and within the substrate. A first dielectric layer is over the contact region. A contact structure is within the first dielectric layer and over the contact region. A first electrode and a second electrode are within the first dielectric layer, wherein at least one of the first electrode and the second electrode is over the contact structure. The first electrode and second electrodes may be laterally or vertically separated. A phase change structure is disposed between the first electrode and the second electrode. The phase change structure includes at least one spacer within the first dielectric layer and a phase change material (PCM) layer over the spacer.