A phase change memory may include self-aligned polysilicon vertical bipolar junction transistors used as select devices. The bipolar junction transistors may be formed with double shallow trench isolation. For example, the emitters of each bipolar transistor may be defined by a first set of parallel trenches in one direction and a second set of parallel trenches in the opposite direction. In some embodiments, the formation of parasitic PNP transistors between adjacent emitters may be avoided.