phase change devices , and particularly multi - terminal phase change devices , include first and second active terminals bridged together by a phase - change material whose conductivity can be modified in accordance with a control signal applied to a control electrode . this structure allows an application in which an electrical connection can be created between the two active terminals , with the control of the connection being effected using a separate terminal or terminals . accordingly , the resistance of the heater element can be increased independently from the resistance of the path between the two active terminals . this allows the use of smaller heater elements thus requiring less current to create the same amount of joule heating per unit area . the resistance of the heating element does not impact the total resistance of the phase change device . the programming control can be placed outside of the main signal path through the phase change device , reducing the impact of the associated capacitance and resistance of the device .