A theory to explain the switching behaviour and variation of the capacitance with temperature and voltage, is proposed for vacuum evaporated metal-glass-metal devices. It is also shown that the observed capacitance of the device could be negative under voltage bias conditions. It is proposed that the voltage Vth at which the device changes from the off state to the on state and the transition voltage Vcn at which the capacitance of the device becomes negative should be related to the glass transition temperature Tg. A study of Al-Ge15Te81Sb2-Al devices reveals that the experimental results are in concurrence with the theoretical predictions.