The voltage and current of the device is a special non-linear relationship. When the applied voltage cross at both ends is lower than the standard rated voltage, the resistance of device is close to infinity and there is almost no current through internal. When the applied voltage cross at both ends is higher than the normal rated voltage, the device will rapidly breakdown, becomes a low resistance situation from a high impedance situation, and the operating current increases rapidly. When the voltage at both ends is lower than standard rated voltage, the device can recover to high impedance. Therefore, the operating mode is an Off-on-to Off.