A Transient voltage suppression diode is a diode which is used to protect a circuit against spikes and transient overvoltages.
Spikes can be caused due to switching, lightning strikes and faults in the circuit. A spike lasts for a small period of time of the order of microseconds. However, the energy dissipated in a spike is such that it can damage components in a circuit.
Hence,it is necessary to protect spikes from reaching the components by shunting through a transient voltage suppression diode. A transient voltage suppression diode is connected in the reverse bias. With the anode to the negative and the cathode to the positive.
During normal supply voltage, the transient voltage suppression diode is in the non-conducting state.
When the voltage rises and exceeds the breakdown voltage of the diode, it starts conducting and the surge is diverted before reaching the component. TVS Diodes can be connected with the anodes facing each other to provide bidirectional protection
When the surge subsides and the voltage returns to normal, the Transient voltage suppression diode comes back to its non-conducting state.
The Transient Voltage Suppression diode has a VI characteristic which is similar to the zener diode. However, the zener is designed to regulate the voltage while the Transient voltage suppression diode is used to protect against overvoltage.
The RoHS is the Restriction on the use of Hazardous Substances in Electrical and Electronic Equipment. 
