An ESD protection diode is a type of Zener diode.
When a diode is reverse-biased, little current flows from cathode to anode. However, when the reverse bias exceeds a certain point (called the reverse breakdown voltage), the reverse current suddenly increases. As the reverse bias is increased, the diode reaches a region of a constant voltage regardless of the current flowing through the diode. The breakdown voltage (Zener voltage) characteristics of Zener diodes are utilized to compose a constant-voltage regulator and suppress surge voltage. Zener voltage regulator diodes are used to maintain a constant voltage whereas ESD protection diodes are used to absorb ESD energy in order to protect an electronic circuit.