The BZM55C20 from Micro Commercial Components is a Zener Diode with Zener Voltage 18.8 to 21.2 V, Zener Current 1.0 mA, Voltage Forward 1.5 V, Current 5.0 mA, Current Reverse Reverse Leakage Current:- 0.1 - 2.0 uA. More details for BZM55C20 can be seen below.