The breakdown is either due to the Zener breakdown effect that occurs below 5.5 V, or impact ionization that occurs above 5.5 V. Both mechanisms result in the same behavior and do not require different circuitry; however, each mechanism has a different temperature coefficient.
The Zener effect has a negative temperature coefficient while the impact effect experiences a positive coefficient. The two temperature effects are almost equal at 5.5 V and cancel out each other to make the Zener diodes rated at around 5.5 V the most stable over a wide range of temperature conditions.
Zener diode specifications
Zener diodes vary in specifications such as nominal working voltage, power dissipation, maximum reverse current, and packaging. Some commonly used specifications include:
- Voltage Vz: The Zener voltage refers to the reverse breakdown voltage—2.4 V to about 200 V; can go up to 1 kV while the maximum for the surface-mounted device (SMD) is about 47 V).
- Current Iz (max.): Maximum current at the rated Zener voltage Vz—200 uA to 200 A).
- Current Iz (min.): Minimum current required for the diode to break down—5 mA and 10 mA.
- Power rating: The maximum power the Zener diode can dissipate; given by the product of voltage across the diode and the current flowing through. Typical values are 400 mW, 500 mW, 1 W, and 5 W; for surface mounted, 200 mW, 350 mW, 500 mW, and 1 W are typical.
- Voltage tolerance: Typically ±5%.
- Temperature stability: Diodes around 5 V have the best stability.
- Package: Leaded devices and the surface mount either as discrete devices or within integrated circuits.
- Zener resistance (Rz): The diode exhibits some resistance as evident from the IV characteristics.