Zener Diode Characteristics Explained

A Zener diode is a special type of semiconductor diode designed to operate reliably in the reverse breakdown region. Unlike a normal diode, which would be destroyed by reverse breakdown, a Zener diode is engineered to handle the current that flows when it reaches its reverse breakdown voltage, known as the Zener voltage (\(V_Z\)).

Voltage Regulation by Zener Diode

The most important characteristic of a Zener diode, especially for applications like voltage regulation, is its ability to maintain a nearly constant voltage across its terminals once it enters the reverse breakdown region. This phenomenon is critical to understanding the question.

• When a reverse voltage applied across the Zener diode reaches the Zener voltage (\(V_Z\)), the diode starts conducting significant current in the reverse direction.
• Beyond this point, even if the reverse current through the diode increases significantly, the voltage across its terminals changes only very slightly. This is because the diode exhibits a very low dynamic resistance (\(R_Z\)) in this region.
• The relationship between a small change in voltage (\(\Delta V_Z\)) and a small change in current (\(\Delta I_Z\)) in the Zener region is given by the dynamic resistance:

$$R_Z = \frac{\Delta V_Z}{\Delta I_Z}$$

Because \(R_Z\) is very small (ideally close to zero, but practically a few ohms), even a large change in current (\(\Delta I_Z\)) will result in only a very small change in voltage (\(\Delta V_Z\)).

Current and Voltage Relationship in Zener Diode

Let's consider the scenario given in the question: when the current through a Zener diode increases by a factor of 2 (i.e., it doubles). Based on the operating principle of the Zener diode in its breakdown region:

• Once the Zener diode is operating in its voltage regulation region, a substantial increase in current (like doubling it) will cause only a negligible change in the voltage across its terminals.
• This is the fundamental property that makes Zener diodes useful as voltage regulators, providing a stable reference voltage regardless of reasonable fluctuations in load current or input voltage.

Therefore, when the current through a Zener diode increases by a factor of 2, the voltage across its terminals is practically unchanged.