For a Zener diode working in reverse bias condition, the Zener current is _________ for supply voltage _________ the Reverse Zener breakdown voltage. 

Zener Diode in Reverse Bias Condition

Definition: A Zener diode is a special type of diode designed to reliably allow current to flow "backward" when a specific, predetermined reverse voltage, known as the Zener breakdown voltage, is reached. In the reverse bias condition, the Zener diode remains non-conductive until the supply voltage exceeds the Zener breakdown voltage. Once the breakdown voltage is reached, the diode conducts a significant current while maintaining a nearly constant voltage.

Working Principle:

When a Zener diode is reverse-biased, it initially behaves like a standard diode, with negligible reverse current flowing. However, as the reverse voltage increases and reaches the Zener breakdown voltage, the diode begins to conduct in the reverse direction. The Zener breakdown occurs due to quantum mechanical tunneling of electrons across the narrow depletion region, and the diode enters a stable breakdown region where the voltage across it remains constant, regardless of further increases in current.

Correct Option Analysis:

The statement under consideration is:

"For a Zener diode working in reverse bias condition, the Zener current is _________ for supply voltage _________ the Reverse Zener breakdown voltage."

The correct option is:

Option 2: Close to zero; less than

Explanation:

• In the reverse bias condition, when the supply voltage is less than the Zener breakdown voltage, the Zener diode does not conduct significantly. The current flowing through the diode is the reverse saturation current, which is extremely small (close to zero).
• As the supply voltage is below the breakdown voltage, the diode remains in the reverse blocking region. The depletion region prevents significant current flow, and the Zener diode behaves like an open circuit.
• Thus, the Zener current is "close to zero" when the supply voltage is "less than" the Zener breakdown voltage.

Important Information:

To better understand the context and eliminate confusion, let’s evaluate the other options:

Option 1: Close to zero; greater than

This option is incorrect. When the supply voltage exceeds the Zener breakdown voltage, the diode enters the breakdown region and begins to conduct significant current. The current is not "close to zero" in this scenario.

Option 3: Close to infinity; greater than

This option is incorrect. While the Zener diode conducts significant current in the breakdown region, the current is not "close to infinity." The diode stabilizes the voltage across its terminals, and the current depends on the external circuit (like the series resistor), ensuring it remains finite and within safe operating limits.

Option 4: Close to infinity; less than

This option is also incorrect. For supply voltages less than the Zener breakdown voltage, the Zener diode remains in reverse bias, and the current is close to zero, not "close to infinity."

Conclusion:

The behavior of a Zener diode in reverse bias is a fundamental concept in electronics. When the supply voltage is less than the Zener breakdown voltage, the current through the diode is negligible, and the diode does not conduct significantly. Once the breakdown voltage is reached, the diode maintains a stable voltage across its terminals and conducts a controlled current. This property makes Zener diodes widely used in voltage regulation applications. The correct option, therefore, is Option 2: "Close to zero; less than."