Zener diode (A-level only): Anode and cathode.

The idea

A Zener diode is a p–n junction with a heavily doped depletion region, so it can conduct safely in reverse breakdown at a well-defined Zener voltage. The anode is the p-type terminal and the cathode is the n-type terminal, marked by a band on the package. In normal forward bias the anode is positive relative to the cathode and the diode conducts at about 0.7 V for silicon. In reverse bias the cathode is positive relative to the anode; beyond the Zener voltage, breakdown occurs and the voltage across the diode stays nearly constant while current changes. For example, a 5.1 V Zener with its cathode to the positive rail and anode to 0 V maintains about 5.1 V across it when reverse current flows. Identifying anode and cathode correctly is essential for predicting conduction and for wiring the diode in a circuit.

What earns the marks

• The anode is the p-type terminal and the cathode is the n-type terminal of the Zener diode.
• The cathode is marked by a band on the diode body; conventional current flows from anode to cathode when forward biased.
• Forward bias means anode positive relative to cathode, giving conduction at about 0.7 V for a silicon Zener.
• Reverse bias means cathode positive relative to anode; the diode blocks until the reverse voltage reaches the Zener voltage.
• In reverse breakdown the voltage across the Zener remains approximately constant while the current through it changes.
• Correct identification of anode and cathode is needed to connect the Zener so that it operates in reverse breakdown for voltage regulation.

Common mistakes and how to put them right

• Assuming the banded end is the anode.

  Put it right: The band marks the cathode, so the banded end connects to the more positive rail in reverse-bias operation.

• Thinking a Zener conducts in reverse bias at any voltage.

  Put it right: It blocks below the Zener voltage and only breaks down at that value.

• Confusing forward and reverse bias.

  Put it right: Forward bias has anode positive relative to cathode, while reverse bias has cathode positive relative to anode.

• Treating the Zener voltage as a forward voltage.

  Put it right: The Zener voltage is a reverse-breakdown value, not the forward drop.

Exam tips

• Sketch the diode symbol and label anode and cathode before answering circuit questions; this reduces polarity errors.
• When explaining Zener action, state the bias condition and the approximate constant voltage across the diode in breakdown.
• Use the band marking to justify connections in circuit diagrams, and check that the cathode faces the positive supply rail for regulation.
• If asked to compare forward and reverse behaviour, give the approximate forward voltage and the named Zener voltage separately.