07 Mar 2026
Basic Electronics: Zener Diode and LED
Zener diode, reverse breakdown, voltage regulation, LED working principle, materials, and applications.
Special purpose diodes are p-n junction devices designed for a particular function. Two important examples in the open elective syllabus are the Zener diode and the light emitting diode (LED).
A Zener diode is used mainly for voltage regulation. An LED is used for converting electrical energy into light.
Zener diode
A Zener diode is a heavily doped p-n junction diode designed to operate safely in reverse breakdown. In ordinary diodes, reverse breakdown may damage the junction. In a Zener diode, the breakdown region is controlled and useful.
The symbol of a Zener diode is similar to a diode symbol, but the cathode line is bent. It is connected in reverse bias when used as a voltage regulator.
Zener breakdown
When reverse voltage reaches a certain value, called the Zener voltage $V_Z$, the reverse current increases sharply. The voltage across the diode remains nearly constant over a range of current.
This nearly constant voltage property makes the Zener diode useful as a voltage regulator.
Zener diode characteristic
The V-I characteristic has:
- normal diode behavior in forward bias;
- very small reverse current before breakdown;
- sharp increase of reverse current at Zener voltage;
- nearly constant voltage in breakdown region.
| Region | Bias condition | Behavior |
|---|---|---|
| Forward region | Forward bias | Conducts like an ordinary diode |
| Reverse leakage region | Reverse bias below $V_Z$ | Very small current |
| Breakdown region | Reverse bias at $V_Z$ | Voltage remains nearly constant |
Zener diode as voltage regulator
In a regulator circuit, the Zener diode is connected in reverse bias parallel to the load. A series resistor is used to limit current.
If the supply voltage changes, the current through the Zener diode changes, but the voltage across the load remains nearly constant at $V_Z$. Thus the load receives a stable voltage.
For an exam answer, draw the circuit and explain:
- Zener diode is reverse biased;
- load is connected parallel to the Zener diode;
- series resistor limits current;
- output voltage is approximately $V_Z$;
- excess current is taken by the Zener diode.
LED
An LED is a p-n junction diode that emits light when forward biased. The full form is light emitting diode.
When electrons from the n-side recombine with holes on the p-side, energy is released. In suitable semiconductor materials, this energy appears as light.
Working principle of LED
The working of an LED is based on electroluminescence. In forward bias:
- electrons and holes are injected across the junction;
- recombination takes place near the junction;
- energy is emitted in the form of photons;
- the color of light depends on the band gap of the semiconductor.
LEDs are commonly made from compound semiconductors such as GaAs, GaP, and GaAsP.
Applications
Zener diode applications:
- voltage regulation;
- voltage reference circuits;
- over-voltage protection;
- clipping circuits.
LED applications:
- indicator lamps;
- display panels;
- optical communication;
- remote controls;
- traffic signals and lighting.
University questions
The 2019 open elective paper asked: βDraw characteristics curve of a Zener diode. Hence, explain its use as a voltage regulator.β This is a standard long-answer question.
Practice questions from this topic:
- Draw and explain the V-I characteristic of a Zener diode.
- Explain Zener diode as a voltage regulator.
- What is reverse breakdown?
- Explain the working principle of LED.
- Mention important applications of Zener diode and LED.
Answer points
- For Zener diode, emphasize reverse breakdown and constant voltage.
- For regulation, mention reverse bias, series resistor, and load voltage.
- For LED, emphasize forward bias and electron-hole recombination.
- Avoid writing LED as an ordinary heating effect; the light is due to radiative recombination.
Discussion