Question
Compare five types of diodes — p-n junction, Zener, LED, photodiode, and solar cell. For each, state the principle, biasing condition in normal use, and one application.
(CBSE 12 + JEE Main pattern)
Solution — Step by Step
| Diode | Principle | Normal Bias | Key Application |
|---|---|---|---|
| p-n junction | Rectification (allows current in one direction) | Forward bias | AC to DC conversion |
| Zener | Controlled reverse breakdown at fixed voltage | Reverse bias | Voltage regulation |
| LED | Electroluminescence (emits light in forward bias) | Forward bias | Displays, lighting |
| Photodiode | Photocurrent generation by incident light | Reverse bias | Light detection, optical communication |
| Solar cell | Photovoltaic effect (light to electricity) | No external bias (zero bias) | Power generation |
p-n junction diode: In forward bias, current flows easily (low resistance). In reverse bias, only a tiny leakage current flows. Used as a rectifier — converts AC to DC.
Zener diode: Designed to operate in reverse breakdown. The voltage across it stays nearly constant at the Zener voltage even as current changes. This makes it a voltage regulator.
LED: When electrons recombine with holes in forward bias, energy is released as photons. The colour depends on the band gap of the semiconductor material (GaAs for IR, GaP for green, GaN for blue).
Photodiode: Operated in reverse bias. Light creates electron-hole pairs in the depletion region, generating a photocurrent proportional to light intensity. Fast response makes it ideal for optical communication.
Solar cell: Essentially a large-area photodiode operated at zero bias. Incident sunlight creates electron-hole pairs, and the built-in electric field of the junction separates them, producing a voltage (~0.5 V per Si cell). No external power needed — it generates power.
flowchart TD
A["Diode Types"] --> B["p-n Junction: rectification"]
A --> C["Zener: voltage regulation"]
A --> D["LED: light emission"]
A --> E["Photodiode: light detection"]
A --> F["Solar Cell: power generation"]
B --> G["Forward bias → conducts"]
C --> H["Reverse bias at VZ → constant voltage"]
D --> I["Forward bias → photon emission"]
E --> J["Reverse bias → photocurrent ∝ light"]
F --> K["Zero bias → generates EMF from light"]Why This Works
All five devices are based on the p-n junction but exploit different properties. The forward-biased junction allows current flow and electron-hole recombination (useful for rectification and light emission). The reverse-biased junction has a strong electric field in the depletion region (useful for breakdown regulation and photocurrent collection).
The LED and photodiode are essentially inverse processes: in an LED, electrical energy is converted to light; in a photodiode, light is converted to electrical current. The band gap determines the photon energy (and thus wavelength) involved.
Alternative Method — Grouping by Energy Conversion
- Electrical to light: LED
- Light to electrical signal: Photodiode
- Light to electrical power: Solar cell
- AC to DC (electrical processing): p-n junction rectifier
- Voltage stabilisation: Zener diode
For JEE Main, know the V-I characteristics of each diode. The Zener has a sharp knee in reverse bias at . The photodiode’s reverse current increases with light intensity (the curve shifts downward in the fourth quadrant). The solar cell operates in the fourth quadrant of the V-I plot — it delivers power rather than consuming it.
Common Mistake
Students confuse the biasing of photodiode and solar cell. A photodiode is operated in REVERSE bias (for fast response and linearity), while a solar cell operates at ZERO bias (or slightly forward biased under load). Using a photodiode in forward bias would work as a slow LED, not as a detector. Also, students sometimes think LEDs work in reverse bias — they emit light only in FORWARD bias when electrons recombine with holes.