Question
What are the different types of diodes, how does each work, and where is each used?
Solution — Step by Step
When p-type and n-type semiconductors join, a depletion region forms at the junction (carriers recombine, leaving behind immobile ions).
- Forward bias (p connected to +, n to -): Depletion region narrows, current flows freely
- Reverse bias (p connected to -, n to +): Depletion region widens, only tiny leakage current flows
This one-way conduction is the basis of all diode applications.
| Diode Type | Key Principle | Use |
|---|---|---|
| Zener diode | Controlled breakdown in reverse bias at specific voltage | Voltage regulation |
| LED | Forward bias causes electron-hole recombination, emitting photons | Lighting, displays |
| Photodiode | Reverse-biased; photons create electron-hole pairs, generating current | Light sensors, cameras |
| Solar cell | Unbiased p-n junction; sunlight generates voltage across the junction | Power generation |
graph TD
A[Diode Types] --> B[P-N Junction - basic rectification]
A --> C[Zener - voltage regulation]
A --> D[LED - light emission]
A --> E[Photodiode - light detection]
A --> F[Solar Cell - power from light]
B --> B1[Used in: rectifiers, clipper, clamper circuits]
C --> C1[Operates in reverse breakdown - constant voltage across it]
D --> D1[Band gap determines colour: larger gap = shorter wavelength]
E --> E1[Reverse biased - photocurrent proportional to light intensity]
F --> F1[No external bias - open circuit voltage = 0.5-0.6V per cell]LEDs emit photons when electrons recombine with holes. The photon energy equals the band gap:
Larger band gap = higher energy photon = shorter wavelength (bluer colour).
| Material | Band Gap | Colour |
|---|---|---|
| GaAs (Gallium Arsenide) | 1.4 eV | Infrared |
| GaP (Gallium Phosphide) | 2.26 eV | Green/Yellow |
| GaN (Gallium Nitride) | 3.4 eV | Blue/Violet |
| InGaN | 2.4-3.4 eV | Blue to Green |
Why This Works
All these diodes are variations of the same p-n junction physics. The depletion region acts as a barrier that can be modulated. Forward bias lowers the barrier (current flows); reverse bias raises it (current blocked). Each specialised diode exploits a specific property: Zener uses controlled avalanche breakdown, LED uses radiative recombination, and photodiodes use photon-generated carriers.
CBSE 12 boards frequently ask: “Differentiate between photodiode and solar cell.” Key differences: photodiode is reverse biased (used as a detector), while solar cell is unbiased (used as a power source). Both convert light to electricity, but their biasing conditions and applications differ.
Alternative Method
To remember which way each diode is biased, use this rule: if the diode PRODUCES electrical output from light (photodiode, solar cell), it works in reverse bias or no bias. If the diode CONSUMES electrical energy to produce light (LED), it works in forward bias.
Common Mistake
Students confuse the biasing of photodiodes and LEDs. An LED operates in forward bias (current flows, light is emitted). A photodiode operates in reverse bias (light falls on it, generating a photocurrent). If you forward-bias a photodiode, it just behaves like a regular diode — no useful light detection happens. Getting the bias direction wrong is a very common error.