LED

LED

1. What an LED Is

LED (Light Emitting Diode) = forward-biased p-n junction that emits light via radiative recombination of injected electrons (conduction) with holes (valence) :

Opposite of photodiode (light → current) and solar cell.

2. Condition for Light — Direct Band Gap

• Direct gap semiconductors (GaAs, GaN, InP) → conduction band minimum aligns in $k$ with valence maximum ⇒ electrons recombine with photon only, efficient.
• Indirect gap (Si, Ge) → needs phonon momentum ⇒ non-radiative, inefficient ⇒ not used for LEDs (Si is poor emitter).

So LED materials: GaAs ($E_g=1.42$ eV ⇒ IR 870 nm), GaAsP (red), GaP (green), GaN ($3.4$ eV ⇒ blue 365 nm, white via phosphor), InGaN for all colours.

3. Working and Characteristics

p  | active region | n (heterojunction confinement)
Forward bias ~2–4 V injects carriers into active region, they recombine ⇒ photons escape through transparent top contact.
Intensity ∝ forward current I_f (more carriers).

• Wavelength:

e.g., $1.42$ eV ⇒ $0.87$ µm.

• I-V: diode forward exponential, light output ∝ $I$ above threshold.

4. Efficiency and Structures

• Internal quantum efficiency $η_{int}$ = photons per injected electron (~high for direct gap).
• Extraction efficiency limited by TIR (high $n$ ~3.5 ⇒ only ~4% escapes without texturing/lens).
• Heterostructure and quantum wells confine carriers to boost recombination.

5. Applications

Indicators, displays, lighting (white LED: blue GaN + yellow phosphor YAG), fibre transmitters (850 nm), backlights, automotive, signalling.

6. Exam Points

• Explain $e-h$ recombination $hν≈E_g$ and why direct gap needed — 5 marks, very frequent.
• Give $λ=1.24/E_g$ and examples GaAs IR, GaN blue — 5 marks, frequent.
• Draw forward-biased band diagram with injection — 5 marks, frequent.
• Contrast LED vs photodiode vs solar (emission vs detection vs power) — 3 marks.