LEDs

LEDs

1. What an LED Is (in this electronics context — same as earlier physics but for BEEE)

LED (Light Emitting Diode) = forward-biased p-n junction of direct band gap semiconductor (GaAs, GaP, GaN) that emits photons via radiative recombination:

Same as earlier LED detail, but in BEEE context compare with normal Si diode (indirect gap, non-emissive).

2. Why Direct Gap Matters for BEEE

• Si, Ge (indirect gap): momentum mismatch ⇒ phonon needed ⇒ non-radiative (heat) ⇒ no light — why Si diode doesn't glow.
• GaAs, GaP, GaN (direct gap): conduction band minimum aligns in $k$ with valence maximum ⇒ photon alone conserves momentum ⇒ efficient light — used for LEDs.

So LED materials are direct gap III-V (e.g., GaAs $1.42$ eV IR, GaP $2.26$ eV green, GaN $3.4$ eV blue).

3. Working (forward bias ~2–3V)

p (GaP) | junction | n (GaP) with forward bias V_f≈2V (red) or 3.5V (blue GaN)
Electrons n→p, holes p→n injected into depletion/active region → recombine → photons → escape through transparent top contact/lens
Intensity ∝ I_f (forward current, e.g., 20mA gives bright).

Wavelength:

e.g., $E_g=2.2$ eV ⇒ $λ≈0.56$ µm (green). Heterostructure and quantum wells boost efficiency.

4. Characteristics in BEEE Lab

• I-V: forward knee ~1.5–3.5V (higher than Si 0.7V due to larger $E_g$), reverse breakdown ~5V (low).
• Light vs $I$: linear above threshold.
• Colours: IR (GaAs), Red (GaAsP), Yellow (AlInGaP), Green (GaP), Blue/White (InGaN + phosphor).

5. Uses (BEEE list)

Indicators, 7-seg displays (next topic), optocouplers, fibre transmitters (850 nm), lighting (white LED: blue + yellow phosphor), displays.

6. Exam Points

• Explain $e-h$ recombination $hν≈E_g$ and why direct gap (GaAs) emits, Si does not — 5 marks, very frequent (the most common LED vs Si diode distinction).
• Give $λ=1.24/E_g$ and examples (GaAs IR, GaN blue) — 5 marks, frequent.
• Show forward bias $V_f≈2$–$3V$ vs Si 0.7V — 3 marks, frequent.
• Draw forward I-V with higher knee — 3 marks.