LED Behavior

Light-Emitting Diode (LED) = a forward-biased p-n junction (made of a direct-bandgap semiconductor) that emits light when electrons recombine with holes in the depletion region.

Emitted photon energy ≈ E_g of the semiconductor ⇒ λ = hc/E_g.

Material chooses colour: GaAs (IR, λ ~ 870 nm), GaAsP (red), GaP (green), GaN/InGaN (blue-UV), GaN+phosphor (white).

Direct-bandgap materials (GaAs, GaN) make efficient LEDs; indirect-bandgap (Si) emit very little light.

Forward voltage V_F depends on colour: red ~1.8 V, green ~2.2 V, blue ~3.0 V, UV ~3.5+ V.

LED current must be limited externally (resistor or constant-current driver) — they have negative dynamic resistance.

Efficiency: modern white LEDs reach 100-200 lm/W (vs incandescent ~15 lm/W).

Applications: indicator lights, displays, traffic signals, solid-state lighting, fibre-optic communications, OLED displays.