LED Behavior
λ = 1240/E_g — 5 materials from GaAs (IR) to InGaN (violet) with photon emission.
Key Notes
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.
Formulas
Emitted wavelength
Determines colour. λ = 1240/E_g (nm).
Forward voltage estimate
Approximate; real diodes have additional ohmic drop.
Current-limiting resistor
Designed for the LED's desired forward current (typically 10-20 mA for indicators).
Internal quantum efficiency
High for direct-bandgap; near zero for indirect.
Important Points
Colour is set by MATERIAL (E_g), not by current or temperature (much).
BLUE LEDs were the hard ones — invented in 1993 (Akasaki, Amano, Nakamura, Nobel 2014). Combined with phosphors made white LEDs feasible.
Always use a series resistor (or driver) with a discrete LED; otherwise it self-destructs at slightly elevated forward voltage.
LEDs degrade gradually (lumen output drops with use) — different from sudden incandescent burnout.
OLEDs use organic semiconductors — flexible, but shorter lifetime.
LED efficiency scales as ~λ/V_F — longer wavelength = higher efficiency. White light via blue + yellow phosphor has typical efficiency 40-50%.
LED Behavior notes from sciphylab (also known as SciPhy, SciPhy Lab, SciPhy Labs, Physics Lab). Class 12 physics revision for JEE Mains, JEE Advanced, NEET UG, AP Physics 1/2/C, SAT, and CUET-UG.