Light-Emitting Diodes (LED)

An LED is fundamentally a forward-biased pn junction made from a direct-bandgap semiconductor. Applying a forward voltage lowers the potential barrier of the depletion region, and electrons from the n-side and holes from the p-side are injected across it – this is known as minority carrier injection. In the active region, both carrier types meet and recombine radiatively, as described in the fundamentals chapter; the photon energy roughly corresponds to the material's bandgap, slightly reduced by the thermal energy distribution of the carriers (which is why the emission spectrum isn't a sharp peak but has a spectral width of typically 20–30 nm at room temperature). Unlike the Si pn junction, which is primarily used as a diode or solar cell, light emission in an LED is the actual purpose, not a side effect.

An LED's forward voltage correlates directly with the bandgap: a rule of thumb is V_f ≈ E_g/e, slightly increased by series resistance and contact losses. A red AlGaInP LED (E_g ≈ 2 eV) therefore has a typical forward voltage around 1.8–2.2 V, while a blue InGaN LED (E_g ≈ 2.7 eV) sits at 2.8–3.4 V – markedly higher than a conventional Si diode (≈0.7 V), which directly affects the operating voltage required by LED circuits. Efficiency depends critically on how many carriers actually recombine radiatively rather than non-radiatively – governed by doping quality, defect density, and interface states in the crystal, which act as recombination centers and "swallow" carriers without emitting a photon.

Emission color follows directly from the material's bandgap, which is why each LED color has its own established system. Red and yellow LEDs are based on AlGaInP heterostructures on GaAs substrate, lattice-matched by choice of the aluminum-gallium ratio. Infrared and simple red LEDs have historically also used GaAsP or AlGaAs. Blue and green LEDs use InGaN on sapphire or SiC substrate – despite substantial lattice mismatch to the substrate (up to 16% for GaN-on-sapphire), which produces high dislocation densities that long limited material quality.

The technological maturity of InGaN in the 1990s (Shuji Nakamura, Nobel Prize in Physics 2014) was the decisive prerequisite for efficient white LEDs, since previously only red and yellow were available with sufficient efficiency – the so-called "green gap" (the efficiency shortfall of green LEDs between the well-mastered InGaN/blue and AlGaInP/yellow systems) remains an active research topic today. White light today is mostly produced not by additive RGB mixing but by a single blue InGaN LED (peak ≈ 450–460 nm), part of whose light is converted by a yellow phosphor (usually Ce:YAG, cerium-doped yttrium aluminum garnet) via photoluminescence into longer-wavelength light (peak ≈ 560 nm). The combination of transmitted blue and excited yellow produces a white color impression for the human eye; phosphor blend and thickness can be tuned to set color temperature (warm white to cool white) and color rendering index (CRI).