Explain working of a LED.

LED (Light Emitting Diode): a PN junction diode that emits light when forward biased (electroluminescence). Made of direct band-gap compound semiconductors such as GaAs (infrared), GaAsP (red/yellow), GaP (red/green), GaN/InGaN (blue/white).

       light out (dome lens)
          \  |  /
     +-----------------+
     |   thin p-layer   |  <- metal ring contact (anode)
     |------------------|  <- junction (recombination region)
     |     n-layer      |
     |   n+ substrate   |
     +-----------------+
       metal contact (cathode)
 Symbol:  A o---|>|---o K  with two outward arrows

Construction: an n-type layer is grown on a substrate and a thin p-type layer is diffused on top so that the light produced at the junction escapes through the thin p-region. A ring/edge metal contact on top (anode) leaves the light-emitting area open; the cathode contact is at the bottom. The chip is placed in a reflector cup and covered by a transparent epoxy dome that acts as a lens.

Working: when forward biased (about 1.5 to 3.5 V depending on material), electrons from the n-side and holes from the p-side are injected across the junction. When a free electron in the conduction band recombines with a hole in the valence band, it falls to a lower energy level and the energy difference (band gap E_g) is released as a photon of light. The wavelength is \( \lambda = \dfrac{hc}{E_g} \approx \dfrac{1.24}{E_g(\text{eV})}\,\mu m \), so the colour depends on the semiconductor material. Brightness is proportional to forward current, so a series current-limiting resistor is used. In reverse bias no light is emitted (low reverse breakdown, about 3 to 5 V).