Q.What is an LED? Give the principle of operation with a diagram.

Step 1 (Definition). An LED (light emitting diode) is a p-n junction diode that emits visible or invisible light when it is FORWARD biased; since electrical energy converts directly into light energy, this process is called electroluminescence.

Step 2 (Principle of operation). Forward bias drives conduction-band electrons from the n-side and valence-band holes from the p-side to diffuse across the junction; once across, they become excess minority carriers and recombine with the majority carriers already present in that region.

Step 3. Recombination releases energy either as heat (non-radiative recombination) or, in materials chosen specifically for this purpose, as a photon of energy $h\nu\approx E_g$ (radiative recombination) -- it is this radiative pathway an LED is engineered to maximise.

Step 4 (Diagram/structure). A commercial LED has a p-layer and n-layer on a substrate, enclosed in a transparent plastic case/window that lets the emitted light escape in the desired direction, with anode and cathode leads; its circuit symbol is the ordinary diode arrowhead-and-bar with two small arrows pointing away from it (light emitted outward). …

Define LED, explain forward-bias recombination (electroluminescence), …

• Describing LED emission as simply 'current flowing through', without mentioning the recombination mechanism. …