In one line
An LED is a p-n junction chip with a thin top layer for light to escape, mounted in a reflector cup inside a transparent epoxy lens with two leads; a series resistor is needed because its current rises very steeply once forward voltage is reached.
Explained simply
Break open a common 5 mm LED and you will see a tiny chip sitting in a small metal cup on one lead (the cathode, usually with the bigger internal post), connected by a hair-thin wire to the other lead (the anode). The plastic dome acts as a lens that focuses the light upward.
Technically, the n-type layer is grown on a substrate, a thin p-type layer is placed on top so that photons generated near the junction escape easily, and metal contacts are kept small (or ring-shaped) so they do not block light. A reflector directs side-emitted light forward.
How it works
- Chip: semiconductor layers (e.g. GaAsP, InGaN) forming the p-n junction.
- Thin top layer: lets the light produced near the junction come out with little absorption.
- Reflector cup: on the cathode lead; reflects light upward.
- Bond wire: fine gold wire from chip to anode lead.
- Epoxy lens: protects the chip and shapes the beam (clear or tinted, round or flat).
- Identification: anode lead is longer; cathode side has a flat edge on the rim.
- Symbol: diode symbol with two arrows pointing outward (light leaving).
Like any diode, an LED's current rises sharply once forward voltage is reached, so a small rise in supply voltage could destroy it. A series resistor (or a constant-current driver for power LEDs) sets a safe current, typically 10–20 mA for small indicator LEDs.
Key formula
\( R = \dfrac{V_S - V_F}{I_F} \)
\( R \) = series resistor (\( \Omega \)), \( V_S \) = supply voltage (V), \( V_F \) = LED forward voltage (V), \( I_F \) = desired forward current (A).
Quick example
A red LED (\( V_F \approx 2 \) V) on a 5 V supply at 15 mA needs \( R = (5-2)/0.015 = 200\,\Omega \); a standard 220 \( \Omega \) is used. Connecting the LED directly to 5 V would push the current far above its rating and burn it out.
Common mistakes
- Drawing the LED symbol with arrows pointing inward (that is a photodiode).
- Identifying the shorter lead as the anode.
- Connecting an LED straight across a battery with no resistor.
How MU asks it
Numericals are not expected in this module (MU syllabus), so questions are theory with diagrams. Typical: "Explain the basic structure of an LED with a neat diagram" (5 marks), sometimes "State advantages of LEDs". For full marks draw a labelled cross-section (chip, p and n layers, bond wire, reflector cup, epoxy lens, anode and cathode leads), draw the symbol, and mention the need for current limiting.