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How does a diode work?

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Quick answer

A diode works by letting current pass in only one direction. Inside, p-type and n-type silicon meet at a junction that forms a thin barrier. A small forward voltage, about 0.7 volts for silicon, collapses that barrier so current flows, while a reverse voltage widens it and blocks almost all current.

A diode works as a one-way valve for electric current, letting it flow easily in one direction and blocking it in the other. It does this with a junction between two differently doped layers of semiconductor, usually silicon, which forms a barrier that only a forward voltage can overcome.

Step by step

  1. P-type and n-type silicon are joined, and carriers near the boundary combine to form a depletion region.
  2. The depletion region leaves a built-in electric field that opposes further movement of electrons and holes across the junction.
  3. Applying forward voltage above about 0.7 volts in silicon overcomes this field, and current flows across the junction.
  4. Applying reverse voltage pulls carriers away from the junction, widening the depletion region and stopping nearly all current.
  5. In an AC circuit, the diode therefore conducts only during the half-cycles that forward-bias it, converting AC into pulsing DC.

Key things to understand

  1. A diode joins p-type semiconductor, which has spare holes, to n-type semiconductor, which has spare free electrons.
  2. At the junction, electrons and holes cancel out, leaving a thin depletion region with no free carriers and a built-in electric field.
  3. A forward voltage of roughly 0.6 to 0.7 volts in silicon shrinks this barrier, and current then flows freely.
  4. A reverse voltage widens the depletion region, so only a tiny leakage current trickles through and the diode effectively blocks.
  5. If the reverse voltage exceeds the breakdown rating, current surges through, which Zener diodes use deliberately to hold a steady voltage.

Labelled diagram

  1. P-type layer

    Silicon doped to carry spare holes, acting as the anode side.

  2. Depletion region

    Thin zone without free carriers whose field opposes current crossing.

  3. N-type layer

    Silicon doped to carry spare electrons, acting as the cathode side.

  4. Forward bias

    Positive on the anode shrinks the barrier, and current flows freely.

  5. Reverse bias

    Positive on the cathode widens the barrier, blocking all but tiny leakage.

A p-n junction diode in forward and reverse bias

Real-world examples

  • A mobile phone charger plugged into India's 230-volt mains uses diodes to rectify alternating current before reducing it to a few volts DC.
  • Indicator lights and LED bulbs are diodes that turn the energy of crossing electrons into visible light.
  • Solar panel junction boxes contain bypass diodes that let current route around a shaded or damaged section of cells.
  • A diode across a relay coil absorbs the voltage spike produced when the coil switches off, protecting the transistor driving it.

Common mistake

✗ Myth: A diode is a perfect switch that conducts with no voltage loss and blocks any reverse voltage.

✓ Fact: A silicon diode drops about 0.6 to 0.7 volts while conducting, leaks a tiny current in reverse, and breaks down if the reverse voltage exceeds its rating.

People also ask

How does a diode convert ac to dc?
Alternating current reverses direction many times a second, 50 times in India and the UK. A diode conducts only when forward-biased, so it passes one half of each cycle and blocks the other. Four diodes in a bridge redirect both halves the same way, and a capacitor then smooths the pulses into steadier DC.
Why does current only flow one way in a diode?
The junction between p-type and n-type silicon forms a depletion region with a built-in electric field. Forward voltage opposes that field and collapses the barrier, letting electrons and holes cross. Reverse voltage adds to the field, pulling carriers away from the junction and leaving almost nothing to carry current.
What happens if a diode is connected backwards?
In most circuits, a reversed diode simply blocks current, so the device does not work; a reversed LED stays dark. If the reverse voltage exceeds the diode's breakdown rating, large current can flow and destroy it. Some circuits deliberately add a diode so that a battery fitted backwards cannot damage the electronics.
How do you tell which way a diode goes?
A diode's cathode, the end current flows out of, is marked with a painted band on the body. In circuit symbols, the triangle points in the direction of conventional current, toward a bar that represents the cathode.
Is an LED a diode?
Yes. A light-emitting diode is a diode made from compound semiconductors such as gallium nitride or gallium arsenide phosphide. When electrons cross the junction and fill holes, they release energy as light, and the material sets the colour.
Why does a diode drop some voltage?
Energy is needed to push carriers over the junction's built-in barrier, so a conducting silicon diode loses about 0.6 to 0.7 volts across it. Schottky diodes use a metal-semiconductor junction with a lower drop, which wastes less power in chargers and power supplies.
What is a bridge rectifier?
A bridge rectifier is four diodes arranged so that both halves of an alternating current wave are steered the same way through the load. The output is a bumpy direct current that a capacitor then smooths into a steadier supply.

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