LED Principle

Part of Lighting

How semiconductor junctions convert electricity directly into photons of light.

Why This Matters

Understanding how LEDs work at a fundamental level is not just academic. It tells you why LEDs fail, what operating conditions damage them, why different colors require different voltages, and why LEDs from salvaged electronics can often be reused even when the original circuit is destroyed. It also explains the relationship between LEDs and transistors — the two technologies emerge from the same semiconductor physics, which means rebuilding LED manufacturing requires the same industrial base as rebuilding computing.

The LED is the most efficient artificial light source humans have ever made. A modern LED converts 40–60% of input electrical energy directly into light. A candle converts about 0.04%. Understanding the physical reason for this efficiency gap helps you appreciate the strategic importance of salvaging and preserving LED technology.

Even if your community never fabricates a semiconductor, understanding LED physics lets you make better decisions about which devices to salvage, how to test unknown components, what failure modes to watch for, and how to explain the technology to others who will maintain and eventually reproduce it.

Semiconductor Basics

All matter falls into three rough conductivity categories: conductors (metals), insulators (glass, rubber, dry wood), and semiconductors (silicon, germanium, gallium arsenide). Semiconductors sit between conductors and insulators, but their key feature is controllable conductivity — their resistance can be precisely adjusted by adding small amounts of specific impurities, a process called doping.

Intrinsic semiconductor: Pure crystalline silicon has four valence electrons per atom, forming a stable lattice. At absolute zero, it is a perfect insulator. At room temperature, a small number of electrons gain enough thermal energy to break free and conduct. Each freed electron leaves a “hole” — a vacancy in the lattice that behaves like a positive charge carrier.

N-type doping: Add atoms with five valence electrons (phosphorus, arsenic) to silicon. The extra electron is not needed for bonding and is free to move. Result: excess negative charge carriers (electrons). N-type material.

P-type doping: Add atoms with three valence electrons (boron, aluminum) to silicon. The missing electron creates a hole in the lattice. Holes move through the material as positive charge carriers. P-type material.

The P-N Junction

When P-type and N-type semiconductor materials are brought together in intimate contact, something remarkable happens at the boundary.

Electrons from the N-side diffuse into the P-side and fall into holes. Holes from the P-side diffuse into the N-side and are filled by electrons. This creates a depletion region at the junction — a zone with no free carriers, swept clean by the recombination process.

The recombination creates fixed ions: positive ions on the N-side (atoms that donated electrons and now lack one), negative ions on the P-side (atoms that accepted electrons and now have an extra one). These fixed ions create an electric field pointing from N to P across the depletion region. This built-in field prevents further diffusion — an equilibrium is reached.

The potential energy difference across the depletion region is the junction potential, typically 0.6–0.7V for silicon or 0.2–0.3V for germanium. This is why silicon diodes require about 0.7V to conduct.

Forward Bias and Current Flow

Applying external voltage with positive terminal to the P-side (forward bias) opposes the built-in field. When the external voltage exceeds the junction potential, the depletion region collapses and current flows freely.

Electrons from the N-side and holes from the P-side pour into the junction. They meet and recombine. In ordinary silicon diodes, this recombination releases energy as heat — the electron simply drops to a lower energy state and vibrates the lattice.

In certain semiconductor materials, the recombination releases energy as a photon instead of heat. This is the LED effect.

Why Some Junctions Emit Light

Whether a semiconductor junction emits light or heat during recombination depends on the band structure of the material.

In silicon and germanium (indirect bandgap semiconductors), the energy transition during recombination must also change the electron’s momentum. This is an unlikely quantum process, and the energy mostly goes to heat via phonons (lattice vibrations). Silicon diodes are hot, not bright.

In gallium arsenide, gallium nitride, indium phosphide, and related III-V compound semiconductors (direct bandgap semiconductors), the energy transition does not require a momentum change. Recombination can occur directly with photon emission. These materials glow.

The photon energy — and therefore its color (wavelength) — equals the energy gap of the semiconductor material:

E_photon = h × f = hc/λ

Where:
  h = Planck's constant
  f = photon frequency
  c = speed of light
  λ = wavelength

Larger bandgap → higher energy photons → shorter wavelength → blue/violet/UV light Smaller bandgap → lower energy photons → longer wavelength → red/infrared light

This is why blue LEDs require higher forward voltage (3.0–3.5V) than red LEDs (1.8–2.2V) — the blue photon carries more energy, requiring more electrical energy per recombination event.

White Light from LEDs

White light cannot come from a single semiconductor junction because no junction material emits across the full visible spectrum simultaneously.

The modern solution is a phosphor conversion white LED:

1. A blue LED die (gallium nitride, typically emitting at 450–460 nm) is coated with a yellow phosphor (usually cerium-doped yttrium aluminum garnet, YAG:Ce)
2. Some of the blue light passes through the phosphor layer unchanged
3. The rest excites the phosphor, which re-emits as broad-spectrum yellow light
4. The combination of blue and yellow is perceived as white by the human eye

The color temperature of the white light depends on the phosphor thickness and composition:

• Thin phosphor → more blue passes through → cool white (5,000–6,500K, bluish)
• Thick phosphor → more yellow emission → warm white (2,700–3,000K, yellowish, like incandescent)

This is why warm white LEDs have a slightly lower forward voltage than cool white — the thick phosphor layer absorbs slightly more blue light, but the junction voltage is essentially the same.

LED Efficiency and Losses

An LED’s efficiency is measured as wall-plug efficiency — optical power out divided by electrical power in.

Modern commercial white LEDs achieve 40–60% wall-plug efficiency. The remaining 40–60% becomes heat at the junction. By comparison:

Light source	Energy to light
Candle	~0.04%
Incandescent bulb	~5%
Fluorescent tube	~22%
LED (typical)	40–60%

The losses in LEDs come from several mechanisms:

• Auger recombination — three-carrier events at high current densities that produce heat instead of light
• Surface recombination — defects at crystal surfaces trap carriers before they can recombine radiatively
• Optical losses — photons produced at the junction that get absorbed before escaping the chip
• Phosphor losses — Stokes shift (energy difference between absorbed and emitted photon) in the phosphor

Failure Modes

Understanding why LEDs fail helps you extend their life and identify salvageable components.

Thermal degradation (most common): Excess junction temperature accelerates all failure mechanisms. Phosphor degrades, changing color temperature toward blue/green. Bonding wire connections oxidize or corrode. The LED dims gradually over thousands of hours. Running at lower current than rated extends life dramatically — a 1W LED run at 350 mA instead of 700 mA may last five times longer.

Electrical overstress: A current spike even briefly exceeding the rated maximum can melt bond wires, crack the junction, or vaporize a local hot spot. Protect every LED with proper current limiting. Capacitors across the power supply help absorb voltage spikes.

Electrostatic discharge (ESD): The tiny junctions in LEDs are vulnerable to static electricity. When handling bare LED chips or strips, touch a grounded metal surface first. For LEDs in circuit board assemblies, ESD is usually not a concern — the board provides some protection.

Reverse voltage: LEDs have very low reverse breakdown voltage (typically 5V or less). A voltage spike in reverse can instantly destroy the junction. In AC circuits or circuits with inductive loads, always include a reverse protection diode or use the LED in a full-wave rectified circuit.

Phosphor degradation: The yttrium aluminum garnet phosphor in white LEDs slowly changes its emission spectrum over time, particularly under thermal stress and UV exposure. The LED shifts toward cooler color temperatures. This is cosmetic only — the LED still provides useful light.