LEDs Explained: How Light Emitting Diodes Produce Color, Work, and Packaged Variants

This video explains the fundamental physics and engineering of light emitting diodes (LEDs). It covers how a PN junction emits photons when forward biased, why different materials produce colors from red to blue to green, and how packaging, polarity, and current control affect performance. The talk also surveys through hole and surface mount LEDs, RGB and bi-color variants, and practical circuits for blinking and white light creation. By the end, you’ll understand why LEDs are energy efficient and how color is selected through semiconductor band gaps and phosphor coatings.

Introduction to LED Function and History

The video begins with the basic question of why LEDs glow and how color depends on the semiconductor material. LEDs are diodes that emit photons when a voltage is applied, a process tied to recombination of electrons and holes in a PN junction. While regular diodes and LEDs share the same core principle, LEDs emit photons in the visible spectrum, whereas many standard diodes emit near infrared light.

PN Junctions, Band Gaps, and Photon Energy

Inside the LED, a thin junction between n-type and p-type materials creates a depletion region and an electric field. When forward biased, electrons cross the junction and recombine with holes, releasing energy as photons. The photon energy, and thus the color, depends on the semiconductor band gap. Initially silicon diodes emit infrared photons (about 1.1 eV, corresponding to ~1127 nm) and are not visible to humans. To achieve visible colors, materials with larger band gaps are used, such as gallium arsenide, gallium phosphide, and gallium arsenide phosphide blends, which can produce wavelengths from red to green to blue.

Materials and Color Tuning

Different materials yield different band gaps: silicon gives near infrared, GaAs-based materials give red to near-infrared light, GaP and GaN allow green to blue wavelengths. By blending gallium arsenide with gallium phosphide, engineers can tune the effective band gap and reach any color in the visible range, enabling red, green, and blue LEDs, and later mixing them to produce white light.

LED Packages, Sizes, and Configurations

LEDs come in various shapes and sizes including standard through-hole (5 mm), smaller SMDs, and larger high-power LEDs for torches and floodlights. The video explains how polarity is identified: the longest lead is the anode, the flat edge marks the cathode, and internal cone shapes or top markings help distinguish terminals. SMD LEDs may have a small dot indicating the anode or cathode, requiring reference to data sheets or testing.

Color Mixing and White Light

White light is produced by combining multiple colors. A common approach is to use a blue LED with a yellow phosphor coating to mix blue light with broad-spectrum yellow, which appears white to the human eye. RGB LEDs contain red, green, and blue diodes that can be driven separately to mix any color, including white, by adjusting voltage and current to each diode.

Practical Circuitry and LED Safety

The video demonstrates how LEDs are driven safely with current limiting. Supplying 9 V to an LED without a resistor will destroy it; a resistor drops excess voltage and sets the current. Data sheets specify forward voltage and current ratings; LEDs are typically rated for around 20 mA, with slight over- or under-driving still possible but hazardous beyond limits. LED drivers in lighting circuits ensure constant current and avoid flicker. Simple USB LED strings show parallel connections where each LED branch can be cut to tailor length and brightness.

Bi-colour and RGB LED Variants

Several LED varieties are covered: bidirectional two-color LEDs use two LEDs in opposite directions, 3-pin bi-color types switch between colors, and 4-pin RGB LEDs share terminals but allow independent control of red, green, and blue dies to mix colors, including white.

From Theory to Practice: What Sets LED Color

The color is determined by the energy difference between the conduction band in the N-type region and the valence band in the P-type region. This energy difference, manifested as the photon energy during electron-hole recombination, determines wavelength and thus color. The video traces a sequence of material choices moving from silicon (infrared) to gallium arsenide (near-infrared) to gallium phosphide (green to blue) and finally blends for full visible color coverage, enabling white light when red, green, and blue are combined.

Summary of LED Strengths and Applications

LEDs are energy efficient, generate less heat than incandescent bulbs, and can be produced in many forms for lighting, displays, and indicators. Understanding PN junctions and band gaps clarifies why different LEDs glow in different colors and how sophisticated color control is achieved in modern electronics and lighting systems.

Note on Sponsorship

Content focuses on LED technology and electronics concepts.