The Structure And Operating Principle Of LED Lamps

We are all very familiar with LED lamps. However, have you ever wondered how they operate and what their structure is? In this article, Phuc Gia Laboratory Corporation invites you to explore this information!

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Table of contents

1. What is an LED?

LED is the acronym for Light Emitting Diode, an electronic component based on a P-N junction. The essence of an LED is a diode. It contains a semiconductor chip doped with impurities to create a P-N junction: the P-channel contains “holes” (electron vacancies), while the N-channel contains electrons. Current flows from the Anode (P-channel) to the Cathode (N-channel). When an electron fills a hole, it generates light radiance. The emitted wavelengths vary in color depending on the specific impurities within the semiconductor chip. LEDs are classified into three primary categories by power range: small-size, medium-size, and large-size.

2. Structure of LED Lamps

Lens – LED light is directional. The standard light distribution angle for an LED lamp is 180 degrees, with light emitted from the upper half of the lamp. On some LED lamps, the distribution angle is adjustable, offering different narrow or wide beams. The illumination angle can be altered using a lens. Polycarbonate lenses are preferred as they minimally obstruct light and are relatively easy to manufacture. The surface quality and shape of the lens are critical for ensuring proper light propagation and limiting losses in overall light output.

LED Chip – this is the component that emits light for the lamp.

The Structure and Operating Principles of LED Lamps — The Structure LED Lamps

Surface Layer: Typically, a Metal Core Printed Circuit Board (MCPCB) is used to mount the LED Lamps. In addition to providing a surface for attaching the LED chip, the metal core also serves to transfer heat to the Heat sink via a broader contact surface.

Contact Layer: Usually a thermal grease or adhesive, this component is used to maximize the contact when mounting the surface layer to the Heat sink, thereby maximizing the heat transfer down to the Heat sink.

Heat Sink: There are two types of heat sinks. Active heat sinks, which are typically fans used to circulate air. Passive heat sinks utilize metal fins to dissipate heat. Active cooling is often more effective, but for most applications, a passive Heat sink is sufficient to maintain the best operating temperature for the luminaire.

3. Light Emission Principle of LED Lamps

When an appropriate voltage is applied across the conductors, electrons recombine with holes and release energy in the form of photons. This effect is known as electroluminescence. The color of the light from LED Lamps is determined by the semiconductor’s energy band gap.

The Luminescence Phenomenon: Electrons near the conduction band minimum can, after a time, transition to an empty state in the valence band, where they recombine with a hole and release energy as a photon.

In a semiconductor, this is a spontaneous radiative recombination process, which is not dependent on the spectral energy density of any external electromagnetic radiation.

Considering a P-N Junction at Zero-Bias: In both the depletion region and the neutral region, the system is in equilibrium. Therefore, the rate of electron recombination equals the rate of electron emission. The resulting photon flux density is minuscule and largely reabsorbed, so no luminescence is observed.

Considering a P-N Junction under Forward-Bias: Within the depletion region, the phenomena of diffusion and carrier injection cause an abrupt increase in the excess carrier concentration (both electrons and holes). To restore equilibrium, these electrons and holes recombine through spontaneous recombination, emitting photons. The applied forward voltage constantly maintains this state of excess carriers in the depletion region. Consequently, a steady photon flux density is emitted from the depletion region, forming a light beam that exits the junction.

In the case of a P-N Junction under Reverse-Bias: The reverse current is a very small flow of minority carriers. This results in an emitted photon flux density that is far too small and is mostly reabsorbed, thus no light is produced.

In summary, the forward voltage applied to an LED creates carrier injection across the junction, which in turn causes a sudden rise in excess carrier concentration. This increase triggers radiative recombination as the system attempts to return to equilibrium. This is the fundamental operating mechanism of an LED.

4. What Factor Determines the Color of LED Lamps?

We often see LED lamps in a wide variety of beautiful and diverse colors. What makes it possible to create such multi-colored arrays of light?

– The Determining Factor for LED Light Color

The decisive factor for an LED’s color is the type of impurities (or semiconductors) used within the lamp. The specific composition and dosage of these impurities will dictate the color of the light produced. Manufacturers leverage this principle to create LED lamps with different colors.

– How are the different colors of LEDs created?

Specifically, the light from an LED that we perceive visually is determined by the light’s energy level. This light energy is generated by the combination of P-holes and N-electrons. The greater the distance between these two elements, the higher the energy.

Light colors with the highest energy include purple and blue, while those with the lowest energy include red and orange. Typically, manufacturers produce LEDs with basic colors like green or red, and then combine various color chips to create multi-colored LED products.

– White Light – A Special Combination of the LED Principle

Many people mistakenly believe white light is an LED’s original color, but this is incorrect. To produce white light, manufacturers combine basic colors such as green, red, and blue. Three different colored chips are generally used in a ratio of 69% green, 21% red, and 10% blue to generate white light.

To create white light, basic color LED chips must be combined

Furthermore, another method to achieve white light from LEDs involves using phosphor (fluorescent powder). Specifically, manufacturers will apply methods such as:

Coating a layer of yellow Nd:YAG phosphor over a blue LED chip.
Coating a layer of 555nm wavelength YAG phosphor onto a 460nm wavelength blue crystal LED chip, and then using a lens to combine the yellow and blue light to produce the color of white light.

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