A light-emitting diode (LED) is a semiconductor device that produces light through the movement of electrons within a solid material. While often perceived as “cool” lights, LEDs do generate heat, though much more efficiently than older lighting technologies like incandescent bulbs. Incandescent bulbs radiate approximately 90% of their energy outward as heat. An LED, however, retains the majority of its heat internally at the light-generating chip, requiring sophisticated engineering to manage the thermal energy.
How LEDs Create Light
The process of light generation begins when an electric current is applied to a semiconductor chip, structured as a positive (P) layer and a negative (N) layer forming a P-N junction. This arrangement allows current to flow in only one direction (forward bias). The applied voltage pushes electrons from the N-type material toward the junction, while positive charge carriers, called “holes,” move from the P-type material toward the same junction.
When electrons and holes meet at the junction, they recombine. This recombination releases energy as photons (light). The specific semiconductor materials used determine the energy difference between the electron and hole, which defines the color and wavelength of the emitted light. This direct conversion of electrical energy to light is known as electroluminescence.
The Source of Waste Heat
Despite the efficiency of electroluminescence, the conversion of electrical energy into light is never 100% perfect, and the unused energy manifests as heat. For a typical LED, around 70% of the electrical input is converted into thermal energy rather than light. This waste heat is generated directly at the tiny P-N junction, often referred to as the “junction point.”
This internal heating is compounded by the electrical resistance present in the circuit and the semiconductor material itself. The inability of the light fixture to transfer this heat away from the junction is quantified by a metric called “thermal resistance,” which acts as a bottleneck in the thermal pathway. Unlike an incandescent bulb, the LED concentrates its heat in a microscopic area. The concentrated heat at the junction must be actively removed to prevent damage to the chip, which is a major engineering challenge.
Managing Internal Heat
Because the semiconductor chip is highly sensitive to temperature, the heat generated at the junction must be quickly conducted away using a dedicated thermal management system. LEDs rely on conduction, transferring heat through direct physical contact. The heat is channeled from the junction through the LED package to a thermal slug, then onto a circuit board, and finally to a heatsink.
Heatsinks are typically constructed from a highly conductive material, such as aluminum, and often feature fins to maximize surface area. These fins facilitate the final transfer of heat to the ambient air through convection, completing the thermal path. Specialized thermal interface materials (TIMs), like thermal grease or pads, are placed between components to eliminate microscopic air gaps and minimize thermal resistance. This engineered pathway ensures the LED chip operates within its safe temperature range.
Impact on LED Lifespan
Poorly managed heat reduces the operating life of the LED. The temperature at the P-N junction is the most important factor determining the reliability and longevity of the light source. For example, a 10°C increase in the junction temperature can reduce the expected lifespan by approximately 50%.
When the junction temperature exceeds its design limit, two primary forms of degradation occur: lumen depreciation and color shift. Lumen depreciation is the gradual dimming of light output over time, often measured by the L70 standard (when output drops to 70% of its initial value). Color shift occurs as high heat degrades the phosphor materials, causing the light to subtly change color, often shifting toward a bluish or greenish tint.