Light Emitting Diodes (LEDs) are based on the semiconductor diode. When the diode is forward biased (switched on), electrons are able to recombine with holes and energy is released in the form of light. This effect is called electroluminescence and the color of the light is determined by the energy gap of the semiconductor. The LED is usually small in area (less than 1 mm2). Applications of LEDs are diverse. They are used as low-energy and also for replacements for traditional light sources in well-established applications such as indicators and automotive lighting.

For LED applications with a constant heat source, the power output of the TEG needs to match or exceed the power input requirements of the sensor. If the heat source is variable, the TEG can be used to trickle charge a battery that powers the LED directly.