Question

A light-emitting diode (LED) is a semiconductor device (p-n junction diode) that emits light when an electric current is passed through it. LEDs are becoming the most popular light sources because they are much more energy efficient than conventional incandescent light sources. While the "white" light produced by a light bulb or the sun is a blend of many different colors, and these sources typically produce a large amount of heat, LEDs release only one particular color of light, and they produce very little heat. Therefore, it is estimated that replacing all the incandescent traffic light signals in the United States with LEDs would save almost 2.5 billion kWh per year (roughly $200 million) or 5 billion pounds of CO2 (from burning fossil fuels to produce electricity) released into the atmosphere. When a LED is activated (i.e., when a suitable voltage is applied to the diode), excited electrons are able to recombine with holes within the device, releasing energy in the form of photons. The color the LED emits, thus, depends on the wavelength of the photons emitted from recombination. The types of semiconductor determine the band gap energy which can create photons of different wavelengths. The voltage across the LED corresponds to the value of the band gap energy. The voltage can be converted to energy (in eV) by simply multiplying by the charge of one electron (e). For example, if the voltage is measured to be 1.2 V, the band gap energy would be 1.2 eV.

a. The voltage across an LED was measured to be 1.67 V. What is the band gap of this semiconductor, what wavelength of light is emitted (thus what color)?

b. Two LEDs are excited with the same amount of electrical energy from a battery and emit the same number of photons. One of these LEDs is emitting green photons and the other is emitting red photons. Which LED is more energy efficient?

c. If a material has a band gap energy in the infrared portion of the spectrum, this material will appear in what color?

d. If you want to create a red cutoff filter (i.e., the filter that transmits only red light of all the colors in the visible region of the spectrum), what should the band gap be to make such a filter out of a semiconductor?

Answer 1