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Optoelectronic Devices
10 questions By Tony R. Kuphaldt
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Question 4 of 10
What determines the color of an LED?
Reveal answerThe type of semiconductor materials used to make the PN junction determine the color of light emitted.
Challenge question: describe the relationship between LED color and typical forward voltage, in terms of photon frequency, energy, and semiconductor band gap.
Notes:Ask your students to identify some common LED materials and colors, and of course cite their sources as they do. The challenge question may be readily answered through experimentation with different LED colors, although a physics-based explanation will take some additional research. This kind of experiment is very easy to conduct in class, together.
If time permits, you might wish to mention Albert Einstein’s contribution to this aspect of physics: his formulation for the energy carried by a photon (a quantum) of light:
E = hf Where,
E = Energy carried by photon, in Joules
h = Planck’s constant, 6.63 × 10−34 J·s
f = Frequency of light, in Hertz ([1/s])
Typical frequencies for visible light colors range from 4 ×1014 Hz for red, to 7.5 ×1014 Hz for violet.
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Question 5 of 10
What is the typical forward voltage drop for a light-emitting diode? What is the typical forward current for an LED?
Reveal answerLED forward current is 20 mA. Forward voltage varies with color.
Notes:Be sure to ask students where they obtained their information, and what some of the forward voltage drops are for different color LEDs.
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Question 6 of 10
Explain the operating principle of a photovoltaic cell, otherwise known as a “solar cell.” What happens within these devices to convert sunlight directly into electricity?
Reveal answerThe energy of photons (light “particles”) striking a PN semiconductor junction creates electron-hole pairs, which then move in the direction that the depletion region’s electric field pushes them.
Challenge question: of what significance is the band gap of the PN junction to the efficiency of the cell?
Notes:There is quite a bit of detail that could be added to the account given in the answer. Ask your students to supply some of this detail! There are many resources for learning how photovoltaic cells work, so your students should have no trouble finding the information on their own.
