5
\$\begingroup\$

Something is bothering me when I consider the forward voltage of tiny monochrome LEDs. Consider the following data from datasheets of tiny SMT monochrome LEDs of different colors (sources red yellow green blue):

Red (source):

excerpt from datasheet showing Vf_max = 2.4V

Yellow:

excerpt from datasheet showing Vf_max = 2.4V

Green:

excerpt from datasheet showing Vf_max = 2.4V

And finally blue, the odd one:

excerpt from datasheet showing Vf_max = 3.5V

Why the discontinuity on blue?

As I understand it, voltage is supposed to be directly proportional to frequency, which kinda explains why blue is higher (at least 2.66V, if I did my math right), but doesn't explain the discontinuity.

\$\endgroup\$
4
  • \$\begingroup\$ maybe it is not the blue LED that is "discontinuous" \$\endgroup\$
    – jsotola
    Aug 2 at 16:49
  • 11
    \$\begingroup\$ Blue LEDs use a different semiconductor technology than red, orange, yellow and green LEDs. \$\endgroup\$ Aug 2 at 16:53
  • 1
    \$\begingroup\$ Evolution of LEDs: lots of colors, recently Blue. Different technology. Just because it is a LED doesn't mean it follows a pattern. \$\endgroup\$ Aug 2 at 16:59
  • 1
    \$\begingroup\$ Note that if you measured the red, yellow, and green, they likely would not all be exactly 2 - 2.4 volts. Try it and see. To further complicate things, our eyes are not linearly-sensitive to the spectrum - green will appear brighter. \$\endgroup\$
    – rdtsc
    Aug 2 at 17:50
15
\$\begingroup\$

@Math Keeps Me Busy hit the nail on the head. Look at the "Chip Technology" for each of the LEDs. Red, yellow, and green are AlInGaP. Blue is InGaN. Different material, different bandgap, different forward voltage. Within the same material, the designer can play around with the parameters to match currents and voltages (but at different brightnesses, note). If you change materials, this gets harder.

\$\endgroup\$
3
  • 7
    \$\begingroup\$ Add there are now "pure green" using the InGaN material, as opposed to the original "pee green" LED's. They are far more brilliant and much brighter. This family also includes any 'violet' or 'purple' LED's you happen across. \$\endgroup\$
    – Kyle B
    Aug 2 at 20:56
  • 16
    \$\begingroup\$ (it's "pea green" not "pee green". You should see a doctor) \$\endgroup\$
    – user253751
    Aug 3 at 8:26
  • \$\begingroup\$ I agree that it's due to different materials (and by extension, device construction). But I don't think this addresses the question's point of confusion, which boils down to the fact that 470 nm = 2.64 eV regardless of material (and perhaps with an implicit assumption of direct band-gap materials). Note the question's math confuses volt with electronvolt. \$\endgroup\$
    – IceGlasses
    Aug 4 at 0:37
4
\$\begingroup\$

Creating a blue LED required finding a semiconductor material that had a direct band gap voltage that was sufficient to create photons with the required energy. There isn't a continuum of such materials, and the material that was selected (Gallium nitride, or GaN) simply had a significantly larger band gap than you might expect from the theory.

\$\endgroup\$
1
  • \$\begingroup\$ Except none of these samples are GaN \$\endgroup\$ Aug 2 at 21:04
3
\$\begingroup\$

In general you are correct, the forward voltage follows the wavelength of light. It could be that the manufacturer of the blue LED wanted to match the forward voltage and included another diode or misreported the forward voltage drop. In general they look like this (higher energy/wavelength, the higher forward voltage):

enter image description here
Source: http://lednique.com/current-voltage-relationships/iv-curves/

There are many new technologies such as OLED's and quantum dots that probably break these rules and have different energy/ voltage conversion factors, I couldn't find any evidence of this from wurth however.

\$\endgroup\$
2
  • 1
    \$\begingroup\$ I am suspicious of the source. Here is an example of actual empirical data from 4 LEDs. Note the log Y-axis? Note yellow? Note about where the Ohmic behavior starts to dominate the data points? If I replotted these on a linear y-axis it wouldn't look that much like what your included chart shows. Do you feel it's a solid chart? If feel it is a little "off" to me. Pretty, though. \$\endgroup\$
    – jonk
    Aug 3 at 3:06
  • \$\begingroup\$ Your graph shows 3.2V in the range of possible forward voltages for blue (if the LED is fairly bright). Why do you think it's wrong? \$\endgroup\$
    – user253751
    Aug 3 at 8:27
3
\$\begingroup\$

Your Specs:

R AlInGaP 2V 60 mcd (20 mA)
Y AlInGaP 2V 80 mcd
G AlInGaP 2V 30 mcd
B InGaN 3.2V 135 mcd

There is something they are not telling you about the general rule of Vf vs frequency of light emission. Nor are they telling you how the colours are modified using the same material.

But a recent breakthrough in GaN construction enter image description here ... demonstrates that within a family of common materials the doping variations do correlate frequency or wavelength with forward voltage, just as they did with old technology using GaP.

enter image description here

A bandgap, also called an energy gap, is an energy range in a solid where no electron states can exist.

It seems we have band gaps in our understanding of electroptical energy.

\$\endgroup\$
1
  • \$\begingroup\$ I would add that the color of light is specifically due to the bandgap of the InGaN in the quantum well, but as the diagram well-illustrates there are a lot more layers impacting the electrical properties. \$\endgroup\$
    – IceGlasses
    Aug 4 at 0:45
0
\$\begingroup\$

A white LED is a blue one with a yellow phosphor on top. Then the forward voltage is the same as a blue one. A modern bright green LED has the same forward voltage as a blue LED because they use the same material.

\$\endgroup\$
1

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.