Why do blue LEDs have a higher forward voltage than expected?

In summary, there is no direct relationship between the band gap energy and the forward voltage of blue LEDs. They are complex devices made up of multiple layers of III-V semiconductors with different energy gaps, resulting in quantum well structures that emit light. This makes it difficult to accurately predict the forward voltage based on the band gap energy alone.
  • #1
madmike159
Gold Member
371
0
My physics/electronics teacher told me some thing really interesting and confusing a wile ago and I have been trying to get my head round it. The forward voltage in a LED (voltage needed to drive the current) is proportional to the band gap energy.
However Blue LEDs have a higher forward voltage than the band gap energy suggests. I haven’t been able to figure out why. Does anyone know?

This is a link to a page with some band gap energies for different semi-conductors. http://en.wikipedia.org/wiki/Bandgap

This is a link to a page on LED’s and what they are made out of http://en.wikipedia.org/wiki/LED

I up loaded a .txt of forward voltages and band gap energies so you don't have to look through lots of internet pages. The blue LED forward voltage defiantly looks too high. If anyone knows a reason for this your help would be appreciated.
 

Attachments

  • LED's.txt
    2.6 KB · Views: 743
Physics news on Phys.org
  • #2
Sorry I ment to put this in General Physics.
 
  • #3
There are really a lot of other factors that go into the forward voltage of an LED, such as doping, the quality of the interfaces, the intrinsic resistive losses of the material, etc. Even if you account for all of these, there isn't really even a well-defined "forward voltage," because the current / light output varies continuously with voltage.

So, you're right that the forward voltage isn't completely proportional to band gap. As a rule of thumb, though, it's close enough.
 
  • #4
Hey madmike, do you think you can point out a specific example that this pertains to? Be as quantitative as possible. If by "forward voltage" you mean the voltage necessary to turn the device on, then this shouldn't be substantially near the bandgap.

On a sidenote (may be of use), I know InGaN is common for blue LEDs. Additionally, there several designs which use LEDs of longer wavelength and then send that light through some sort of upconverter to get blue light out of the system.
 
  • #5
cmos said:
Hey madmike, do you think you can point out a specific example that this pertains to? Be as quantitative as possible. If by "forward voltage" you mean the voltage necessary to turn the device on, then this shouldn't be substantially near the bandgap.

I didn't say near to I said proportional.

Thanks i though there would of been something I would of missed.
 
  • #6
Blue LEDs are not -as far as I know- simple "bandgap devices" in the same way as red LEDs. Instead, blue and UV LEDs are quite complicated devices built up from several layers of III-V semiconductors with different energy gaps. The transitions that emitt the light are due to these layers forming a quantum well. I.e. there is no direct correlation between the bandgap of the material and the frequency of the emitted light.

Building quantum wells is fairly easy in theory (it basically involves "designing" the right potential using the Schroedinger equation, it is a very direct application of basic quantum mechanics) but in real devices you also need buffer layers etc. This is why reliable, cheap blue LEDs haven't been around for more than a decade or so.
Blue laser diods are even more complicated, but the basic principle is the same as for LEDs.
 

1. What are Blue LEDs and how do they work?

Blue LEDs, or blue light-emitting diodes, are semiconductor devices that emit blue light when an electric current is passed through them. They typically consist of a layer of gallium nitride (GaN) sandwiched between layers of other semiconductor materials. When an electric current is applied, electrons in the GaN layer are excited and release energy in the form of photons, creating blue light.

2. How are Blue LEDs different from other types of LEDs?

Blue LEDs are different from other types of LEDs because they emit blue light instead of other colors like red or green. They also require a higher band gap energy, which is the minimum amount of energy needed for electrons to move from the valence band to the conduction band, compared to other LEDs. This is due to the unique properties of the semiconductor material used in blue LEDs, such as gallium nitride.

3. What is band gap energy and why is it important for Blue LEDs?

Band gap energy is the energy difference between the valence band, where electrons are bound to atoms, and the conduction band, where electrons are free to move and conduct electricity. For Blue LEDs, a higher band gap energy is needed because the electrons need to release more energy in the form of photons to create blue light. This is why specific semiconductor materials with higher band gap energies, such as gallium nitride, are used in blue LEDs.

4. What are some applications of Blue LEDs?

Blue LEDs have a wide range of applications, including in electronic displays, traffic lights, and automotive lighting. They are also used in lighting for indoor and outdoor spaces, as well as in medical and scientific equipment. In recent years, blue LEDs have also been used in the development of new technologies, such as Blu-ray players and high-efficiency solar cells.

5. What is the future of Blue LEDs?

The future of Blue LEDs looks promising, as researchers continue to improve the technology and find new applications. One potential development is the use of blue LEDs in horticulture, as they have been shown to promote plant growth and increase crop yields. Blue LEDs are also being explored for use in data communication and quantum computing. As technology advances, we may see even more innovative uses for blue LEDs in the future.

Similar threads

How Does Voltage Affect LED Color Through Band Gap Adjustments?
    • Introductory Physics Homework Help
Replies
1
Views
350
Why is the band offset of an LED equal to the band gap?
    • Atomic and Condensed Matter
Replies
1
Views
2K
I Relationship between the depletion region and the band gap of a diode?
    • Atomic and Condensed Matter
Replies
3
Views
5K
B Valence band, conduction band
    • Atomic and Condensed Matter
Replies
1
Views
3K
Why Does Band Gap Energy Increase Higher Up the Periodic Table?
    • Atomic and Condensed Matter
Replies
1
Views
5K
Question about band gaps in 2D
    • Atomic and Condensed Matter
Replies
22
Views
3K
Does Ohmic Contact Affect the Energy Gap Shape of a P-N Junction?
    • Atomic and Condensed Matter
Replies
1
Views
2K
Why do doped semiconductors have a decreased band gap?
    • Atomic and Condensed Matter
Replies
1
Views
2K
I Built-in potential in pn junction
    • Atomic and Condensed Matter
Replies
2
Views
3K
Why conductors bonds in such way that leaves the valence band not full
    • Atomic and Condensed Matter
Replies
3
Views
4K

Hot Threads

Recent Insights

Back
Top