Simple Explanation of Laser Diodes?

[awlego]

All this working with lasers make me want to know how they work...

I have tried to understand how laser diodes work, but most of the sites I come across are way too technical for me. Could anyone provide a simple explanation for how they work?

I know it has to do with electricity passing over two differently dopped materials (p and n I think). What exactly is dopping? And why does this create laser light?

Thanks!

-Awlego

 

[bobhaha]

p and n type materials are semiconductors. A P-type semiconductor (P for Positive) is obtained by carrying out a process of doping, that is adding a certain type of atoms to the semiconductor in order to increase the number of free charge carriers. This is usually done with the elements of silicon and boron. This forms an electron "hole" which is due to the boron not having enough electrons to fill all the holes, when bonded with silicon.

N-type semiconductors (n for negative) is obtained the same way, but it creates an excess of electrons. This is usually done with the elements of silicon and phosphorus. The extra electrons are created because when the two elements bond, there are more electrons then holes, therefore creating excess electrons in the material.

When a P and N type semiconductor are sandwiched together (pressed together) it forms a resistance barrier. This is where the excess electrons from the N type fills the holes in the p type. To overcome this electrical resistance barrier electrons must physically jump the gap between the P and N type semiconductor, in order to conduct. This is a basic diode, in a nut shell.

For a laser diode, it is much more complex. I'm not 100% sure of the make up of the laser diode, but I would imagine it would consist of very similar elements. In this case as electrons jump from one semiconductor to another they impart some of their energy to the material. It then undergoes a stimulation state in which the energy imparted creates light. But it is not like typical light, the light from a stimulated laser diode produces light in one direction, instead of in all directions (like a light bulb for example). This light that is emitted will travel in a straight line out of the laser diode and is called laser light.

This is a simplistic explanation but I hope you understand a bit more about how a laser works now!

Hope that helped! -Adrian

 

[pullbangdead]

There's no such thing as a true "simple" explanation. In some ways, these are some of the most complicated electronic devices around. (in some ways though, they're actually some of the simplest). When I have some time later this week, I'll write some stuff up.

bobhaha hit some of the high points, but as they say, the devil is in the details.

Lasers are MUCH more complicated than regular p-n diodes: in structure, in the materials they're made of, and in the physics.

In materials: Laser diodes aren't ever made of silicon (although bobhaha did correctly point out the doping materials used in silicon, giving you an idea of how doping works, with P or B sitting on Si sites and contributing holes or electrons). Laser diodes use III-V semiconductors. Si is a group IV semiconductor (look at your periodic table), you can see it's neutral and highly covalent in nature, with each Si sharing its 4 electrons with its 4 nearest neighbors, giving full valence of 8 electrons per atom. By combining a group III element (like gallium) with a group V element (like nitrogen), you see how you get the same thing, with full valence for each atom and highly covalent bonding. But with a III-V semiconductor, you can see how doping gets more complicated. The material is even more complicated because it's rarely ever just 2 elements, it's often alloys like InGaN, InGaAsP, etc. Still charge-neutral III-V highly-covalent solids, and then doped to p- or n-type, but much more complicated.

In structure: Laser diodes aren't simple p-n diode. They're typically p-i-n diodes (i meaning intrinsic, or not doped), and typcially quantem wells at that. Additionally, they're heterostructures, meaning the p-type region is a completely different material from the i-type region, which is a completely different material from the n-type region. For example, an InGaN "intrinsic" (actually unintentionally doped, but beyond the scope of this discussion) region sandwiched between p-type and n-type GaN layers. It's a quantum well because the InGaN layer will only be single-digit nanometers thick.

In the physics: Well, you get the idea.

So yeah, maybe later I'll type up a full, real response on how electrons and holes combine to make coherent light, when I have some more time. At least you have some more topics to start with now, in the mean time.

 

[bobhaha]

WOW! thanks PBD! you know quite a lot in this field.. as far as my physics coarse went, laser diodes only consisted of PN junction.. I have never heard of a PIN junction.. but I will be researching into it. I never knew that the laser diode uses III-V semiconductors... this would explain the the need of a 3rd or even 4th element for the alloy.

I'm waiting to read your full response! -Adrian

 

[Meatball]

I can't wait to read the full response either... this has always been kind of mystery to me as well!

 

[awlego]

Wow thanks! I too look forward to the full response, it will be much appreciated. What has been provided has been great. Thanks!
-Awlego

 

[Benm]

Well, as far as simple explanations go, there have been simple laser diodes that use common silicon based semiconductor technology too. These devices operated in the infrared, but for the principle that doesn't matter.

It's important to note that all semiconductor diodes in fact emit light, although usually only very little, and infrared (not observable by eye). LED's are diodes that emit a lot of light compared to ordinary ones. The first LED's where also infrared, but using different materials got into the visible range.

In the very basis, a laser diode is a LED made of materials such that population inversion can be achieved, and with mirrors around the light emitting area to form a laser. So it actually is like a LED with mirrors on either side of the chip, in its very essense.


Real world laser diodes used today are far more complex than these basics, using more exotic materials and structures built in serveral P- N-, metallic and intrinsic layers, using quantum wells and all, but you dont need to understand all of that to grasp the basic principle of operation.