LED Construction

The active region is where photons are generated as electrons jump to a lower energy level across the diode junction.

The substrate is un-doped semiconductor. The P layer is P doped semiconductor. The N layer is N doped semiconductor. The N and P layers form a diode.

This is my understanding, but I may be a bit out of date with some of it and could be wrong in some of the detail.

The substrate of most active devices (ICs, transistors, diodes, LEDs, etc) comes from the large, circular wafer of semiconductor material that the subsequent processing steps are done to. In the very early days other methods were used, but now wafer processing dominates (because it lends itself to handling the fabrication of large numbers of devices in an efficient way).

The n-type layer is formed in the substrate by gas diffusion in an oven (doping) or, for the kind of IC where the features need to be very small, ion implantation from an ion beam. Then the p-type region is diffused into the n-type. Once you have done those steps you have three layers (you'd have many more for an integrated circuit). At this point, you'd only call the base layer of original, untouched semiconductor material the substrate.

The active region forms around the join between the n-type and p-type material when the diode is forward biased. If you want to know how that occurs, then any semiconductor physics textbook will tell you [I'm not going to because I can't remember it in enough detail - sorry]. It's the same process for any p-n diode. There is nothing special about an LED other than that the radiation given off in the active region is of a frequency that is useful (which is done by choice of semiconductor and the dopants) and effort has been made to get as much as possible out of the device rather than have it reabsorbed and ending up as heat. Although getting the radiation out sounds easy, in practice it is difficult and a lot of work goes into improving the process (if you have any good ideas, run off and patent them quickly). High intensity LEDs use techniques like having arrays of holes etched into the surface that use interference effects to move the light across what is effectively an impedance mismatch between the material and air, and clever stuff like that.

Drawings don't usually give you an idea of scale - the diffused layers are very thin, so the substrate is way taller and serves as the mechanical support for the whole thing.

There is an exception to this for some kinds of LEDs. They have a non-semiconductor substrate (usually sapphire, I think) that a very thin semiconductor layer is grown on. The sapphire distorts the atomic lattice of the semiconductor material and that results in a different frequency of light emission to what you'd get with an ordinary substrate of that material.

Edit

That will teach me to post from memory without looking it up. I was making up my own physics there. The recombination is different for LEDs ('considerable amount of direct recombination') to what happens in an ordinary diode ('mostly via traps'). The difference is down to the type of semiconductor.

Thanks for the reply!

A few follow up questions:

What is the purpose of the substrate?

What is the active region made of? is it doped?

Hi Nicholas,

WARNING I have an alternative view of electronics.

Now, if you are still reading, I have been doing some research into subatomic physics and have concluded that it is photon flow, not electron flow that makes everything in the universe work.

For you questions, the Substrate is usually a piece of silicon on which the specific semiconductor is constructed. In most cases this is "pure" silicon.

The active region is another piece of silicon in which specific "doping" atoms are added to the crystal structure to create areas where current flow is easy or current flow is inhibited.

From my theory, the doping atoms create areas within the crystal matrix where photons flow differently than they do for the silicon atoms. This natural flow of photons reduces the resistance of the silicon substrate so that it facilitates photon flow when a bias voltage is applied to the junction.

The LED "light" is emitted at the junction between the substrate and the active region itself.

Again, my view is that the transfer of photons across the different silicon material results in specific types of photons to be released by the atoms as other photons flow between the atoms. Essentially the doping atoms behave very predictably under these conditions to free of photons of a narrow range of frequencies, which is by you get emission of Red, Blue, Green, Yellow, UV or IR wavelengths. The only thing that changes is the type of doping material and the density of the doping atoms.

In most cases, each LED also emits an IR component. In some cases the manufacturer will put an IR blocking filter inside the LED so that only the visible light photons escape.

Conventional theory will tell you that the photons are emitted because the electrons are changing orbital levels around each doping atom. I dispute this view because nobody has ever proven that electrons can orbit an atom let alone change energy levels by the absorption or emission of photons. My work has demonstrated that each atom is really surrounded by a very large number of captured photons, which when voltages are applied across these atoms some of the captured photons are emitted while other photons are absorbed by each atom.

Either way, the light does come out of the LED at very specific frequencies.

DAB

The substrate is the starting wafer of semiconductor material before it has been processed to make diodes. The P-N layers are just the areas at the top of the substrate after it has had small amounts of impurities infused into it. They call these P-N infused areas "layers" because the impurities only infuse to a shallow depth, but they are still all just part of the substrate. The uninfused or un-doped areas are called the substrate - they are conductive or semi conductive, but not doped.