How do LEDs work?
How and why are they so much more efficient than a standard light bulb? Tom O'Hanlon went to find out at the Cambridge Centre for Gallium Nitride with Rachel Oliver...
Rachel - Welcome to the lab. First things first we need to put the lab coats on…
Tom - Just do up the poppers...
Rachel - Now I’m sure lots of people have seen an LED bulb but what they actually look like these days? It’s very much like a normal light bulb, maybe with a sort of frosted glass cover. And it’s a bit difficult to tell why this is different to a normal light bulb that you might have been buying for years and years. So what we thought we might do this morning is take one apart and find out what’s inside. Now we’re going to do that using a diamond saw over here…
We’ve got here an LED bulb that’s really in several parts. We’ve got this yellow globe - it looks a bit like a small ping pong ball and then underneath we can see a load of circuit boards and things like that.
Tom - Okay. So the LEDs are those little chips, maybe about 5 mm across?
Rachel - Yes, something like that. This bulbs got 6 LEDs in. Different bulbs have different numbers. They’re quite small chips; they’re actually quite small devices, and they’re made from a material called gallium nitride. That’s a material that you don’t find in nature. LED manufacturers grow it in big crystal growth systems and we actually have one of those big crystal growth systems here where we make experimental LED structures and try and improve them.
Now we’re in the growth lab and what we’re looking at here is one of our crystal growth systems. It’s about the size of a transit van but a lot of the bulk is actually just things like control electronics and systems that handle gases; they move gases around.
Tom - The gallium nitride is built up by reacting a gas called trimethyl gallium, which supplies the gallium, with ammonia, which supplies the nitrogen. These gases are passed through a surprisingly familiar piece of plumbing…
Rachel - The gallium and the nitrogen come through a structure called a shower head, which looks like a shower head.
Tom - Like the nice ones you get in fancy hotels?
Rachel - Yes, exactly. Admittedly, it doesn’t look much like my shower head at home which is white and plastic.
Tom - Below the shower head is a wafer similar to what you get with ice cream. But instead of biscuit, it’s made from either of man-made sapphire or silicon. Rachel heats these wafers up to a thousand degrees Celsius and the gases split apart on the wafer surface and react to build up the LED crystal layer by layer.
Rachel - The gallium nitride layers that are the base that we grow for the LED, they’re typically about microns thick, so a micron is 1/1,000th of a millimetre. The actual light emitting layers (the bit the light comes out from) are much, much thinner somewhere in the order of a millionth of a millimetre. It’s very, very thin light emitting layers.
Tom - We’ve spoken about how you make the material used to make these LEDs, but how do they actually emit light?
Rachel - Well, I’ve got something in my office which might help you to understand that so why don’t we walk back through there…
Tom - To my surprise, Rachel showed my a wooden game I hadn’t seen in years - it’s called Labyrinth. Do you remember it?
You have to roll a ball bearing through a maze, twizzling knobs on the side to tilt the board up, down, left, and right trying to avoid it falling down some holes. How is this anything like an LED, though?
Well, it comes down to these layers. On one side of the LED, the layer is tweaked so there are extra electrons which are negative. On the other side, some electrons are missing leaving behind holes which are positively charged. When you flick the light switch on, the negative electrons and the positive holes are pushed towards each other.
Rachel - So, we can think of our electron as being the ball bearing and the holes in the toy are just the holes in the crystal and where the electron and the hole meet, the electron basically falls down into the hole just as the ball bearing falls into the holes in the toy.
Tom - So how are your skills at this Labyrinth game Rachel?
Rachel - Not great. Shall we see how far I can keep it going before my electron recombines with my hole. Let’s have a go… I’ve got to get round my first corner now and I’m not sure I’m going to make it. Uh, I’ve managed one corner. You heard there the process by which my ball bearing, which is my electron, lost some of its energy. It fell down and it went clunk at the bottom and some of the energy it had, when it was at the higher level, got converted into sound energy. In the LED, obviously, the electron and hole meeting gives us light, but the basic principle is just the same.
Tom - In these LEDs, the electron falling into the hole creates blue light. To get white light you need what Rachel described earlier as a yellow ping pong ball. It’s coated with chemicals called phosphors which convert some of the blue light to yellow. Yellow light, plus blue, makes white.
Rachel also told me about another trick LEDs use to make the light more efficiently…
Rachel - It’s all to do with these very, very thin layers I talked about which actually give the light out. The reason they’re very, very thin is that we’re using them to trap the electron and hole together in the same place. Now it’s like if you and I needed to meet up to have a cup of coffee, if I said to you “great, let’s meet up tomorrow and we’ll meet in Cambridge,” well we might find each other but it’s not very likely. If I say to you “let’s meet up and have a coffee in the common room at the Department of Material Science,” it’s quite likely we’ll actually manage to find each other and have our coffee and our chat. So we’re doing the same thing with the electron and the hole and because they meet quickly and efficiently and combine to give out light, there’s no time for them to wander off and find maybe mistakes in the crystal where that process by which they combine could go wrong and give out heat.
Really, in terms of the research we do, one of the key things we do is think about the structures that give out the light. They’re very, very small and we try and engineer them so that that process where the electron and the hole meet is as fast and efficient as possible, and that makes the overall device as efficient as possible.