So, the next question, is why are these LEDs blue? And what if we want a different colour?
In the crystal, we have two types of particles flowing around. Electrons and holes. Holes aren't 'real' particles, but they still exist: much like bubbles in a bottle of water. It's an awkward analogy, but we can imagine two bottles of water: one that's completely full except for a few bubbles, and another that is higher in energy that's almost empty apart from a few drops of water. These drops of water sloshing around are the electrons moving through the crystal. What happens when an electron (water droplet) and hole (bubble) meet? They annihilate each other, and in the process give off a little spark of energy in the form of a piece of light. For gallium nitride, the light's blue because of the difference in energy between the electrons and the holes.
To get higher energy light (more blue->purple->ultra violet) we can replace some of the gallium with aluminium. To get lower energy light (green) we replace some of the gallium with indium. So far, so good.
One of many outstanding problems though is that LEDs have much poorer efficiencies when we want to emit green light. Lots of different combinations and permutations have been tried, but none are great, and we're still looking for a better solution–this might be in the form of finding a brand new material, or of growing nitride-based crystals in more unusual forms, such as nanowires (imagine a forest of crystals standing on a sapphire 'floor') or quantum dots (tiny little pyramids). This can help because electrons act in very different ways when they're confined in certain dimensions, e.g. if they can only move in along a straight line, and this can be exploited to make better devices. So lots of people are trying this, not just for green LEDs, but for all manner of different devices.
So this is what we do when we want to intentionally change the colour that's emitted. But there's lots that happens to unintentionally change the colour too, which is a problem if you're trying to make thousands of light bulbs that should all look the same! This is the result of much more subtle problems that exist on the very smallest atomic length scales...
In the crystal, we have two types of particles flowing around. Electrons and holes. Holes aren't 'real' particles, but they still exist: much like bubbles in a bottle of water. It's an awkward analogy, but we can imagine two bottles of water: one that's completely full except for a few bubbles, and another that is higher in energy that's almost empty apart from a few drops of water. These drops of water sloshing around are the electrons moving through the crystal. What happens when an electron (water droplet) and hole (bubble) meet? They annihilate each other, and in the process give off a little spark of energy in the form of a piece of light. For gallium nitride, the light's blue because of the difference in energy between the electrons and the holes.
To get higher energy light (more blue->purple->ultra violet) we can replace some of the gallium with aluminium. To get lower energy light (green) we replace some of the gallium with indium. So far, so good.
One of many outstanding problems though is that LEDs have much poorer efficiencies when we want to emit green light. Lots of different combinations and permutations have been tried, but none are great, and we're still looking for a better solution–this might be in the form of finding a brand new material, or of growing nitride-based crystals in more unusual forms, such as nanowires (imagine a forest of crystals standing on a sapphire 'floor') or quantum dots (tiny little pyramids). This can help because electrons act in very different ways when they're confined in certain dimensions, e.g. if they can only move in along a straight line, and this can be exploited to make better devices. So lots of people are trying this, not just for green LEDs, but for all manner of different devices.
So this is what we do when we want to intentionally change the colour that's emitted. But there's lots that happens to unintentionally change the colour too, which is a problem if you're trying to make thousands of light bulbs that should all look the same! This is the result of much more subtle problems that exist on the very smallest atomic length scales...