LED - Land of electrons

what is an LED?

An LED is a light-emitting diode. This is a diode that when given the correct voltage and current, will emit light. They are designed to emit light in a narrow bandwidth, so that LEDs with different colours can be made. Essentially, an LED is similar to a PN junction diode, except that it is designed to emit light – passing current in the forward direction and blocking current in the reverse direction! LEDs convert electrical energy into light energy.

how do they work?

LEDs are made from a very thin layer of semiconductor material that is doped to give a colour of a specific wavelength. When an LED is forward-biased electrons from the conduction band recombines with holes in the valence band, releasing photons.

construction

The PN junction of an LED is surrounded by a transparent hard plastic body, that allows the light to be seen and protects the junction. The plastic body also serves another, very important purpose, to focus the light. The PN junction doesn’t emit much light, the plastic body focusses the light like a lens, towards the domed top. This is the reason why an LED looks brighter at the top, than at the sides!

identification

On most LEDs there are a couple of ways of identifying the connections. The cathode (-) usually has a flat side on the body and a shorter lead.

advantages of LEDs

One of the advantages of LEDs over other light sources is that they produce a lot less heat. This reduction in heat also means better efficiency because less energy is wasted as heat and more is used to produce light. Overall, LEDs last longer than other light sources, such as lightbulbs.

different colours

The different colours of LEDs are achieved by using different compounds. Table 1 shows a table that I have put together of some of the compounds that are used.

compound	colour emitted
Gallium arsenide phosphide Aluminium gallium indium phosphide Gallium phosphide	RED
Gallium arsenide phosphide Aluminium gallium indium phosphide Gallium phosphide	AMBER
Gallium arsenide phosphide Aluminium gallium indium phosphide Gallium phosphide	YELLOW
Gallium phosphide Gallium nitride Indium gallium nitride	GREEN
Indium gallium nitride Aluminium gallium nitride Zinc selenide	BLUE

table 1. compounds used to achieve different wavelengths

current limiting resistor

There is a maximum amount of current that you can pass through an LED. If you exceed it by too much, it will be destroyed! To prevent this from happening, we connect a resistor in series with the LED. To calculate the value of resistor required, we need to know the voltage drop of the LED. LEDs have different voltage drops. Figure 1 shows part of a datasheet of an LED and we can see that the voltage drop (forward voltage) is 2V.

voltage drop forward voltage of LED — figure 1. voltage drop of LED

If we imagine that we have a 5V supply connected across the resistor and LED, we can calculate the voltage drop across the resistor:

VR = V - VLED
   = 5 - 2
   = 3V

Now, we need to know what current we would like to pass through the LED. Looking again at the datasheet, we can see that the forward current at the forward voltage of 2V was tested at 20mA. We now have the forward voltage and forward current and we can use Ohm’s law to calculate the resistance that we require:

R = V / I
  = 3 / 0.0002
  = 150Ω

We know now that to use our LED safely, we need to have a resistor in series with it, with a resistance of 150Ω.