LED Sensing ~ BI-Directional LEDs Part – I
Sunday, 3 May, 2009 § Leave a comment
In my quest to find other methods of input detection for multi-touch and multi-modal hardware i came across another technique made famous by Jeff Han it is the use of LED s as bi-directional sensors. This technique is in no means new, it know fact that LED s can behave in such a way that, they act as sensors, when they are reverse biased. The theory behind how this all works is explained below.
It is a little known fact that an LED can be used not only to output light but also to sense it. From a basic electronic standpoint a diode is simply a PN junction, so are light emitting diodes (LEDs) and photo diodes, therefore essentially all diodes are the same thing in that they are a PN junction.
Any semiconductor can be doped with one of two types of impurities (from either group 3 or group 5 in the periodic table) that result in material that conducts electricity in two different ways. One is with an excess of electrons is called N type and the other with a shortage of electrons is called P type, this varience is due to the nature of the doping element which either create an excess or shortage in electrons. This is because a shortage of electrons can behave, as far as electricity conduction is concerned, just like an excess of positive charges. These positive charges are called holes in recognition of what they are, the absence of an electron. So electricity can be conducted by the flow of holes in P type material or electrons in N type.
So when a P and N type material come together to form a junction some of the holes in the P type materiel move into the N type material where the combine and cancel each other out. Similarly some electrons at the junction move into the P type material and cancel each other out. So around the junction an area of materials builds up that has neither electrons or holes and so will not conduct electricity at all. This is known as in intrinsic region or I type. This seals off the path for the migration of any more holes and electrons and the I region stabilises.
To get electricity to flow you have to apply an external voltage that adds more electrons to the N side and more holes to the P side. This builds up a potential that is big enough to break through this intrinsic layer. However, if you put a smaller voltage across it you just put an extra charge or electrostatic pulling power across the I-Region.
Now if a single photon (light particle) enters into this I-Region it interacts with an atom and forces it release energy (i.e. it causes the electron to jump into a higher orbit). This frees up an electron for conduction and it also leaves a hole behind where the electron originally was.
we say we have generated an electron hole pair, that is one electron (free) and one hole. These would normal recombine due to there oppsing nature, but the electric field across the region pulls them apart causing a tiny current to flow. The more photons that enter this light I-Region the more the current adds up and so you have a current that is proportional to the light falling on the diode, this is called a photo current.