In our last technical blog we discussed how semiconductor junctions can be used to create electrical signal. This is a good start, and you can certainly imagine that if you made a two dimensional array of such photodiodes you would have an imaging devices. There is one problem however. The device that we described pumps out electricity at some rate. While this rate can be adjusted, it gets hard to integrate signal for large amounts of time like we do with photographic film. For this purpose the charge coupled device, the CCD, comes to our rescue.
There are a lot of fascinating and important points to consider about CCDs. The first is how do they store your image until you have achieved sufficient exposure to read it out. Take a look at Figure 1, which shows schematically the structure of a single pixel element of a CCD array.on. First of all, notice that we have a gate, basically a metal electrode. For starters it is held at a positive voltage. Next we have a layer of silicon dioxide. Yes sand! And that’s an insulator that does not carry electrons. It acts like a barrier that keeps the electrons away from the gate. Then we have a thin layer (called an epitaxal layer) of n-type silicon. This is the photosensitive region, where electrons are easily stripped off by light. The epitaxal layer sits on a layer of p-type silicon. A negative voltage is applied to the back side of the p-type silcon layer.
What about the photoelectrons? They can’t go to the gate. They won’t go to the negative electrode on the back side. So they start to fill up the holes in the p-type silcon layer below the gate. The longer the device is exposed to light the more electrons are created and bound up by holes until there are no more holes available. You can think of the holes as forming a well that collects electrons, and then the maximum number of electrons that the well can hold is the well capacity.
We seem to have just what the doctor order. It is a device which stores charge proportionally to the light exposure. as a function of exposure. It is effectively an electronic film. You can certainly see how you can build up an array of these pixel. So really the big question that remains is how to read it out. That is the subject of our next technical blog.