Figure 1 – Schematic of Bayer filter overlain on a CCD (top) also showing how rede, blue, and green light is separated by the filter. From the Wikicommons and in the public domain.

Now that we understand how a charge coupled device or CCD works and how it reads out, we are in a position to consider how exactly one goes about making a color digital camera sensor out of one.The first question is how does one make an intrinsically black and white sensitive sensor sensitive to color.  If you followed our discussion about autochromes and additive color, the answer is obvious.  You overlay the pixels with some kind of set of red, green, and blue filters.  Because everything needs to be electronically addressable and translatable from imaging device to display, say a computer screen, it’s better not to use a random distribution of filters, like in an autochrome, but rather an ordered one.  Figure 1 shows schematically such a filter system, referred to as a Bayer filter.

Figure 2 – Image acquired with a CCD chip through a Bayer filter. From the Wikicommons and in the public domain.

Notice that we have twice as many green filters as we do red or blue filters.  The reason for this is that the human eye as twice as many green sensitive cells as it does red or blue sensitive ones.  Figure 2 shows an example of an image, where we have focussed in very closely so that we can see the individual filters of the Bayer filter.  You can do the same thing at home with  a digital TV or computer screen and a magnifying glass.  So far so good, but there’s a second very important question to consider – the question of how to make the sensor as sensitive as possible.  Even though the intrinsic quantum efficiency of a CCD can be remarkably high (as high as almost one electron per photon) several factors conspire against us.  There is dead space between pixels and off angle light isn’t going to make it to the photosensitive region.

If you look at Figure 3 you see a schematic of a cross section of a pixel that illustrates several aspects of camera sensor design.  Let’s walk through this.

1.  is the incoming light.
2.  is a lenslet that collects light to the pixel.  It can be shaped so that even light over dead zones makes it to the sensor pixel.
3. is the Bayer filter element.
4. is the transparent voltage gate.
5. is the silicon dioxide insulator.
6. is the n-doped silica layer
7. is the p/n silica layer
8. is the p-doped silica layer.

Figure 3 – Cross section of a pixel on a color digital camera CCD sensor. From the Wikicommons and in the public domain.

The lenslets are very important elements.  Without them the device can become quite insensitive.  This is especially true as we push greedily for more and more pixels.  If you have a certain amount of light striking a pixel and then cut the pixel size in half, so that you have four pixels, where previously there was one, then each pixel gets only a quarter as much light.  The well depth similarly goes down.  So the bottom line is that you want as much signal as possible.  Signal compared to what?  Yes you guessed it.  Signal compared to noise!

Figure 1 – Charge transfer in a CCD array. By Michael Schmid from the Wikicommons and under creative commons license.

The real secret of a charge coupled device is how it uses programmed voltages to shift the stored charge between pixels.  This is illustrated schematically in Figure 1.  Once the exposure is complete.  The device systematically shifts the charge from pixel to pixel until it reaches the readout point.  This is accomplished as shown in the animation by taking a pixels voltage to zero which allows it to transfer to the adjacent pixel held at a voltage V. The process, illustrated in Figure 2 is not unlike a “bucket brigade.”

Figure 2 – The march of the electrons in a CCD sensor. (c) DE Wolf 2013

By the way, I think that you can see the advantage of using a shutter on a CCD camera.  It protects the sensor from light while it is in the process of reading out.

At the output the accumulated charge acts as a voltage.  A device known as a analogue to digital converter (A/D) converts this voltage to a digital signal.  Significantly the signal is analogue up until this point, when it becomes digitized.

Figure 3 – A CCD sensorarray. From NASA via the Wikicommons and in the public domain.

There’s a lot to talk about regarding CCDs.  However, let’s focus for now on two points, just to give a flavor.

• As charge is transferred between pixels there’s a little slop over, just like spilling water in a bucket brigade.  As a result the readout process introduces a source of noise in the signal.  This means that the amount of charge transferred per pixel is variable because of the readout, in addition to all the other reasons that it can be variable.
• The dynamic range of the device, that is the number of grey levels that it can be divided into is defined, not by the number of bits in the digitizer, but rather by the number of electrons.  We discussed many months ago the concept of  photon counting noise.  This is controlled by the number of electrons in the well, by the well depth.

You can also see that I am very focused on sources of noises.  This is what it’s all about.  If you want to know how sensitive of CCD is, or if you want to discuss proper exposure, or dynamic range, or ISO you’ve got to worry about what physicists and engineers call the signal-to-noise ratio.

But for now, we are getting ahead of ourselves.  And I’d like finally to show you a typical CCD sensor array, which is shown in Figure 3.

Figure 1 – Schematic of the structure of a pixel element of a CCD array (c) DE Wolf 2013

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.

Figure 1 – schematic of the p-n junction of an LED. Top shows distribution of electrons and holes in the two regions. Bottom shows the conductance and valence bands. From the Wikicommons by S-kei and in the public domain under creative common license.

We have previously discussed how a p-n junction can be used to produce a light emitting diode (LED) and how an array of LEDs can be used to create a digital display.  I redisplay Figure 1 from that blog to remind you.  So lets recall a few critical points.  First, the n-type semiconductor is rich in electrons.  Second, the p-type semiconductor is deficient in electrons, that is is rich in holes.  Second the application of a voltage across the diode (p side positive, n side negative) causes a current to flow and drives electrons from the n-type semiconductor to the junction, where they combine with holes; thus falling out of the conduction band into the valence band.  When they do this, they emit light.  Hence, we have a light emitting diode.

Now the whole process can, in fact, be reversed.  Physicists have a really cool expression.  They say that “the system is invariant under reversal of time.”  Whoa! Time reversal. Shades of H. G. Wells.  Actually this whole question of time reversal in physics is rather fun.  In our everyday world time is an arrow.  It moves from the past to the present and then on to the future – in inexorably.   But the equations of physics they don’t care they can go either way.

Figure 2 – Examples of discrete photodiode light detectors. From the Wikicommons and in the public domain under GNU licsense

And for our little p-n junction this means that if we can apply a current (aka electricity) and create light we can apply light and get electricity.  So if you have a p-n junction and shine light on it (at the correct wavelength), electrons get lifted from the valence band to the conductance band.  What happens next depends on whether you have a battery hooked up to it.  If you don’t then the electrons build up at the junction creating an electric field or a voltage.  The size of the voltage depends upon the amount of light.  This is the so-called photovoltaic mode.  On the other hand if you attach a battery in the opposite to that shown in Figure 1 (meaning that the plus side is attached to the n-type material and the minus side to the p-type material, referred to as reverse bias) then the as electrons rise to the conductance band they are swept to the positive battery terminal, a current is created.

The reason that the battery needs to be set up reverse bias or backwards to that shown in Figure 1 has to do with our desire to run the system in reverse time.  If running electrons one way causes light, we expect that light will cause the electrons to run the other way.

So we can use a p-n junction to create light, a light emitting diode.  And we can use a p-n junction to create either a voltage or a current.  The latter means that we can use a p-n junction to measure light.  Some example of discrete (stand alone) photodiodes are shown in Figure 2. These devices are at the heart (eyes?) of how many light meters on cameras work.  And we are one step further to understanding how a digital camera’s detector array works.