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 now have the background information about semiconductors that we need to start to talk about digital photography – about digital array displays and digital sensor arrays.  Let’s start with a discussion of LED (light emitting diode) displays.  If you sigh at this point and try to escape to elsewhere on the web, chances are that an LED display will still be staring you in the face.  If you instead try to escape by watching television, the chances are still very high that an LED display is staring you in the face.  So under the assumption that it’s worth knowing as much about your computer as it knows about you, let’s proceed.

In my last technical blog, I discussed p and n type semiconductors.  In and of themselves, they’re not too exciting.  However, the fun begins when we start to combine them.  Suppose we put a p type semiconductor right up against an n type semiconductor.  This is what is called a p-n junction.  This is also called a diode and is shown graphically in Figure 1, where we’ve even allowed for a small transition region.  The p region is shown in blue and the n region in green.  You’ll note that we’ve connected this to a battery.  The plus terminal to the p side and the negative terminal to the n side, as one might expect.  Once the battery is connected, the electrons start to flow towards the plus terminal (remember that opposites attract).  Remember also that electrons can flow.  Holes really only appear to flow because the electrons are flowing.  So there is a current in the semiconductor, with electrons flowing from the n region into the p region and on to the plus terminal of the battery.  Of course, electrons exit the negative terminal of the battery and enter the n side of the diode.  If you hooked the battery up backwards there would be no current.  A diode is unidirectional.

As an aside, in the center we see the symbol for a diode, with its big honking arrow showing the direction that current flows.  This is because back in the eighteenth century no less than Benjamin Franklin made the unlucky guess that the charge carrier was not the negative but the positive charge, and this view is perpetrated by convention in circuit theory today, even though everyone knows that it is wrong.

Figure 2 – Large size red, green, and blure LEDs the fundamental components of an LED display. From the Wikicommons by PiccoloNamek and in the public domain under creative common license.

The key thing about the light emitting diode (LED) is that when the electrons enter the p region they recombine with and neutralize holes.  This represents as shown in the lower portion of the schematic a transition from the conductance band back to the valence band with the emission of light whose color depends on the transition energy.  Hence, electrical energy is converted to light at the junction.

In Figure 2, I show diodes of the three primary colors: red, green, and blue.  If you take a magnifying glass up to your LED display, you will see minute arrays of these red, green, and blue LEDs each of which is individually addressable, meaning the light intensity of each can be individually controlled so that whatever color you and intensity you desire is created by additive color.  That is basically, how an LED display works.

Figure 1 – Schematic of a semiconductor crystal at absolute zero where all of the electrons are bound up by holes.  (c) DE Wolf 2013.

As we have seen, the fundamental silver halide chemistry of analogue photography depends upon the physics of semiconductors.  And yet, it was developed before anyone knew about semiconductors or even uttered the word.  Indeed, in those days they didn’t need to utter the word “analogue” in relation to photography, because there was no digital photography to contrast it with.  The same is true for watches.  In the “dinosaur ages,” when I was a lad, there were just watches not analogue watches and certainly not digital watches.  Indeed, the time is fast approaching when there may be no watches except during retro-fashion fads.

But, as I promised, semiconductors will explain a lot of the technical aspects of digital photography.  To get there we need to discuss one more aspect of semiconductor physics – qualitatively I promise.

At absolute zero the valence electrons have no excess kinetic energy and are all sitting in their valence bands.  This is shown in Figure 1.  All of the positively charged holes are paired up with negatively charged electrons – very boring.  Now as we heat up the semiconductor to room temperature, some of the thermal (heat) energy gets absorbed by the electrons and they can escape to the conduction band where they are free to move.

Figure 2 – Schematic of a semiconductor crystal at room temperature where some of the electrons have escaped the lattice and are in the conductance band. There are functionally two types of charge carriers: the free electrons and the positive holes. (c) DE Wolf 2013.

Free to move means that if I attach a battery to two sides of the crystal current will flow.  Recognize, that as they flow in the current some electrons may recombine with holes.  But then new holes appear as new electrons escape and the net effect is that electrons move form the negative side of the battery to the positive side (remember that opposites attract).

But wait! Imagine that you are watching this process from  a distance.  Is it the electrons that are moving or is it the holes.  They both appear to be moving.  It’s like the sensation of being on a train and suddenly the platform appears to be moving.  Of course, we know that the electrons have escaped the crystal lattice, while the atoms (holes) have not.  But it does look like the holes are moving, and you can even calculate a speed for this motion.

The next question that we need to ask is whether there is a way of modifying the semiconductor crystal so that has more electrons or more holes?  Well, remember that whether a material is a conductor, a semiconductor, or an insulators depends on how tightly the valence electrons are bound to the positive atomic nuclei.  Remember also that the solid-state crystal acts as a unit not as a set of individual atoms.  The net-net of all of this is that if we add a small amount of a material that tends to donate electron the crystal will have more free electrons (in the conductance band).  If we add a small amount of a material that tends to bind up electrons (in the valence band) it will have more holes.  The materials added are called “doping agents.”  Semiconductors with excess negative charge (electrons) are called “n-type semiconductors.”  Semiconductors with excess positive charge (holes) are called p-type semiconductors.

The cool thing is that doping can be a very localized process, and as a result, you can build up some very complex semiconductors capable, for instance, of creating a computer and displaying your photographs on a light emitting diode display.  That will be the topic of our next technical blog.