The lowly diode, a device with only two leads, can nonetheless do many things. Diodes can detect, rectify, suppress, emit light, detect light, change capacitance, emit microwaves and more. This wide range of use means diodes are included in almost every design and it’s well worth learning more about the inner workings of all kinds of diodes.
My introduction to diodes started like many of my generation with a homemade crystal radio set. My first diode was a piece of pencil graphite in contact with an old fashion safety razor with the joint of the two dissimilar materials — graphite and steel — creating the diode. In this configuration the diode is said to be “detecting” which is the act of turning a weak radio signal into a weak audio signal. At least in my home town of Marion Indiana, one radio station was stronger than the other so that I didn’t have to listen to two stations at once.
I eventually learned about “real” diodes and the 1N34A Germanium diode was my “goto” diode into my teens. Nowadays looking into a modern version of the 1N34A you can still see the semblance of the old “cat’s whisker” by looking carefully into the diode.
A quick and somewhat inaccurate semblance of the way a diode works can be demonstrated with marbles and jacks representing negative electrons and positive “holes”. Holes are basically an atom missing an electron due to the combination of elements, a process known as doping. Join me after the break for the explanation.
At rest some of the electrons and holes combine to produce an electrically neutral atom that acts as an insulator since electrons cannot readily jump to and from the atom now that it’s locked up.
If we apply a voltage in a reverse direction, holes and electrons pull away from the center and no current flows, the central zone, known as the depletion zone, gets larger. Connecting a voltage in a way that makes more electrons, (more “potential”) available to the N material starts to push electrons across neutral barrier a process known as breakdown. In the video I show the electrons (marbles) pushing the depletion off one end, in real life there is a crystal lattice and holes are flowing in one direction while electrons are flowing in the other, the goal was to show a simple difference about why a reverse connected diode works different that a forward biased diode.
Schottky and Zener Diodes
Schottky Diodes are available that offer a lower Forward Voltage Drop than silicon which also means less power dissipation. As a quick visual of the difference between a silicon diode and a Schottky diode we subjected the devices to a 1 volt ramp signal and visually demonstrate the differences as shown below.
It should be noted that this isn’t a common way to use these devices, this was mostly to show the proportion of a 1 volt signal that the diode selection can affect. In the older days we mostly saw Schottky in RF applications as the low capacitance and fast recovery time worked well at high frequencies.
Another piece of information that I picked up while young was that a good number of Zener Diodes were not really true Zener Diodes they were in fact avalanche diodes. With that said they did pretty act much like their Zener cousins and so the lack of visible differences allowed users to indiscriminately use either.
Alas, back then in the before time, there was no Internet to look up or verify facts like this, so one pretty much had to commit brains cells for storing the semi-useful facts on the off chance it would make be needed some day. Also I must admit that I was curious if I could tell the difference between the two so out came the 5 digit Keithly VOM and a heat gun.
As it turns out the identification between a Zener and Avalanche diode can be ascertained by checking whether it has a positive or negative coefficient (reaction to heat). Shown are two different voltage diodes after the meter has been zeroed and the diodes heated. An interesting fact is that at around 5.6V the breakdown voltage is due to roughly half from the Zener effect and half from the Avalanche which results in the temperature characteristics tending to cancel out.
If this discussion got you thinking about the effects of temperature change, make sure you watched my video about calculating component heat.
Crude Voltage Regulation
A very common voltage regulator was made back then with a power transistor in a Common Base configuration with the base voltage set by the 5.6v Zener. The circuit shown here is a somewhat crude regulator and didn’t have the kind of load regulation and other qualities we assume that we get with our 3 terminal regulators.
Whether the diode is a Zener or Avalanche (or both) isn’t real important if the part is specified correctly, with the exception that transient voltage suppression diodes tend to be Avalanche style.
Finally when working with analog circuits where the forward voltage drop is significant, an OpAmp can be used to overcome the drop while still providing a directional conduction. In this case the diode is included in a negative feedback loop where most of the undesirable effects are canceled out. Shown working here as a simple rectifier, variations of this circuit find their ways into peak and hold circuits and audio detectors.