The characteristic curve of a junction diode is also called an I-V Curve. It is typically a graph showing the current flow at different voltages. The current is typically on the y-axis, and the voltage on the x-axis.
This type of graph provides engineers with a visual record of the operating characteristics of the component. This information enables them to use the component more appropriately within a circuit.
There are many different types of diodes, and they all have different characteristics curves and applications. Here are some diodes you might come across in the future: Zener, Germanium, Gunn, Tunnel, and Schottky. This article looks at a typical silicon diode.
All of these pretty graphs are indicating one thing. They are indicating that a forward-biased diode is not a linear device. In simple language, the current that flows through it is not proportional to the applied voltage. If you can remember this bit, then you can go to the top of the class.
A junction diode is just a slab of p-type semiconductor material in contact with an n-type semiconductor material. Together they form a junction.
You have not got the first clue about these positive and negative signs in the semiconductor material. You do not know what they mean or how to visualise them, so let me explain.
The p-type semiconductor material has plus symbols. These represent holes that are lacking an electron. When an atom has an electron missing, or a shortage of electrons, its overall charge becomes positive. That is what the plus signs mean in the diagram!
The n-type semiconductor material has negative symbols. These represent an excess of electrons. When an atom has an excess of electrons, its overall charge becomes negative.
In scientific literature, and if you want to sound clever, holes can also be called carriers, because they carry the electrons away. As you can see in the animation, the red circles are positive, representing holes. The blue represent negative electrons, which fill up the holes. They move across the semiconductor material by filling the holes… Kind of like stepping stones, except that it happens very fast.
A diode is considered forward biased when the cathode (n-type silicon) is connected to a negative potential, and the anode (p-type silicon) is connected to the positive potential.
In this situation, the negatively charged electrons at the cathode end are attracted to the opposite side which has the positive potential. Hey, do not call a NASA engineer just yet, this is sounding too easy, you might think. Well it is!
The movement of these electrons does not happen straight away because of a barrier at the junction of the diode – shown in yellow. This barrier will prevent the flow of electrons until the potential difference across the junction reaches around 0.6 V. Therefore the barrier determines the cut-off voltage!
Once the electrons jump over to the opposite side, they occupy the space represented by the plus symbols. These are the holes, or carriers, that carry the electrons away...
As the voltage is increased further, to ~0.7 V, and ~0.8 V, the diode reaches a fully conducting state, and the current flow through it rises very fast. As you can see in the graph, the curve is almost vertical at around 0.8 V.
Diodes have a limit to the amount of current that they can pass. For example a common garden variety 1N4148 can pass a maximum of 200 mA. At the maximum current, it will begin dissipating heat, and the curve will drift. This means that diodes are temperature dependent. Their characteristics can change in different temperatures.
Barriers are everywhere. The class barrier, colour barrier, ticket barrier, and this is a junction barrier. Like all barriers, they are not always intentionally made, but simply form themselves, and are generally a right pain.
Understanding the barrier is an important part of understanding the silicon diode characteristic curve. This is because the barrier determines the 0.6 V cut-off regions on the graph.
If you are building a crystal radio, then you will be interested to know that a germanium signal diode has a cut-off region of only 0.3 V. This means that the germanium diode can operate at much lower forward voltages than its silicon counterpart.
If the applied voltage across the junctions is enough to overcome the barrier voltage, then the electrons will be able to flow, and the diode will conduct.
Due to the non-linear nature of the graph, the precise threshold voltage is sometimes found through a little geometry with the curve.
Actually, it is a very simple and non-painful procedure. There is a straight part to the diode characteristic graph. All you have to do is to extend this part until it crosses the x-axis (Vf). The point of intersection is the threshold voltage.
A diode is considered reverse biased when the cathode (n-type silicon) is connected to a positive potential, and the anode (p-type silicon) is connected to the negative potential.
If the diode was an ideal component, it would not conduct at all in this situation, however no component is ideal and the diode does conduct a negligible amount. The current that flows is very small and called the leakage current. It is typically in the micro-ampere range, which is why the reverse current axis scale is different and best expanded when plotting the characteristic curve.
The leakage current has the effect of transforming the barrier regions back into their respective p-type and n-type properties. This occurs because the electrons and holes re-align to their original locations. Electrons move back to the n-type material, and the holes move back to the p-type material. This in turn causes heat within the diode which has the effect of lowering its resistance.
As you can see in the graph, there comes a point when the reverse current will no longer increase. Any increase in reverse bias voltage will keep the reverse current to almost the same.
Then something interesting happens at the knee part of the curve. This point is called the breakdown voltage. Suddenly there is an increased flow in reverse current. No matter how much reverse voltage is applied, the voltage across the diode does not change.
In specification sheets some manufacturers refer to the reverse current, or leakage current, as the point near the knee, just before breakdown occurs.
Useful Diode Parameters
The maximum forward bias current (If) of a diode is a very useful parameter. Different diodes are capable of carrying different amount of current. If you are placing a protection diode into a circuit, you will need to know how much current your circuit will draw, and whether the diode is up to the job of carrying it.
The forward voltage drop (Vf), also known as the cut-off voltage, is a very useful parameter worth knowing. This parameter is defined at a specified current and temperature in datasheets. This value tells you that the diode will continue conducting even when the voltage falls as low as that figure.
What most people do not realise is that the forward voltage drop is ideally specified when the forward current is zero. This is when the curve cuts the x-axis and the current is zero.
If you were a crystal radio enthusiast, you would be interested in this parameter because it determines how sensitive your crystal radio diode is.
Diodes are very sensitive to heat and their resistance can change creating a drift. This is why datasheets always mention the parameters taken at a specific temperature. Typically it is at 25 °C.
The Flaming Diode!
The diode has been the bane of all high-school students and teachers. I remember sitting in class and the teacher was going on about forward bias, and reverse bias, and breakdowns… And even he was about to have one as well…
The person to blame is of course Fleming, or more accurately, Sir Ambrose Fleming. Of course, he never saw the characteristic graphs on an oscilloscope. They did not have oscilloscopes back then. The apparatus they used closely resembled to what you would see in vintage Frankenstein movies, with thunder bolts of lightening, and huge lever switches being engaged.
Fleming was trying to invent the Oscillation Valve, but ended up inventing the diode instead. Of course, my mother always said, as I am sure yours must have too, that to reach for the moon, you should always aim for the stars. That way, when you crash and burn, you will at least have some moon dust on your head, when it hits the rocks…
Most of this stuff is very useful, but of course, you’ll get your degree through your overpriced education, and forget all about diodes. Then one day, you’ll be building a crystal radio for your children and finding out that the germanium diodes you ordered from ding dong on eBay are in fact silicon! Then of course, you are going to have to go through all of this again to figure out the difference between germanium and silicon diodes!
So what do I think? I figured I might as well make an article! It might even get top rank!