Diode

Diodes

A diode, or "rectifier,"  is any device through which electricity can flow in only one direction. The first diodes were crystals used as rectifiers in home radio kits. A weak radio signal was fed into the crystal through a very fine wire called a cat's whisker.  The crystal removed the high frequency radio carrier signal, allowing the part of the signal with the audio information to come through loud and clear. The crystal was filled with impurities, making some sections more resistant to electrical flow than others. Using the radio required positioning the cat's whiskers over the right kind of impurity to get electricity to flow through the crystal to the output below it.

At the time, though, no one really understood about the impurities -- then in 1939 Russell Ohl accidentally discovered that it was the boundary between sections of different purity that made the crystal work. Now that the way they work is understood, manufacturers make crystal diodes that work much more consistently than the ones in those original radio kits.

A crystal diode is made of two different types of semiconductors right next to each other. One side is easy for electrons to travel through; one side is much tougher. It's something like trying to swim through a pool filled with water and then a pool filled with mud: swimming through water is easy; swimming through mud is next to impossible. To an electron some semiconductors seem like water, some like mud.  (For more information, read about semiconductors in Everything You Ever Wanted to Know about Conduction.) 

One side of the semiconductor boundary is like mud, one like water. If you try to get electricity to move from the mud side to the water side, there's no problem. The electrons just jump across the boundary, forming a current. But try to make electricity go the other way and nothing will happen. Electrons that didn't have to work hard to travel around the water side just don't have enough energy to make it into the mud side. (In real life, there are always a few electrons that can trickle in the wrong direction, but not enough to make a big difference.) 

This boundary has turned out to be crucial for our daily lives. Diodes change the alternating current that comes from your wall outlet into the direct current that most appliances need. And transistors need two such boundaries to work.

A semiconductor diode consists of a semiconductor PN junction and has two terminals, an anode (+) and a cathode (-). Current flows from anode to cathode within the diode (according to the high to low circuit analysis method), but only when there is at least a certain amount of forward voltage applied. When positive voltage is applied across the diode, it is called a forward bias, whereas a negative voltage is called a reverse bias.

A diode is best described as a one way valve, since it only allows current to flow from anode to cathode. For example, if you applied a reverse bias to the diode with a magnitude of 5 volts, current would not flow. If you applied 5 volts with a positive bias, current would flow.

This seems pretty simple, but there are exceptions to the one way valve analogy. For example, diodes have a minimum forward voltage level to allow current to flow. In most cases, about .7 volts are needed to trigger current flow. You can see this from figure 1.3 below. The current does not start to flow until a certain amount of forward voltage is applied.

Another exception is the breakdown voltage. All diodes have a point where, if the reverse voltage is high enough, the semiconductor structure will break down, allowing current to flow. This value is usually fifty volts or higher and when the breakdown voltage is reached, it generally damages or destroys the diode.

Why is a diode useful? Because it can be used for rectification, protection of components from reverse voltage, and creating interesting wave shapes. For example, say you have an electrolytic capacitor that can only withstand 10V of reverse bias voltage. All you have to do is place a diode in front of it and it will block most reverse voltages from destroying the capacitor. Rectification is the process of converting an alternating current signal into a direct current signal and is used in all AC to DC converters and power supplies.

An ideal diode's operation, shown above, would cause full current to flow immediately upon application of forward voltage. However, this is not the case in the real world, and with real diodes the forward current increases with additional forward voltage and there is also a limit to reverse voltage before the diode breaks down and allows reverse current. The operation of a real diode is shown below.