Scientists have found that certain materials will act as an insulator, but can be tricked into conducting when the circumstances are correct. These materials are called semiconductors. A semiconductor will usually be made from two similar materials; one has a slightly positive ion, the other a slightly negative ion. The point where the two materials meet is called a junction. The slightly positive material is the P-junction, and the more negative material is the N-junction. The unit is therefore called a PN Junction.
 

A simple junction with no voltage applied.
No Battery At rest.		No Battery At rest.
The material on the left is called "N" while the material on the right is called "P". The "N" material contains a higher number of negative ions, therefore it has a slightly negative charge on an atomic level. The "P" material has more positive ions, so it has a more positive charge when compared to the "N" material. The ions are happy to rest on either side of the junction.
The junction in a reverse bias condition.
Battery (+) No current flow.		Battery (-) No current flow.
Opposite charges attract, like charges repel. Since the material in the "N" section of the junction has a slightly (-) charge to it, the junction widens as the negative ions are attracted to the positive charge. The same attraction occurs on the "P" side of the junction, the positive ions are attracted to the negative charge of the battery. The resistance of the semiconductor goes up very high and almost no current flows across the junction. The material turns into a good insulator. This is known as a REVERSE BIAS condition.
The junction in a forward bias condition.
Battery (-) Current flows!		Battery (+) Current flows!
With the polarity of the battery matching the materials, the negative charge repels the negative ions so that they move closer to the junction point. From the other end the positive ions are also repelled and travel to the junction point. Since the ions are forced to be very close together, a small voltage will cause the current to flow across the junction. Most diodes begin current flow once the voltage level reaches about 0.6 volts. (Depending on the material used in the junctions.) This is known as a FORWARD BIAS condition.

Since the diode has a forward bias voltage of about 0.6 volts, it will not conduct until this voltage is reached. Once the voltage goes above 0.6 volts, the diode will act like a low value resistor in the forward direction. If the current is reversed the diode will snap shut and not allow the current to flow. It will act like an ultra high value resistor.

Each diode is rated to carry a certain amount of current, once this level is exceeded, the diode will begin to overheat. Once the diode gets hot enough, the junction can melt. If this happens, the diode can start to conduct on both directions and will be destroyed. Another thing to remember is that even though a diode won't allow current to flow in the opposite direction under normal conditions, it is possible to introduce enough voltage that it jumps across the junction anyway. This will cause damage to the diode and it will not function any more. (The junction may vaporize!)
 
 

Diode Types
	Switching diodes are used to convert low current AC into DC. Because they only allow current to flow in one direction they can force the AC voltage to only swing in the positive direction.
This is an AC waveform. Notice how the voltage swings above and below the 0v reference? This means current would flow forward and then backwards through the circuit. This is a switching diode and its schematic symbol. When this diode is used to trim off the negative voltage it only effects half the waveform. It is called a half wave rectifier. Here you can see the resulting waveform from a half wave rectifier. You will notice that the negative part of the waveform has been removed. Again we start with an AC waveform that has both positive and negative voltage. This combination of four diodes is called a full wave rectifier because it effects both the positive and negative waveforms. The negative waveforms are routed into positive waveforms. As you can see, the waveform consists of all positive waves, both the positive and negative waveforms from the input have been recovered.
	Zenner diodes are special in that they have a higher threshold for switching into the forward bias mode. Where standard diodes require about 0.6v, zenner diodes are available in different values. (3 volts in this example.) This makes the Zenner diode useful in voltage regulators.
	The LED or Light Emitting Diode is constructed so that when voltage is applied, light is produced. Most LED's operate from 2.0 to 3.0 volts. Some of the newer blue LED's operate at 4.0 volts. As the current in the LED increases so does it's light output. Once you go above the maximum current for the LED however it will overheat, become dimmer, and eventually fail. It is possible to drive them slightly above the recommended level if they are pulsed so that they are on half the time and off half the time. LED's have very defined colors because of the way they are manufactured. LED's that operate in the infrared bandwidth are invisible to our eyes. These are used for burglar alarms, television remote controls, and sensors in robots. Another special LED that has been produced emits a powerful beam of light which is highly coherent, this is called a LASER Diode.
	All diodes are sensitive to light to some degree, the photodiode is very sensitive to light. It has a large resistance in darkness, then when light is shined onto the junction, it's resistance lowers depending on the amount of light. Many photodiodes are made into a metal container with a clear opening. Some are available which look like an LED, many are sensitive to infrared ( IR ) light. These are used with the IR LED's mentioned above. Interesting note: A solar cell is simply a VERY LARGE photo diode! The surface area is large enough that the electricity it produces is enough to overcome the 0.6 volt bias level and have enough remaining be used as an energy source.