Diode is one of semiconductor devices here is a piece of information about it.
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.)
