Diodes are a two-terminal electronic component that conducts electric current in only one direction. The term usually refers to a semiconductor, the most common type today. This is a crystalline piece of semiconductor material connected to two electrical terminals.
A vacuum tube type (now little used except in some high-power technologies) is a vacuum tube with two electrodes: a plate and a cathode.
The most common function of a semiconductor is to allow an electric current to pass in one direction while blocking current in the opposite direction (the reverse direction). Thus, the semiconductor can be thought of as an electronic version of a check valve.
This unidirectional behavior is called rectification, and is used to convert alternating current to direct current, and to extract modulation from radio signals in radio receivers.
Below is an illustration of a semiconductor symbol. Remember that the (Anode=positive) is the triangle and the (Cathode=negative)is the straight line.
A good way to remember this is that the "A" in the anode is shaped like the triangle in the symbol.
The cathode is always symbolized by a line stamped on the end of the semiconductor and by a straight line in the electronic symbol for the semiconductor.
However, semiconductors can have more complicated behavior than this simple on-off action. This is due to their complex non-linear electrical characteristics, which can be tailored by varying the construction of their P-N junction.
These are exploited in special purpose semiconductor that perform many different functions. For example, some specialized Semiconductors such as Zener types, are used to regulate voltage.
Varactors are used to electronically tune radio and TV receivers , Tunnel types generate radio frequency oscillations , and Light-emitting types produce light. Tunnel types exhibit negative resistance, which makes them useful in some types of circuits.
"Diodes" were the first semiconductor electronic devices. The discovery of crystals' rectifying abilities was made by German physicist Ferdinand Braun in 1874.
The first semiconductors, called cat's whisker, developed around 1906, were made of mineral crystals such as galena. Today most semiconductors are made of silicon, but other semiconductors such as germanium are sometimes used.
Here are some common electronic symbols for various semiconductors. These semiconductors will be covered in more detail in later tutorials.
Schematic symbols for semiconductors.