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Diodes function as the electronic version of a check valve - they allow electric current to flow through them in one direction, but block current from flowing through them in the opposite direction.

Although it is not completely accurate, a diodes electrical characteristics can be roughly approximated by two linear regions. Below a certain difference in potential between the two leads, the diode can be thought of as an open (non-conductive) circuit. As the potential difference is increased, at some stage the diode will become conductive and allow current to flow, at which point it can be thought of as a potential "source" with zero (or at least very low) resistance.

Diodes are sometimes known as rectifiers for their use to "rectify" alternating current electricity into direct current.

The first diodes were vacuum tube devices (also known as "thermionic valves"), arrangements of electrodes surrounded by vacuum within a glass envelope, superficially similar to incandescent light bulbs. The arrangement of a filament and plate as a diode was invented in 1904 by John Ambrose Fleming scientific adviser to the Marconi company based on an observation by Thomas Edison. Like light bulbs, vacuum tube diodes have a filament through which current is passed, heating the filament. In its heated state it is ready to release electrons into the vacuum. These electrons are electrostatically drawn to a positively charged outer metal plate called the anode, or more commonly just the "plate"'. Electrons do not flow from the plate back toward the filament, even if the charge on the plate is made negative, because the plate is not heated.

Although vacuum tube diodes are still used for a few specialist applications, most modern diodes are based on semiconductors. A semiconductor diode consist of an N-doped region adjacent to a P-doped region, creating a junction. (See the semiconductor article for an explanation of these terms.)

In a normal silicon diode, the drop in potential across a conducting diode is approximately 0.6 to 0.7 volts. The value is different for other diode types - Schottky diodes can be as low as 0.2V and Light Emitting Diodes (LEDs) can be 1.4V or more.

There are several types of semiconductor junction diodes:

Normal Diodes: which opperate as described above.
Zener Diodes: a Zener diode can be made to conduct "backwards". This break down occurs at a precise voltage, and they are used as precision voltage references.
Light Emitting Diodes (LEDs): as the electrons cross the junction they emit photons. In most diodes, these are reabsorbed, and are at frequencies that can not be seen (usually infrared). However, with the right materials and geometry, the light become visible.
Photo Diodes: these have wide, transparent junctions. Photons can push electroncs over the junction, causing a current to flow. Photo diodes can be used as solar cells.
Schottkey Diodes
Snap Diodes: these can provide very fast voltage transitions.
Esaki or Tunnel Diodes: these have a region of operation showing negative resistance caused by quantum tunneling, thus allowing amplification of signals and very simple bi-stable circuits.

There are other types of diode, which all share the basic function of allowing electrical current to flow in only one direction, but with different methods of construction.

Point Contact Diode: This works the same as the junction semiconductor diodes described above, but it's construction is simpler. A block of n-type semiconductor is built, and a conducting contact made of some group-3 metal is built to a sharp point which is placed in contact with the semiconductor. Some metal migrates into the semiconductor to make a small region of p-type semiconductor near the contact. I'm not sure if these are still used much. They were fairly widely used for small power applications and digital electronics circa 1960 to 1980. They were not so popular in analogue applications due to high noise and non-linearity.

Tube or Valve Diode: This is the simplest kind of vacuum tube device (referred to as valves in the UK). Electrons will move from a heated metal surface treated with barium oxide into a vacuum. After they are off the surface, they can be attracted to positively charged cool surface (anode). However, electrons are not easily released from a cold non treated surface when the voltage polarity is reversed and hence any flow is a very small current. These are probably completely obsolete by 2001, but for much of the 20th century were used in analogue signal applications, power supply applications, and (approx 1950 to 1960) for digital electronic logic.

Gas Discharge Diode: There are two electodes, not touching, in some kind of gas. One electrode is very sharp. The other has a smoothly curved finish. If a strong negative potential is applied to the sharp electrode, the electic field near the sharp edge or point is enough to cause an electrical discharge in the gas, and a current flows. If the reverse potential is applied, the electrical field strengh around the smooth electrode is not enough to start a discharge. (The discharge can only start easily at the negative end because electrons are much more mobile than positive ions.) These are sometimes used for high-voltage high-current rectification in power supply applications.

/Talk

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 Although it is not completely accurate, a diodes electrical characteristics can be roughly approximated by two linear regions.  Below a certain difference in potential between the two leads, the diode can be thought of as an open (non-conductive) circuit.  As the potential difference is increased, at some stage the diode will become conductive and allow current to flow, at which point it can be thought of as a potential "source" with zero (or at least very low) resistance.
+Diodes are sometimes known as '''rectifiers''' for their use to "rectify" [[alternating current]] electricity into [[direct current]].
-The first diodes were [[vacuum tube]] devices (also known as "thermionic valves"), arrangements of electrodes surrounded by vacuum within a glass envelope, superficially similar to incandescent light bulbs. The arrangement of a filament and plate as a diode was invented in [[1904]] by [[John Ambrose Fleming]]  scientific adviser to the [[Marconi]] company based on an observation by [[Thomas Edison]]. Like light bulbs, vacuum tube diodes have a ''[[filament]]'' through which current is passed, heating the filament. In its heated state it is ready to release electrons into the vacuum. These electrons are electrostatically drawn to a positively charged outer metal plate called the ''[[anode]]'', or more commonly just the ''plate''. Electrons do not flow from the plate back toward the filament, even if the charge on the plate is made negative, because the plate is not heated.
+The first diodes were [[vacuum tube]] devices (also known as "thermionic valves"), arrangements of electrodes surrounded by vacuum within a glass envelope, superficially similar to incandescent light bulbs. The arrangement of a filament and plate as a diode was invented in [[1904]] by [[John Ambrose Fleming]]  scientific adviser to the [[Marconi]] company based on an observation by [[Thomas Edison]]. Like light bulbs, vacuum tube diodes have a [[filament]] through which current is passed, heating the filament. In its heated state it is ready to release electrons into the vacuum. These electrons are electrostatically drawn to a positively charged outer metal plate called the [[anode]], or more commonly just the "plate"'. Electrons do not flow from the plate back toward the filament, even if the charge on the plate is made negative, because the plate is not heated.
-Although other forms of diode exist, most diodes are based on semiconductors. A semiconductor diode consist of an N-[[doped]] region adjacent to a P-doped region, creating a [[junction]]. (See [[semiconductor]] for an explanation of these terms.)
+Although vacuum tube diodes are still used for a few specialist applications, most modern diodes are based on [[semiconductor]]s. A semiconductor diode consist of an N-[[doped]] region adjacent to a P-doped region, creating a [[junction]]. (See the [[semiconductor]] article for an explanation of these terms.)
 In a normal silicon diode, the drop in potential across a conducting diode is approximately 0.6 to 0.7 [[volt]]s.  The value is different for other diode types - Schottky diodes can be as low as 0.2V and Light Emitting Diodes (LEDs) can be 1.4V or more.