A diode functions as the electronic version of a one-way valve. By restricting the direction of movement of charge carriers, it allows an electric current to flow in one direction, but blocks it in the opposite direction.
A diode's current-voltage, or I-V, characteristic can be approximated by two regions of operation. 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 connection with zero (or at least very low) resistance.
A special arrangement of four diodes that will transform an alternating current into a direct current, using both phases of the alternating current, is known as a diode bridge, or single-phase bridge rectifier.
The first diodes were vacuum tube devices (also known as thermionic valves), arrangements of electrodes surrounded by a vacuum within a glass envelope, similar in appearance 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 can now emit electrons into the vacuum. These electrons are electrostatically drawn to a positively charged outer metal plate called the anode, or 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 specialized 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 p-n junction. (See the semiconductor article for an explanation of these terms, especially under the heading p-n junction.)
The Shockley ideal diode equation (named after William Bradford Shockley) can be used to approximate the p-n diode's I-V characteristic.
where I is the diode current, IS is a scale factor called the saturation current, q is the charge on an electron (the elementary charge), k is Boltzmann's constant, T is the absolute temperature of the p-n junction and VD is the voltage across the diode. The term kT/q is the thermal voltage, sometimes written VT, and is approximately 26 mV at room temperature. n (sometimes omitted) is the emission coefficient, which varies from about 1 to 2 depending on the fabrication process.
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:
Point Contact Diode: This works the same as the junction semiconductor diodes described above, but its construction is simpler. A block of n-type semiconductor is built, and a conducting sharp-point contact made with some group-3 metal 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 analog applications due to high noise and non-linearity.
Tube or Valve Diode: This is the simplest kind of vacuum tube device (referred to as a valve 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 untreated surface when the voltage polarity is reversed and hence any flow is a very small current. These were probably completely obsolete by 2001, but for much of the 20th century were used in analog signal applications, power supply applications, and (approx 1950 to 1960) for digital electronic logic.
Gas Discharge Diode: There are two electrodes, 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 electric 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 strength 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.
SCR or Silicon Controlled Rectifier: a diode that does not conduct until it is triggered by applying a very small voltage, usually 1 to 20 volts, to its "gate" . Once triggered it can only be shut off by stopping the electric current flowing through it. SCRs are very useful in the control of A.C. (alternating current) because they shut off automatically at the moment the current drops to zero, every half cycle (0.008333 seconds for house current at 60 hertz) and then can be "triggered" once again. They are manufactured in a range of sizes, with power-handling capacities from a few watts to tens of megawatts.
Triac: two SCR's that are joined end to end or back to back while their gates are connected together. This results in a two directional "diode" electric switch where the current can flow in both directions when it is triggered (turned on). Lower power ones are used everywhere, especially appliances.