Diode

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.

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 semiconductor PN junctions. A semiconductor diode consists of a PN junction and has two(2) terminals, an anode(+) and a cathode(-). Current flows from anode to cathode within the diode.

In this discussion the term diode and rectifier will be used interchangeably; however, the term diode usually implies a small signal device with current typically in the milliamp range; and a rectifier, a power device, conducting from1 to 1000 amps or even higher. Many diodes or rectifiers are identified as 1NXXXX .

An ideal diode is like a light switch in your home. When the switch is closed, the circuit is completed; and the light turns on.When the switch is open, there is no current and the light is off.

However, the diode has an additional property; it is unidirectional, i.e.current flows in only one direction(anode to cathode internally).When a forward voltage is applied, the diode conducts; and when a reverse voltage is applied, there is no conduction. A mechanical analogy is a ratchat, which allows motion in one direction only.

However, a typical diode characteristic is more like the following:

Notice that the diode conducts a small current in the forward direction up to a threshold voltage, 0.3 for germanium and 0.7 for silicon ; after that it conducts as we might expect. The forward voltage drop, Vf, is specified at a forward current, If.

In the reverse direction there is a small leakage current up until the reverse breakdown voltage is reached. This leakage is undesireable, obviously the lower the better, and is specified at a voltage less the than breakdown; diodes are intended to operate below their breakdown voltage.

The current rating of a diode is determined primarily by the size of the diode chip, and both the material and configuration of the package, Average Current is used, not RMS current. A larger chip and package of high thermal conductivity are both conducive to a higher current rating.

The switching speed of a diode depends upon its construction and fabrication. In general the smaller the chip the faster it switches, other things being equal. The chip geometry, doping levels, and the temperature at nativity determine switching speeds . The reverse recovery time, trr, is usually the limiting parameter; trr is the time it takes a diode to switch from on to off.