Diode | Research & Encyclopedia Articles

Diode

Many people have heard of and encountered light-emitting diodes, also called LEDs; they are used often in electronic toys and readouts. The first diodes were invented to serve as rectifiers--that is, they were used to convert alternating current (AC) into direct current (DC). AC is very easy to generate using a transformer; however, AC moves in a sine-wave pattern, moving first in one direction and then doubling back in the opposite direction. Many electronic devices require a current that moves in only one direction, such as DC. A device was needed that would eliminate the sine-wave motion of AC. Karl Ferdinand Braun, a German physicist, noticed that certain crystalline substances served as rectifiers, allowing current to pass very easily in one direction but inhibiting its motion in the opposite direction. When an AC current was passed through these crystals, it was easily converted into DC. Braun patented his crystal rectifier in 1874, but its use was limited to crystal set radios. The crystal rectifier was replaced as an electronic tool in 1906 when the vacuum-tube transistor became availble. The vacuum tube (invented by John Ambrose Fleming) was essentially the first true diode. It consisted of two metal poles, an anode and a cathode (thus the word di-ode), encased in a glass vacuum tube. The cathode was usually made from cesium (or some other alkali metal) that released electrons when heated, while the anode was made from a metal that did not boil off electrons when hot. When a current was applied through the diode so that the cathode was negatively charged, it would heat up and release electrons; these electrons would flow across the vacuum to the anode, thus completing the circuit. This is called the forward current direction and can be thought of as the diode's "on" position. However, when a current was applied so that the anode was negatively charged, it, too, would heat up, but would not release any electrons.

Consequently, no electrons would flow to the cathode and the circuit would be incomplete or in the "off" position. The vacuum tube, or valve diode, would thus allow current to flow in one direction only, just as the crystal rectifier did. If a third type of electrode--the control grid--is added to the system, the device is called a triode. Triodes are used to control the passage of electrons and can often be used to amplify the current as it passes through. Today, most valve diodes have been replaced by semiconductor diodes. A semiconductor, such as silicon or germanium, is a material, usually crystalline, that does not carry current as well as a true conductor. In order to make a semiconductor diode, the crystal is sliced and coated at each end with different substances, so that each end of the crystal has different electronic properties. One end is called the n-type, because its material gives it a greater number of free, negatively-charged electrons. The other end is called the p-type, because it has a greater number of positively-charged "holes." Both the electrons and the holes are allowed to move within the crystal, but they are strongest near the ends. The middle of the crystal is called the p-n junction; this is where electrons and holes exist in equal amounts. When a current is applied to the crystal such that the n-type side is stronger (more negative), more electrons will flow past the p-n junction. However, when the voltage is reversed, the p-type side will become stronger and the p-n junction will act as a barrier. Just like the crystal rectifier and the valve diode, the semiconductor diode only allows current to flow freely in one direction. In order to make an LED, gallium arsenide or a similar substance must be used as the semiconductor. When treated with phosphorus, the LED will glow in the "on" position. If no phosphorus is used the LED will glow in the invisible infrared spectrum.

An evolving development is that of molecular diodes, whereby data could be stored, transmitted and retrieved via molecules. Such a process is expected to allow for the decrease in size and increase in speed of certain electronic instruments.