Description

fig.2 - Diodes

Diode, electronic device that allows the passage of current in only one direction. How things evolved … The first such devices were vacuum-tube diodes, consisting of an evacuated glass or steel envelope containing two electrodes - a cathode and an anode. Because electrons can flow in only one direction, from cathode to anode, the vacuum-tube diode could be used as a rectifier . The diodes most commonly used in electronic circuits today are semiconductor diodes. The simplest of these, the germanium point-contact diode, dates from the early days of radio, when the received radio signal was detected by means of a germanium crystal and a fine, pointed wire that rested on it. In modern germanium (or silicon) point-contact diodes, the wire and a tiny crystal plate are mounted inside a small glass tube and connected to two wires that are fused into the ends of the tube.

What can a diode do …

Because this electronic device allows passage of current in only one direction , it is used in rectification, process of converting an alternating current (AC), which flows back and forth in a circuit, to direct current (DC), which flows only in one direction. Light-emitting diodes are used in many common devices, such as the tuning indicator on a radio. Any number from 0 to 9 may be displayed on a seven-segment arrangement (an arrangement often used on calculators and digital watches), in which each LED lights one segment.

In this picture 3 zeros from a digital display

Types of diodes

Junction-type diodes consist of a junction of two different kinds of semiconductor material. The Zener diode is a special junction-type diode, using silicon, in which the voltage across the junction is independent of the current through the junction. Because of this characteristic, Zener diodes are used as voltage regulators. Another special junction-type diode is used in solar cells; a voltage appears spontaneously when the junction is illuminated.

	In light-emitting diodes (LEDs), on the other hand, a voltage applied to the semiconductor junction results in the emission of light energy.
fig.4 - LED

3.How you can identify a diode

All diodes have polarity, but unlike other components, the polarity marking is for the negative lead.

Diodes are most commonly identified by: one band and "1N" markings one band on a glass-like body small colored light (LED) small colored light (LED)

Unlike resistors and capacitors, diodes are not identified by value, wattage or voltage. They are given a JEDEC (Joint Electronic Device Engineering Council) identification number. Each Kind of diode (i.e. rectifier, tunnel, zener) has its own JEDEC number, but may have the same component shape or body. The only way to positively identify them is by their JEDEC number. Because all diodes are formed by one junction of dissimilar semiconductor types called "N" and "P", the prefix "1" or " 1N'is used in the JEDEC system (for example, 1N4004, 1N914, 1N4733).

The pn Junction

This junction , used by diodes , is obtained by doping semiconductors. The difference in the number of valence electrons between the doping material, or dopant (either donors or acceptors of electrons), and host gives rise to negative (n-type) or positive (p-type) carriers of electricity. This concept is illustrated in the accompanying diagram of a doped silicon (Si) crystal. Each silicon atom has four valence electrons (represented by dots); two are required to form a covalent bond. In n- type silicon, atoms such as phosphorus (P) with five valence electrons replace some silicon and provide extra negative electrons. In p-type silicon, atoms with three valence electrons such as aluminum (Al) lead to a deficiency of electrons, or to holes, which act as positive electrons. The extra electrons or holes can conduct electricity. When p-type and n-type semiconductor regions are adjacent to each other, they form a semiconductor diode, and the region of contact is called a p-n junction .The conductance properties of the p-n junction depend on the direction of the voltage, which can, in turn, be used to control the electrical nature of the device. Series of such junctions are used to make transistors and other semiconductor devices such as solar cells, p-n junction lasers, rectifiers, and many others.

Understanding N-TYPE AND P-TYPE semiconductors

A germanium or silicon crystal, containing donor-impurity atoms, is called a negative, or n-type, semiconductor to indicate the presence of excess negatively charged electrons. The use of an acceptor impurity produces a positive, or p-type, semiconductor, so called because of the presence of positively charged holes.

A single crystal containing both n-type and p-type regions may be prepared by introducing the donor and acceptor impurities into molten germanium or silicon in a crucible at different stages of crystal formation. The resultant crystal has two distinct regions of n-type and p-type material, and the boundary joining the two areas is known as an n-p junction. Such a junction may be produced also by placing a piece of donor-impurity material against the surface of a p-type crystal or a piece of acceptor-impurity material against an n-type crystal and applying heat to diffuse the impurity atoms through the outer layer.

An n-p junction (also known as a diode) will only allow current to flow in one direction. The electrons from the n-type material can pass to the right through the p-type material, but the lack of excess electrons in the p-type material will prevent any flow of electrons to the left. Note that the current is defined to flow in a direction that is opposite to the direction of the flow of the electrons.

fig.4 - Understanding the n-p jonction

Useful links

Rectification | The PN junction | Media Gallery (np led diode)