Wednesday, September 22, 2010

Diode Types and Their Uses

There are many different types of diodes that are available for use in electronics design. Different semiconductor diode types can be used to perform different functions as a result of the properties of these different diode types.

Semiconductor diodes can be used for many applications. The basic application is obviously to rectify waveforms. This can be used within power supplies or within radio detectors. Signal diodes can also be used for many other functions within circuits where the "one way" effect of a diode may be required.

Diodes are not just used as rectifiers, as various other types of diode can be used in many other applications. Some other different types of diodes include: light emitting diodes, photo-diodes, laser diodes and more as detailed in the list below.

Many of the different types of diodes mentioned below have further pages providing in-depth information about them including their structures, method of operation, how they may be used in circuits, and precautions and tips for using them in electronics design.

Types of diodes
It is sometimes useful to summarise the different types of diodes that are available. Some of the categories may overlap, but the various definitions may help to narrow the field down and provide an overview of the different diode types that are available.

Avalanche diode: The avalanche diode by its very nature is operated in reverse bias. It uses the avalanche effect for its operation. In general the avalanche diode is used for photo-detection where the avalanche process enables high levels of sensitivity to be obtained, even if there are higher levels of associated noise.
Laser diode: This type of diode is not the same as the ordinary light emitting diode because it produces coherent light. Laser diodes are widely used in many applications from DVD and CD drives to laser light pointers for presentations. Although laser diodes are much cheaper than other forms of laser generator, they are considerably more expensive than LEDs. They also have a limited life. See related articles list in left hand margin.
Light emitting diodes: The light emitting diode or LED is one of the most popular types of diode. When forward biased with current flowing through the junction, light is produced. The diodes use component semiconductors, and can produce a variety of colours, although the original colour was red. There are also very many new LED developments that are changing the way displays can be used and manufactured. High output LEDs and OLEDs are two examples. See related articles list in left hand margin.
Photo diode: The photo-diode is used for detecting light. It is found that when light strikes a PN junction it can create electrons and holes. Typically photo-diodes are operated under reverse bias conditions where even small amounts of current flow resulting from the light can be easily detected. Photo-diodes can also be used to generate electricity. For some applications, PIN diodes work very well as photo detectors. See related articles list in left hand margin.
PIN diode: This type of diode is typified by its construction. It has the standard P type and N-type areas, but between them there is an area of Intrinsic semiconductor which has no doping. The area of the intrinsic semiconductor has the effect of increasing the area of the depletion region which can be useful for switching applications as well as for use in photo diodes, etc. See related articles list in left hand margin.
Point contact diode: This type of diode is one of the most basic forms of diode in terms of its construction but it performs in the same way as a PN junction diode. This type of diode consists of a piece of N-type semiconductor, onto which a sharp point of a specific type of metal wire (group III metal) is placed. As this physical junction is formed, some of the metal from the wire migrates into the semiconductor and produces a PN junction. Point contact diodes have a very low level of capacitance because the resulting junction is very small. As such this type of diode is ideal for many radio frequency (RF) applications. The downside of the small junction is that they cannot carry high levels of current but they have the advantage that they are very cheap to manufacture, although their performance is not particularly repeatable.
PN Junction: The standard PN junction may be thought of as the normal or standard type of diode in use today. These diodes can come as small signal types for use in radio frequency, or other low current applications which may be termed as signal diodes. Other types may be intended for high current and high voltage applications and are normally termed rectifier diodes. See related articles list in left hand margin.
Rectifier diode: This definition refers to diodes that are used in power supplies for rectifying alternating power inputs. The diodes are generally PN junction diodes, although Schottky diodes may be used if low voltage drops are needed. They are able to rectify current levels that may range from an amp upwards.
Schottky diodes: This type of diode has a lower forward voltage drop than ordinary silicon PN junction diodes. At low currents the drop may be somewhere between 0.15 and 0.4 volts as opposed to 0.6 volts for a silicon diode. To achieve this performance they are constructed in a different way to normal diodes having a metal to semiconductor contact. They are widely used as clamping diodes, in RF applications, and also for rectifier applications.
Signal diode: This for of diode is used for small signal applications where small values of current are drawn. Diodes with the description of signal diode are generally the standard PN junction diode types.
Step recovery diode: A form of microwave diode used for generating and shaping pulses at very high frequencies. These diodes rely on a very fast turn off characteristic of the diode for their operation.
Tunnel diode: Although not widely used today, the tunnel diode was used for microwave applications where its performance exceeded that of other devices of the day. See related articles list in left hand margin.
Varactor diode or varicap diode: This type of diode is used in many radio frequency (RF) applications. The diode has a reverse bias placed upon it and this varies the width of the depletion layer according to the voltage placed across the diode. In this configuration the varactor or varicap diode acts like a capacitor with the depletion region being the insulating dielectric and the capacitor plates formed by the extent of the conduction regions. The capacitance can be varied by changing the bias on the diode as this will vary the width of the depletion region which will accordingly change the capacitance. See related articles list in left hand margin.
Zener diode: The Zener diode is a very useful type of diode as it provides a stable reference voltage. As a result it is used in vast quantities. It is run under reverse bias conditions and it is found that when a certain voltage is reached it breaks down. If the current is limited through a resistor, it enables a stable voltage to be produced. This type of diode is therefore widely used to provide a reference voltage in power supplies. Two types of reverse breakdown are apparent in these diodes: Zener breakdown and Impact Ionisation. However the name Zener diode is used for the reference diodes regardless of the form of breakdown that is employed. See related articles list in left hand margin.

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