diode-types_lect-01

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Lecture-Diodes

The UNIVERSE consists of two main parts-matter and energy.

MATTER is anything that occupies space and has weight. Rocks, water, and air are examplesof matter. Matter may be found in any one of three states: solid, liquid and gaseous. It can alsobe composed of either an element or a combination of elements.

An ELEMENT is a substance that cannot be reduced to a simpler form by chemical means.Iron, gold, silver, copper, and oxygen are all good examples of elements.

A COMPOUND is a chemical combination of two or more elements. Water, table salt, ethylalcohol, and ammonia are all examples of compounds.

A MOLECULE is the smallest part of a compound that has all the characteristics of thecompound. Each molecule contains some of the atoms of each of the elements forming thecompound.

The ATOM is the smallest particle into which an element can be broken down and still retain allits original properties. An atom is made up of electrons, protons, and neutrons. The number andarrangement of these particles determine the kind of element.

An ELECTRON carries a small negative charge of electricity.

The PROTON carries a positive charge of electricity that is equal and opposite to the charge ofthe electron. However, the mass of the proton is approximately 1,837 times that of the electron.

The NEUTRON is a neutral particle in that it has no electrical charge. The mass of the neutronis approximately equal to that of the proton.

An ELECTRON'S ENERGY LEVEL is the amount of energy required by an electron to stay inorbit. Just by the electron's motion alone, it has kinetic energy. The electron's position inreference to the nucleus gives it potential energy. An energy balance keeps the electron in orbitand as it gains or loses energy, it assumes an orbit further from or closer to the center of the

atom.

SHELLS and SUBSHELLS


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are the orbits of the electrons in an atom. Each shell can contain a maximum number ofelectrons, which can be determined by the formula 2n2. Shells are lettered K through Q, startingwith K, which is the closest to the nucleus. The shell can also be split into four subshells labeleds, p, d, and f, which can contain 2, 6, 10, and 14 electrons, respectively.

IONIZATION is the process by which an atom loses or gains electrons. An atom that loses

some of its electrons in the process becomes positively charged and is called a POSITIVE ION.An atom that has an excess number of electrons is negatively charged and is called aNEGATIVE ION.

ENERGY BANDS are groups of energy levels that result from the close proximity of atoms in asolid. The three most important energy bands are the CONDUCTION BAND, FORBIDDENBAND, and VALENCE BAND.

CONDUCTORS, SEMICONDUCTORS, and INSULATORS are categorized as such by usingthe energy band concept. It is the width of the forbidden band that determines whether amaterial is an insulator, a semiconductor, or a conductor. A CONDUCTOR has a very narrow

forbidden band or none at all. A SEMICONDUCTOR has a medium width forbidden band. AnINSULATOR has a wide forbidden band.


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COVALENT BONDING is the sharing of valence electrons between two or more atoms. It is thisbonding that holds the atoms together in an orderly structure called a CRYSTAL.

The CONDUCTION PROCESS in a SEMICONDUCTOR is accomplished by two different typesof current flow: HOLE FLOW and ELECTRON FLOW. Hole flow is very similar to electronflow except that holes (positive charges) move toward a negative potential and in anopposite direction to that of the electrons. In an INTRINSIC semiconductor (one which

does not contain any impurities), the number of holes always equals the number ofconducting electrons.

DOPING is the process by which small amounts of selected additives, called impurities, areadded to semiconductors to increase their current flow. Semiconductors that undergo thistreatment are referred to as EXTRINSIC SEMICONDUCTORS.

An N-TYPE SEMICONDUCTOR is one that is doped with an N-TYPE or donor impurity (animpurity that easily loses its extra electron to the semiconductor causing it to have an excessnumber of free electrons). Since this type of semiconductor has a surplus of electrons, theelectrons are considered the majority current carriers, while the holes are the minority currentcarriers.

A P-TYPE SEMICONDUCTOR is one which is doped with a P-TYPE or acceptor impurity (animpurity that reduces the number of free electrons causing more holes). The holes in this typesemiconductor are the majority current carriers since they are present in the greatest quantitywhile the electrons are the minority current carriers.


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The SEMICONDUCTOR DIODE, also known as a PN JUNCTION DIODE, is a two-elementsemiconductor device that makes use of the rectifying properties of a PN junction to convertalternating current into direct current by permitting current flow in only one direction.

A PN JUNCTION CONSTRUCTION varies from one manufacturer to the next. Some of the

more commonly used manufacturing techniques are: GROWN, ALLOY or FUSED-ALLOY ,

DIFFUSED, and POINT-CONTACT.

CURRENT FLOW in an N-TYPE MATERIAL is similar to conduction in a copper wire. That is,

with voltage applied across the material, electrons will move through the crystal toward thepositive terminal just like current flows in a copper wire.


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CURRENT FLOW in a P-TYPE MATERIAL is by positive holes, instead of negativeelectrons. Unlike the electron, the hole moves from the positive terminal of the P materialto the negative terminal.

JUNCTION BARRIER is an electrostatic field that has been created by the joining of a sectionof N material with a section of P material. Since holes and electrons must overcome this field tocross the junction, the electrostatic field is commonly called a BARRIER. Because there is alack or depletion of free electrons and holes in the area around the barrier, this area hasbecome known as the DEPLETION REGION.


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FORWARD BIAS is an external voltage that is applied to a PN junction to reduce its barrier and,therefore, aid current flow through the junction. To accomplish this function, the external voltageis connected so that it opposes the electrostatic field of the junction.

REVERSE BIAS is an external voltage that is connected across a PN junction so that its voltageaids the junction and, thereby, offers a high resistance to the current flow through the junction.


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The PN JUNCTION has a unique ability to offer very little resistance to current flow in theforward-bias direction, but maximum resistance to current flow when reverse biased. For thisreason, the PN junction is commonly used as a diode to convert ac to dc.

The PN JUNCTION'S APPLICATION expands many different areas-from a simple voltage

protection device to an amplifying diode. Two of the most commonly used applications for thePN junction are the SIGNAL DIODE (mixing, detecting, and switching signals) and theRECTIFYING DIODE (converting ac to dc).


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The METALLIC RECTIFIER or dry-disc rectifier is a metal-to-semiconductor device that actsjust like a diode in that it permits current to flow more readily in one direction than the other.Metallic rectifiers are used in many applications where a relatively large amount of power isrequired.

DIODE CHARACTERISTICS is the information supplied by manufacturers on different types ofdiodes, either in their manuals or on specification sheets.

DIODE RATINGS are the limiting value of operating conditions of a diode. Operation of the

diode outside of its operating limits could damage the diode. Diodes are generally rated for:MAXIMUM AVERAGE FORWARD CURRENT, PEAK RECURRENT FORWARD CURRENT ,MAXIMUM SURGE CURRENT, and PEAK REVERSE VOLTAGE.

A diode is a two-terminal device, having two active electrodes, between which it allows the

transfer of current in one direction only. Diodes are known for their unidirectional current

property, wherein, the electric current is allowed to flow in one direction. Basically, diodes are

used for the purpose of rectifying waveforms, and can be used within power supplies or within

radio detectors. They can also be used in circuits where 'one way' effect of diode is required.

Most diodes are made fromsemiconductorssuch assilicon

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Light Emitting Diode (LED): It is one of the most popular type of diodes and when this diode

permits the transfer of electric current between the electrodes, light is produced. In most of the

diodes, the light (infrared) cannot be seen as they are at frequencies that do not permit visibility.

When the diode is switched on or forward biased, the electrons recombine with the holes and

release energy in the form of light (electroluminescence). The color of light depends on the

energy gap of the semiconductor.

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

Avalanche Diode: This type of diode operates in the reverse bias, and used avalanche effect

for its operation. The avalanche breakdown takes place across the entire PN junction, when the

voltage drop is constant and is independent of current. Generally, the avalanche diode is used

for photo-detection, wherein high levels of sensitivity can be obtained by the avalanche process.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 different from the LED type, as it produces coherent light.

These diodes find their application in DVD and CD drives, laser pointers, etc. Laser diodes are

more expensive than LEDs. However, they are cheaper than other forms of laser generators.

Moreover, these laser diodes have limited life.

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.

Schottky Diodes: These diodes feature lower forward voltage drop as compared to the

ordinary silicon PN junction diodes. The voltage drop may be somewhere between 0.15 and 0.4

volts at low currents, as compared to the 0.6 volts for a silicon diode. In order to achieve this

performance, these diodes are constructed differently from normal diodes, with metal to

semiconductor contact. Schottky diodes are used in RF applications, rectifier applications and

clamping 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.


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Zener diode: This type of diode provides a stable reference voltage, thus is a very useful type

and is used in vast quantities. The diode runs in reverse bias, and breaks down on the arrival of

a certain voltage. A stable voltage is produced, if the current through the resistor is limited. In

power supplies, these diodes are widely used to provide a reference voltage.

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.

Photodiode: Photodiodes are used to detect light and feature wide, transparent junctions.

Generally, these diodes operate in reverse bias, wherein even small amounts of current flow,

resulting from the light, can be detected with ease. Photodiodes can also be used to generate

electricity, used as solar cells and even in photometry.

The photo-diode is used for detecting light. It is found that when light strikes a PNjunction it can create electrons and holes. Typically photo-diodes are operated underreverse bias conditions where even small amounts of current flow resulting from thelight can be easily detected. Photo-diodes can also be used to generate electricity. Forsome applications, PIN diodes work very well as photodetectors

Varicap Diode or Varactor Diode or Tuning Diodes

: This type of diode feature a reverse bias placed upon it, which varies the width of the depletion

layer as per the voltage placed across the diode. This diode acts as a capacitor and capacitor

plates are formed by the extent of conduction regions and the depletion region as the insulatingdielectric. By altering the bias on the diode, the width of the depletion region changes, thereby

varying the capacitance.

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.

Rectifier Diode: These diodes are used to rectify alternating power inputs in power supplies.

They can rectify current levels that range from an amp upwards. If low voltage drops are

required, then Schottky diodes can be used, however, generally these diodes are PN junction

diodes.

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.


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Diodes are used widely in the electronics industry, right from electronics design to production, to

repair. Besides the above mentioned types of diodes, the other diodes are PIN diode, point

contact diode, signal diode, step recovery diode, tunnel diode and gold doped diodes

PIN diode: This type of diode is typified by its construction. It has the standard P typeand N-type areas, but between them there is an area of Intrinsic semiconductor whichhas no doping. The area of the intrinsic semiconductor has the effect of increasing thearea of the depletion region which can be useful for switching applications as well as foruse in photodiodes, etc.

PN Junction: The standard PN junction may be thought of as the normal or standardtype of diode in use today. These diodes can come as small signal types for use in radiofrequency, 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 arenormally termed rectifier diodes.

Signal diode: This for of diode is used for small signal applications where small valuesof 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 ofthe diode for their operation.

Tunnel diode: Although not widely used today, the tunnel diode was used formicrowave applications where its performance exceeded that of other devices of the day

Cat's whisker: As already mentioned, this type of diode was the earliest type to gain

widespread acceptance. It consisted of a small wire placed on a piece of mineral crystal. This

created a small point contact diode, which although unreliable was sufficiently good to enable

radio transmissions to be heard when used in a "crystal set."

Tunnel Diode

The higher the doping level, the greater the shift. In the tunnel diode, the doping levels

are so high that the Fermi levels in both halves of the crystal have been pushed

completely out of the forbidden zone and into the valence and conduction bands.

as the forward bias increases, the applied voltage shifts the levels apart, and gradually back to

the more usual diode energy pattern. Over this applied forward voltage range, diode current

actually decreases as applied voltage increases.

The SEMICONDUCTOR IDENTIFICATION SYSTEM is an alphanumerical code used to

distinguish one semiconductor from another. It is used for diodes, transistors, and many other

special semiconductor devices.

The first number in the system indicates the number of junctions in the semiconductor device

and is a number, one less than the number of active elements. Thus 1 designates a diode; 2

designates a transistor (which may be considered as made up of two diodes); and 3 designates


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a tetrode (a four-element transistor). The letter "N" following the first number indicates a

semiconductor. The 2- or 3-digit number following the letter "N" is a serialized identification

number. If needed, this number may contain a suffix letter after the last digit. For example, the

suffix letter "M" may be used to describe matching pairs of separate semiconductor devices or

the letter "R" may be used to indicate reverse polarity. Other letters are used to indicate

modified versions of the device which can be substituted for the basic numbered unit. Forexample, a semiconductor diode designated as type 1N345A signifies a two-element diode (1)

of semiconductor material (N) that is an improved version (A) of type 345

Semiconductor identification codes.

Semiconductor diode marking


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Take, for example, a diode with brown, orange, and white bands at one terminal and figure out

its identification number. With brown being a "1," orange a "3," and white "9," the device would

be identified as a type 139 semiconductor diode, or specifically 1N139.

Semiconductor diode color code system.

The following is a list of some of the special safety precautions that should be observed whenworking with diodes:

Never remove or insert a diode into a circuit with voltage applied. Never pry diodes to loosen them from their circuits. Always be careful when soldering to ensure that excessive heat is not applied to the

diode.

When testing a diode, ensure that the test voltage does not exceed the diode'smaximum allowable voltage.

Never put your fingers across a signal diode because the static charge from your bodycould short it out.

Always replace a diode with a direct replacement, or with one of the same type. Ensure a replacement diode is put into a circuit in the correct direction.

Symbols


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, Light Emitting Diode (LED)

P-I-N Diode

Varactor Diode

Tunnel Diode

Zener Diode

Schottky Barrier Diode

Photodiodes

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