different types of diodes-main

8/2/2019 Different Types of Diodes-main

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Resurreccion, Matthew L.

BS EE 3-A

Engr. Sevilla M. Tuazon

Professor

Special Diodes



Light-Emitting Diodes (LED)

Overview

The increasing use of digital displays in calculators, watches, and allforms of instrumentation has contributed to an extensive interest in structures

that emit light when properly biased.

In Si and Ge diodes the greater percentage of the energy converted during

recombination at the junction is dissipated in the form of heat within the structure, and the

emitted light is insignificant.

For this reason, silicon and germanium are not used in the construction of LED devices. On the other hand:

Diodes constructed of GaAs emit light in the infrared (invisible) zone during the

recombination process at the p-n junction.

Through other combinations of elements a coherent visible light can be

generated. In the table below provides a list of common compound

semiconductors and the light they generate. In addition the typical range of

forward bias potentials for each is listed.

Color ConstructionTypical Forward

Voltage (V)

Amber AlInGaP 2.1

Blue GaN 5.0

Green GaP 2.2

Orange GaAsP 2.0

Red GaAsP 1.8

White GaN 4.1Yellow AlInGaP 2.1

Construction Diagram

The basic construction of an LED appears in the figure below with the

standard symbol used for the device. The smaller external metallic conducting

surface is connected to the p-type material and the larger external metallic

conducting surface is connected to the n-type material.



Schematic Diagram

Characteristic Diagram



The level of V D under forward bias condition is listed as V F and extends from

2.2 to 3 V. In other words, one can expect a typical operating current of about

10 mA at 2.3V for good light emission, as shown in the figure.

In the figure, the normalized level is taken at I F = 10 mA. Note that therelative luminous intensity is one at I F = 10 mA. The graph quickly reveals that

the intensity of the light is almost doubled at a current of 15 mA and is almost

three times as much at a current of 30 mA. It is important to therefore note that:

The light intensity of an LED will increase with forward current until a point of

saturation arrives where any further increase in current will not effectively increase the

level of illumination.

Operation of the Device

Process of electroluminescence in the LED

As the name implies, the light-emitting diode that gives off visible or

invisible (infrared) light when energized. In any forward-biased p-n junction

there is, within the structure and primarily close to the junction, a

recombination of holes and electrons. This recombination requires that the



energy possessed by the unbound free electrons be transferred to another state.

In all semiconductor p-n junctions some of this energy is given off in the form

of heat and some in the form of photons.

The external metallic conducting surface connected to the p – type material is

smaller to permit the emergence of the maximum number of photons of the

light energy when the device is forward-biased. Note in the figure that the

recombination of the injected carriers due to the forward-biased junction results

in emitted light at the site of the recombination. There will, of course, be some

absorption of the packages of photon energy in the structure itself, but a very

large percentage can leave, as shown in the figure.

Application of the Diode

Even thought the light in not visible, infrared LEDs have numerous

applications where visible light is not a desirable effect.

These include:

security systems

industrial processing

optical coupling

safety controls

Such as:

on garage door openers

in home entertainment centers

Where the infrared light of the remote control is the

controlling element.



Zener Diode

Overview

A zener diode is a silicon pn junction device that is designed for operation inthe reverse-breakdown region. The breakdown voltage of a zener diode is set by

carefully controlling the doping level during manufacture.


Zener diodes are constructed using a large-area PN junction which has been

affixed between two heat sinks to protect the junction during impulse

conduction.

Schematic Diagram

The Zener diode symbol is presented in the figures to ensure that the

direction of conduction of is clearly understood together with the required

polarity of the applied voltage. The direction of the conduction is opposite to

that of the arrow in the symbol, as pointed out in the introduction to this section.




The characteristic drops in an almost vertical manner at a reverse-bias

potential denoted V Z . The fact that the curve drops down and away from the

horizontal axis rather than up and away for the positive-V D region reveals the

current in the Zener region has a direction opposite to that of a forward-biased

diode. The slight slope to the curve in the Zener region reveals that there is a

level of resistance to be associated with the Zener diode in the conduction

mode.




Zener diode characteristic with the equivalent model for each region

Since some applications of Zener diodes swing between the Zener region

and the forward-bias region, it is important to understand the operation of the

Zener diode in all regions. As shown in the figure, the equivalent model for a

Zener diode in the reverse- bias region below V Z is a very large resistor (as for

the standard diode). For most applications the resistance is so large it can be

ignored and the open circuit equivalent employed. For the forward-bias region

the piecewise equivalent is the same as described in earlier sections.When a diode reaches reverse breakdown, its voltage remains almost

constant even though the current changes drastically, and this is the key to zener

diode operation. Two types of reverse breakdown in a zener diode are

avalanche and zener. The avalanche effect occurs in both rectifier and zener

diodes at a sufficiently high reverse voltage. Zener breakdown occurs in a

zener diode at low reverse voltages. A zener diode is heavily doped to reduce

the breakdown voltage. This causes a very thin depletion region. As a result, an

intense electric field exists within the depletion region. Near the zener

breakdown voltage (V Z), the field is intense enough to pull electrons from theirvalence bands and create current.

Zener diodes with breakdown voltages of less than approximately 5 V

operate predominately in zener breakdown. Those with breakdown voltages

greater than approximately 5 V operate predominately in avalanche

breakdown. Both types, however, are called zener diodes. Zeners are



commercially available with breakdown voltages from less than 1 V to more

than 250 V with specified tolerances from 1% to 20%.

Notice that as the reverse voltage (V R) is increased, the reverse current ( I R)

remains extremely small up to the “knee” of the curve. The reverse current isalso called the zener current, I Z. At this point, the breakdown effect begins; the

internal zener resistance, also called zener impedance ( Z Z), begins to decrease

as the reverse current increases rapidly. From the bottom of the knee, the zener

breakdown voltage (V Z) remains essentially constant although it increases

slightly as the zener current, I Z, increases.

Zener Regulation The ability to keep the reverse voltage across its terminals

essentially constant is the key feature of the zener diode. A zener diode

operating in breakdown acts as a voltage regulator because it maintains a nearly

constant voltage across its terminals over a specified range of reverse-currentvalues. A minimum value of reverse current, I ZK, must be maintained in order to

keep the diode in breakdown for voltage regulation. You can see on the curve

that when the reverse current is reduced below the knee of the curve, the

voltage decreases drastically and regulation is lost. Also, there is a maximum

current, I ZM, above which the diode may be damaged due to excessive power

dissipation. So, basically, the zener diode maintains a nearly constant voltage

across its terminals for values of reverse current ranging from I ZK to I ZM. A

nominal zener voltage, V Z, is usually specified on a datasheet at a value of

reverse current called the zener test current.


The zener diode is widely used as a voltage regulator because of its capacity

to maintain a constant voltage over a sizeable range of currents. It can be used

as a single component across the output of a rectifier or incorporated into one of

the variety of one-chip regulators.

http://hyperphysics.phy-astr.gsu.edu/hbase/solids/zener.html#c3

http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/zenereg.html#c1

http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/rectifiers.html#c1

http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/chipreg.html#c1

http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/chipreg.html#c1

http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/rectifiers.html#c1

http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/zenereg.html#c1

http://hyperphysics.phy-astr.gsu.edu/hbase/solids/zener.html#c3



Schottky Barrier (Hot-Carrier) Diodes

Overview

There has been increasing interest in a two-terminal device referred to as aSchottky-barrier, surface-barrier, or hot-carrier diode. Schottky diodes are high-

current diodes used primarily in high-frequency and fast-switching applications.

The term hot-carrier is derived from the higher energy level of electrons in the

n region compared to those in the metal region.


General construction

A Schottky diode is formed by joining a doped semiconductor region(usually n-type) with a metal such as gold, silver, or platinum. Rather than a p-n

junction, there is a metal-to-semiconductor junction. As shown in the first

figure.

Its construction is quite different from the conventional p-n junction in that a

metal-semiconductor junction is created such as shown in the second figure.

The semiconductor is normally n -type silicon (although p -type silicon is

sometimes used), whereas a host of different metals, such as molybdenum,

platinum, chrome, or tungsten, are used. Different construction techniquesresult in a different set of characteristics for the device, such as increased

frequency range, lower forward bias, and so on. Priorities do not permit an

examination of each technique here, but information will usually be provided

by the manufacturer. In general, however, Schottky diode construction results in

a more uniform junction region and a high level of ruggedness.



Passivated Schottky diode

Schematic Diagram

The equivalent circuit for the device (with typical values) and a commonlyused symbol appear in the figure below.


The application of a forward bias as shown in the first quadrant of the figure

below will reduce the strength of the negative barrier through the attraction of

the positive potential for electrons from this region. The result is a return to the

heavy flow of electrons across the boundary, the magnitude of which is

controlled by the level of the applied bias potential. The barrier at the junction

for a Schottky diode is less than that of the p-n junction device in both the

forward- and reverse-bias regions. The result is therefore a higher current at the

same applied bias in the forward- and reverse-bias regions. This is desirable

effect in the forward-bias region but highly undesirable in the reverse-bias

region.



Comparison of characteristics of hot-carrier and p-n diodes


In both materials, the electron is the majority carrier. In the metal, the level

of minority carriers (holes) is insignificant. When the materials are joined, the

electrons in the n -type silicon semiconductor material immediately flow into the

adjoining metal, establishing a heavy flow of majority carriers. Since theinjected carriers have a very high kinetic energy level compared to the electrons

of the metal, they are commonly called “hot carriers”. In the conventional p-n

junction, there was the injection of minority carriers into the adjoining region.

Here the electrons are injected into a region of the same electron plurality.

Schottky diodes are therefore unique in that conduction is entirely by majority

carriers. The heavy flow of electrons into the metal creates a region near the

junction surface depleted of carriers in the silicon material – much like the

depletion region in the p-n junction diode. The additional carriers in the metal

establish a “negative wall” in the metal at the boundary between the two

materials. The net results is a “surface barrier” between the two materials,

preventing any further current. That is, any electrons (negatively charged) in the

silicon material face a carrier-free region and a “negative wall” at the surface of

the metal.




Its area of application was first limited to the very high frequency range due

to its quick response time (especially important at high frequencies) and lowernoise figure (a quantity of real importance in high-frequency applications). In

recent years, however, it is appearing more and more in low-voltage / high-

current power supplies and ac-to-dc converters. Other areas of application of

the device include radar systems, Schottky TTL logic for computers, mixers and

detectors in communication equipment, instrumentation, and analog-to-digital

converters.



Varactor (Varicap) Diodes

Overview

Varactor [also called varicap, VVC (voltage-variable capacitance), ortunning] diodes are semiconductor, voltage-dependent, variable capacitors. A

diode that always operates in reverse bias and is doped to maximize the inherent

capacitance of the depletion region.Their mode of operation depends on the

capacitance exists at the p-n junction when the element is reverse-biased.


The depletion region acts as a capacitor dielectric because of its

nonconductive characteristic. The p and n regions are conductive and act as thecapacitor plates.

Schematic Diagram

The symbols most commonly used for the varicap diode are shown in the

figures below.




The characteristics of a typical commercially available varicap diode appear

in the figure. Note the initial sharp decline in C T with increase in reverse bias.

The normal range of V R for VVC diodes is limited to about 20 V.


As the reverse-bias voltage increases, the depletion region widens,

effectively increasing the plate separation, thus decreasing the capacitance.

When the reverse-bias voltage decreases, the depletion region narrows, thus

increasing the capacitance.

In a varactor diode, these capacitance parameters are controlled by the

method of doping near the p-n junction and the size and geometry of the diode’sconstruction. Nominal varactor capacitances are typically available from a few

picofarads to several hundred picofarads.




The varactor diode is employed in a tuning network. For example, VHF,

UHF, and satellite receivers utilize varactors. Varactors are also used in cellular

communications.




The almost equal spacing between the curves for the same increment in

luminous flux reveals that the reverse current and the luminous flux are almost

linearly related. In other words, an increase in light intensity will result in a

similar increase in reverse current. A plot of the two to show this linear

relationship appears in the figure below for a fixed voltage V λ of 20 V. On

relative basis, we can assume that the reverse current is essentially zero in the

abscence of incident light.

I λ (μA) versus f C (at V λ = 20 V) for the photodiode of the first figure




Basic biasing arrangement

The reverse saturation current is normally limited to a few microamperes. It

is due solely to the thermally generated minority carriers in the n- and p- type

materials. The application of light to the junction will result in a transfer of

energy from the incident travelling light waves (in the form of photons) to the

atomic structure, resulting in an increased number of minority carriers and an

increased level of reverse current. This is clearly shown in the characteristic

diagram for different intensity levels.

The dark current is that current that will exist with no applied illumination.

Note that the current will only return to zero with a positive applied bias equal

to V T .


The photodiode can be employed in an alarm system. The reverse current I λ will continue to flow as long as the light beam is not broken. If the beam is

interrupted, I λ drops to the dark current level and sounds the alarm.

It can also use to count items on a conveyor belt. As each item passes, the

light beam is broken, I λ drops to the dark current level, and the counter is

increased by one.



Photoconductive Cells

Overview

The photoconductive cell is a two-terminal semiconductor device whoseterminal resistance varies (linearly) with the intensity of the incident light for

obvious reasons; it is frequently called a photoresistive device.


The typical construction of a photoconductive cell is provided in the figure

below.

The photoconductive materials most frequently used include cadmium

sulphide (CdS) and cadmium selenide (CdSe). The photoconductive cell does

not have a junction like the photodiode. A thin layer of the material connected

between terminals is simply exposed to the incident light energy.



Schematic Diagram

The most common schematic diagram of a photoconductive cell is provided

in the figure below.


The sensitivity curve for a typical photoconductive devices appears in te

figure below.


As the illumination on the device increases in intensity, the energy state of a

larger number of electrons in the structure will also increase because of the

increased availability of the photon packages of energy. The result is anincreasing number of relatively “free” electrons in the structure and a decrease

in the terminal resistance.



The peak spectral response occurs at approximately 5100 A for CdS and at

6150 A for CdSe. The response time of CdS units is about 100 ms and of CdSe

cells is 10 ms.


Photoconductive cells are used in many different types of circuits and

applications.

Analog Applications

Camera Exposure Control

Auto Slide Focus - dual cell

Photocopy Machines - density of toner

Colorimetric Test Equipment

Densitometer,Electronic Scales - dual cell

Automatic Gain Control - modulated light source

Automated Rear View Mirror

Digital Applications

Automatic Headlight Dimmer

Night Light Control Oil Burner Flame Out

Street Light Control,Absence / Presence (beam breaker)

Position Senso



IR Emitters

Overview

Infrared-emitting diodes are solid-state gallium arsenide devices that emit abeam of radiant flux when forward-biased.


The basic construction of the device is shown in the figure below.

Schematic Diagram

The schematic diagram of an IR emitter is provided in the figure below.




The radiant flux in milliwatts versus the dc forward current for a typical

device appears in the figure below. Note the almost linear relationship between

the two.


When the junction is forward-biased, electrons from the n-region recombine

with excess holes of the p-material in a specially designed recombination region

sandwiched between the p- and n-type materials. During this recombination



process, energy is radiated away from the device in the form of photons. The

generated photons are either reabsorbed in the structure or leave the surface of

the device as radiant energy, as shown in the figure.


Areas of application for such devices include:

Card and paper-tape readers

Shaft encoders

Data-transmission systems

Intrusion alarms



Solar Cells

Overview

In recent years, there has been increasing interest in the solar cell as analternative source of energy. When we consider that the power density received

from the sun at sea level is about 100 mW/cm2

(1 kW/m2), it is certainly an

energy source that requires further research and development to maximize the

conversion efficiency from solar to electrical energy.

Selenium and silicon are the most widely used materials for solar cells,

although gallium arsenide, indium arsenide, and cadmium sulphide, among

others, are also used. Silicon has a higher conversion efficiency and greaterstability and is less subject to fatigue. Both materials have excellent temperature

characteristics. That is, they can withstand extreme high or low temperatures

without a significant drop-off in efficiency.


The basic construction of a silicon p-n junction solar cell appears in the

figure. As shown in the figure below, every effort is made to ensure that the

surface area perpendicular to the sun is at maximum. Also note that the metallicconductor connected to the p-type material and the thickness of the p-type

material.



Schematic Diagram




Since V = 0 anywhere on the vertical axis and represents a short-circuit

condition, the current at this intersection is called the short-circuit current and is

represented by the notation I SC. Under open-circuit conditions (id = 0), the

photovoltaic voltage V OC will result. This is a logarithmic function of the

illumination, as shown in the figure below. V OC is the terminal voltage of a

battery under no-load (open-circuit) conditions. Note, however, in the same

figure that the short-circuit current is a linear function of the illumination. That

is, it will double for the same increase in illumination ( and ), whereas the

change in V OC is less for this region. The major increase in V OC occurs for lower

level increases in illumination. Eventually, a further increase in illumination

will have very little effect on V OC, although I SC will increase, causing the power

capabilities to increase.




The thickness of the p-type material is such that they ensure that a maximum

number of photons of light energy will reach the junction. A photon of light

energy in this region may collide with a valence electron and impart to it

sufficient energy to leave the parent atom. The result is a generation of free

electrons and holes. This phenomenon will occur on each side of the junction.In the p-type material, the newly generated electrons are minority carriers and

will move rather freely across the junction as explained for the basic p-n

junction with no applied bias. A similar discussion is true for the holes

generated in the n-type material. The result is an increase in the minority-

carriers flow, which opposite on direction to the conventional forward current

of a p-n junction.


To make practical use of the solar-generated energy, the electricity is most

often fed into the electricity grid using inverters (grid-connected photovoltaic

systems); in stand-alone systems, batteries are used to store the energy that is

not needed immediately. Solar cells can be used to power or recharge portable

devices.

http://en.wikipedia.org/wiki/Photovoltaic_system








Tunnel Diodes

Overview

The tunnel diode exhibits a special characteristic known as negativeresistance.

The use of tunnel diodes in present-day high-frequency systems has been

dramatically stalled because of the availability of manufacturing techniques for

alternative devices. Its simplicity, linearity, low power drain, and reliability

ensure its continued life and application.


The tunnel diode is fabricated by doping the semiconductor materials that

will form the p-n junction at a level 100 to several thousand times that of a

typical semiconductor diode. This results in a greatly reduced depletion region,

of the order of magnitude of 10-6 cm, or typically about

the width of this

region for a typical semiconductor diode.

The tunnel diode is similar to a standard p-n junction in many respects

except that the doping levels are very high. Also the depletion region, the area

between the p-type and n-type areas, where there are no carriers is very narrow.

Typically it is in the region of between five to ten nano-metres - only a few

atom widths.

As the depletion region is so narrow this means that if it is to be used for

high frequency operation the diode itself must be made very small to reduce the

high level of capacitance resulting from the very narrow depletion region.



The semiconductor materials most frequently used in the manufacture of

tunnel diodes are germanium and gallium arsenide.

Schematic Diagram

The symbols most frequently employed for tunnel diodes.


Its characteristics, shown in the figure, are different from any diodediscussed thus far in that it has a negative-resistance region. In this region, an

increase in terminal voltage results in a reduction in diode current. For a

comparison purposes, a typical semiconductor diode characteristic is

superimposed on the tunnel-diode characteristic of the figure.




Because of the thin depletion region, through which many carriers can

“tunnel” rather than attempt to surmount, at low forward-bias potentials that

accounts for the peak in the curve as shown in the characteristic diagram.

This reduced depletion region results in carriers “punching through” atvelocities that far exceed those available with conventional diodes.



Electrons at the same level on both sides

of the junction. No net current.

As shown in the figure, the equilibrium energy level diagram of a tunnel

diode with no bias applied. Note that the valence band of the p-materialoverlaps the conduction band of the n-material. The majority electrons and

holes are at the same energy level in the equilibrium state. If there is any

movement of current carriers across the depletion region due to thermal energy,

the net current flow will be zero because equal numbers of current carriers flow

in opposite directions.

Electrons on right side are raised until

they are opposite empty states on left

side. Strong current flows from right to

left.

As shown in the figure, the energy diagram of a tunnel diode with a small

forward bias (200 millivolts) applied. The bias causes unequal energy levels

between some of the majority carriers at the energy band overlap point, but notenough of a potential difference to cause the carriers to cross the forbidden gap

in the normal manner. Since the valence band of the p-material and the

conduction band of the n-material still overlap, current carriers tunnel across at

the overlap and cause a substantial current flow. Note in the figure that the

amount of overlap between the valence band and the conduction band decreased

when forward bias was applied.



Electrons on the right raised still farther.

Some are opposite “forbidden band gap,”

some opposite empty states. Current

decreases.

Electrons all are opposite forbidden gap.Very small current.

As shown in the figure, the energy diagram of a tunnel diode in which the

forward bias has been increased to 400 millivolts. As you can see, the valence

band and the conduction band no longer overlap at this point, and tunneling can

no longer occur. The portion of the curve in the characteristic diagram from

peak point to valley point shows the decreasing current that occurs as the bias is

increased, and the area of overlap becomes smaller. As the overlap between the

two energy bands becomes smaller, fewer and fewer electrons can tunnel across

the junction. The portion of the curve between peak point and valley point in

which current decreases as the voltage increases is the negative resistance

region of the tunnel diode. If the energy diagram of a tunnel diode in which the

forward bias has been increased even further, then the energy bands no longer

overlap and the diode operates in the same manner as a normal pnjunction.



Electrons raised until they spill over

barrier. Current increases.


The tunnel diode can be used in high-speed applications such as computers,where switching times in the order of nanoseconds or picoseconds are desirable.

It is also useful in oscillator and microwave amplifier applications, because of its

negative resistance.



PIN Diode

Overview

A PIN diode is a diode with a wide, lightly doped 'near' intrinsicsemiconductor region between a p-type semiconductor and an n-type

semiconductor region. The p-type and n-type regions are typically

heavily doped because they are used for ohmic contacts.

The wide intrinsic region is in contrast to an ordinary PN diode. The wide

intrinsic region makes the PIN diode an inferior rectifier (one typical function

of a diode), but it makes the PIN diode suitable for attenuators, fast switches,

photodetectors, and high voltage power electronics applications.


Basic PIN diode structure

The instrinic layer of the PIN diode is the one that provides the change in

properties when compared to a normal PN junction diode. The intrinsic region

comprises of the undoped, or virtually undoped semiconductor, and in most PIN

diodes it is very thin - of the order of between 10 and 200 microns.

PIN diode with a planar construction

There are a two main structures that can be used, but the one which is

referred to as a planar structure is shown in the diagram. In the diagram, the

intrinsic layer is shown much larger than if it were drawn to scale. This hasbeen done to better show the overall structure of the PIN diode.

http://en.wikipedia.org/wiki/Diode

http://en.wikipedia.org/wiki/Intrinsic_semiconductor


http://en.wikipedia.org/wiki/P-type_semiconductor

http://en.wikipedia.org/wiki/N-type_semiconductor


http://en.wikipedia.org/wiki/Doping_(semiconductor)

http://en.wikipedia.org/wiki/Ohmic_contact

http://en.wikipedia.org/wiki/P-n_junction

http://en.wikipedia.org/wiki/P-n_junction

http://en.wikipedia.org/wiki/Ohmic_contact

http://en.wikipedia.org/wiki/Doping_(semiconductor)



http://en.wikipedia.org/wiki/P-type_semiconductor



http://en.wikipedia.org/wiki/Diode



Schematic Diagram


Forward series resistance characteristic

When forward-biased, it acts like a current-controlled variable resistance.

Reverse capacitance characteristic



When reverse-biased, the pin diode acts like a nearly constant capacitance.


It is found that at low levels of reverse bias the depletion layer become fully

depleted. Once fully depleted the PIN diode capacitance is independent of the

level of bias because there is little net charge in the intrinsic layer. However the

level of capacitance is typically lower than other forms of diode and this means

that any leakage of RF signals across the diode is lower.

When the PIN diode is forward biased both types of current carrier are

injected into the intrinsic layer where they combine. It is this process that

enables the current to flow across the layer.

The particularly useful aspect of the PIN diode occurs when it is used with

high frequency signals, the diode appears as a resistor rather than a non linear

device, and it produces no rectification or distortion. Its resistance is governed

by the DC bias applied. In this way it is possible to use the device as an

effective RF switch or variable resistor producing far less distortion than

ordinary PN junction diodes.


The pin diode is used as a dc-controlled microwave switch operated by rapid

changes in bias or as a modulating device that takes advantage of the variable

forward-resistance characteristic. Since no rectification occurs at the pn

junction, a high-frequency signal can be modulated (varied) by a lower-

frequency bias variation. A pin diode can also be used in attenuator applications

because its resistance can be controlled by the amount of current. Certain types

of pin diodes are used as photodetectors in fiber-optic systems.

The PIN diode is used in a variety of different applications from lowfrequencies up to high radio frequencies. The properties introduced by the

intrinsic layer make it suitable for a number of applications where ordinary PN

junction diodes are less suitable.

In the first instance the diode can be used as a power rectifier. Here the

intrinsic layer gives it a high reverse breakdown voltage, and this can be used to

good effect in many applications.

Although the PIN diode finds many applications in the high voltage arena, itis probably for radio frequency applications where it is best known. The fact

that when it is forward biased, the diode is linear, behaving like a resistor, can

be put to good use in a variety of applications. It can be used as a variable

resistor in a variable attenuator, a function that few other components can

achieve as effectively. The PIN diode can also be used as an RF switch. In the

forward direction it can be biased sufficiently to ensure it has a low resistance



to the RF that needs to be passed, and when a reverse bias is applied it acts as

an open circuit The fact that the PIN diode has a low level of capacitance

because of the additional intrinsic layer in the diode, means that it can switch

more effectively than other forms of diode.



Gunn Diode

Overview

The Gunn diode is a unique component - even though it is called a diode, itdoes not contain a PN diode junction. The Gunn diode or transferred electron

device can be termed a diode because it does have two electrodes. It depends

upon the bulk material properties rather than that of a PN junction. The Gunn

diode operation depends on the fact that it has a voltage controlled negative

resistance.

The Gunn diode is not like a typical PN junction diode. Rather than having

both p-type and n-type semiconductor, it only utilises n-type semiconductorwhere electrons are the majority carriers.


Cross section of only N-type semiconductor diode.

Gunn diodes are fabricated from a single piece of n-type semiconductor. The

most common materials are gallium Arsenide, GaAs and Indium Phosphide,

InP. However other materials including Ge, CdTe, InAs, InSb, ZnSe and others

have been used. The device is simply an n-type bar with n+ contacts. It isnecessary to use n-type material because the transferred electron effect is only

applicable to electrons and not holes found in a p-type material.

Within the device there are three main areas, which can be roughly termed

the top, middle and bottom areas. as shown in the figure below.



The most common method of manufacturing a Gunn diode is to grow and

epitaxial layer on a degenerate n+ substrate. The active region is between a few

microns and a few hundred micron thick. This active layer has a doping level

between 1014

cm-3

and 1016

cm-3

- this is considerably less than that used for the

top and bottom areas of the device. The thickness will vary according to the

frequency required.

The top n+ layer can be deposited epitaxially or doped using ion

implantation. Both top and bottom areas of the device are heavily doped to give

n+ material. This provides the required high conductivity areas that are needed

for the connections to the device.

A discrete Gunn diode with the active layer

mounted onto a heatsink for efficient heat transfer

Devices are normally mounted on a conducting base to which a wireconnection is made. The base also acts as a heat sink which is critical for the

removal of heat. The connection to the other terminal of the diode is made via a

gold connection deposited onto the top surface. Gold is required because of its

relative stability and high conductivity.

Schematic Diagram



The Gunn diode symbol used in circuit diagrams varies. Often a standard

diode is seen in the diagram, however this form of Gunn diode symbol does not

indicate the fact that the Gunn diode is not a PN junction. Instead another

symbol showing two filled in triangles with points touching is used as shown

below.


As voltage is increased, conduction increases due to electrons in a low

energy conduction band. As voltage is increased beyond the threshold of

approximately 1 V, electrons move from the lower conduction band to thehigher energy conduction band where they no longer contribute to conduction.

In other words, as voltage increases, current decreases, a negative resistance

condition.

This negative resistance region means that the current flow in diode

increases in the negative resistance region when the voltage falls - the inverse of

the normal effect in any other positive resistance element. This phase reversal

enables the Gunn diode to act as an amplifier and oscillator.

Operation of the device

The operation of the Gunn diode can be explained in basic terms. When a

voltage is placed across the device, most of the voltage appears across the inner



active region. As this is particularly thin this means that the voltage gradient

that exists in this region is exceedingly high.

The device exhibits a negative resistance region on its V/I curve as seen

below. This negative resistance area enables the Gunn diode to amplify signals.

This can be used both in amplifiers and oscillators. However Gunn diode

oscillators are the most commonly found.


Gunn diode oscillators are used for many purposes and typical applications

include local oscillators, klystron replacement, transmit and receive oscillators

for radio communications, military and commercial radar sources, police radar,

sensors for detecting: velocity, direction, proximity, or fluid levels, alarms,pumps for parametric amplifiers, wireless Lans, collision avoidance and

intelligent cruise control, and others.

A Gunn diode can be used to amplify signals because of the apparent

"negative resistance". Gunn diodes are commonly used as a source of high

frequency and high power signals. A bias tee is needed to isolate the bias

current from the high frequency oscillations. Since this is a single-port device,

there is no isolation between input and output.

A bias tee is a three port network used for setting the DC bias point of some

electronic components without disturbing other components.

http://en.wikipedia.org/wiki/Bias_tee



http://en.wikipedia.org/wiki/Biasing

http://en.wikipedia.org/wiki/Biasing




Laser Diode

Overview

The laser diode is a further development upon the regular light-emitting

diode, or LED. The term “laser” itself is actually an acronym, despite the fact

its often written in lower-case letters. “Laser” stands for Light Amplification

by Stimulated Emission of Radiation.

Laser light is monochromatic, which means that it consists of a single color

and not a mixture of colors. Laser light is also called coherent light , a single

wavelength, as compared to incoherent light, which consists of a wide band of

wavelengths. The laser diode normally emits coherent light, whereas the LED

emits incoherent light.


Basic laser diode construction.

The basic construction of a laser diode is shown in the figure. A p-n junction

is formed by two layers of doped gallium arsenide. The length of the p-n

junction bears a precise relationship with the wavelength of the light to be

emitted. There is a highly reflective surface at one end of the p-n junction and a



partially reflective surface at the other end, forming a resonant cavity for the

photons. External leads provide the anode and cathode connections.

The laser diode is consists of heavily doped n+ and p+ regions. For

manufacture it is normal to start with an n+ substrate and then the top layer can

be grown onto this. The doping can be included in a variety of ways, either by

diffusion, ion implantation or even deposited during the epitaxy process. A

variety of materials can be used for laser diodes, although the most common

starting substrates are Gallium Arsenide (GaAs) and Indium Phosphate (InP).

These are known as type III-V compounds because of their places in the

chemical periodic table of elements. Whatever material is used, it must be

possible to heavily dope it as either a p type or n type semiconductor. This rules

out most of the type II-VI materials, leaving the group III-V materials as the

ideal option.

Apart from the basic semiconductor requirements, there are a number of

optical requirements that are needed to enable the laser diode to operate. It

needs an optical resonator. This must occur in the plane of the required light

output. To achieve this the two walls of the laser diode that form the resonator

must be almost perfectly smooth, forming a mirror surface from which the light

can be reflected internally. One of the walls is made slightly less reflecting to

enable the light to come out from the laser diode. Another requirement is that

the two mirror surfaces must be perfectly perpendicular to the junction,

otherwise the laser action does not occur satisfactorily. The two other surfaces

perpendicular to the one of the required light output are roughened slightly to

ensure that the laser action does not occur in this plane as well. In this way a

resonant optical cavity is created. Although it is many wavelengths long it still

acts as a resonant cavity.

Schematic Diagram

The laser diode symbol used for circuit diagrams is often the same one used

for light emitting diodes. This laser diode circuit symbol uses the basic

semiconductor diode symbol with arrows indicating the generation and

emanation of light.


One of the most commonly used and important laser diode specifications or

characteristics is the L/I curve. It plots the drive current supplied against the

light output.



This laser diode specification is used to determine the current required to

obtain a particular level of light output at a given current. It can also be seen

that the light output is also very dependent upon the temperature.

Laser diode L-I Characteristic

From this characteristic, it can be seen that there is a threshold current below

which the laser action does not take place. The laser diode should be operated

clear of this point to ensure reliable operation over the full operating

temperature range as the threshold current rises with increasing temperature. Itis typically found that the laser threshold current rises exponentially with

temperature.

The laser diode specification for the forward voltage across the diode is

required in a number of areas of the design. Often laser diode manufacturers

prefer to place the voltage on the vertical axis.

Laser diode V-I Characteristic



From the diagram it can be seen that the voltage across the laser diode is

typically around 1.5 volts, although it is necessary to check for the particular

laser diode in question. The forward voltage specification will vary according to

the materials used in the diode, current, etc.

Although the forward voltage does vary with temperature, this is not

normally a major consideration.


Basic laser diode operation.

The basic operation is as follows. The laser diode is forward-biased by an

external voltage source. As electrons move through the junction, recombination

occurs just as in an ordinary diode. As electrons fall into holes to recombine,

photons are released. A released photon can strike an atom, causing another

photon to be released. As the forward current is increased, more electrons enter

the depletion region and cause more photons to be emitted. Eventually some of

the photons that are randomly drifting within the depletion region strike the

reflected surfaces perpendicularly. These reflected photons move along the

depletion region, striking atoms and releasing additional photons due to the

avalanche effect. This back-and-forth movement of photons increases as thegeneration of photons “snowballs” until a very intense beam of laser light is

formed by the photons that pass through the partially reflective end of the pn

junction.



Each photon produced in this process is identical to the other photons in

energy level, phase relationship, and frequency. So a single wavelength of

intense light emerges from the laser diode, as indicated in Figure 3 – 48(c). Laser

diodes have a threshold level of current above which the laser action occurs and

below which the diode behaves essentially as an LED, emitting incoherent light.


With laser diodes being lending themselves to use in many areas of

electronics from CD, DVD and other forms of data storage through to

telecommunications links, laser diode technology offers a very convenient

means of developing coherent light.

Laser diodes and photodiodes are used in the pick-up system of compactdisk (CD) players. Audio information (sound) is digitally recorded in stereo on

the surface of a compact disk in the form of microscopic “pits” and “flats.” Alens arrangement focuses the laser beam from the diode onto the CD surface.

As the CD rotates, the lens and beam follow the track under control of a

servomotor. The laser light, which is altered by the pits and flats along the

recorded track, is reflected back from the track through a lens and optical

system to infrared photodiodes. The signal from the photodiodes is then used to

reproduce the digitally recorded sound. Laser diodes are also used in laser

printers and fiber-optic systems.



Current Limiting Diode (CLD)

Overview

A Current Limiting Diode, also known as a “Current Regulating Diode” or a“Constant Current Diode”, performs quite a unique function. Similar to a zener

diode, which regulates voltage at a particular current, the CLD limits or

regulates current over a wide voltage range. The CLD is diffused using a “FieldEffect” process similar to the diffusion techniques used in manufacturing JFETs

with the electrical characteristics optimized for high output impedance and

current regulating capability.


CLD cross section

Schematic Diagram




From Figure, it can be seen that the device performs as a constant current

generator after about some voltage level of more is applied from anode (drain)

to cathode (source) of the device until breakdown occurs. Whereas, in thereverse direction it operates as a forward biased diode.

IL - Limiting Current: 80% of IP minimum

used to determine Limiting Voltage VL.

IP - Pinch-off Current: Regulator current atspecified Test Voltage (VT = 25V).

POV - Peak Operating Voltage: Maximum voltage

to be applied to device.

TC - Current Temperature Coefficient.

VAK - Anode to Cathode Voltage.

VK - Knee Impedance Test Voltage: Specified

voltage used to establish Knee Impedance (ZK).

VL - Limiting Voltage: Measured at IL, VL

together with knee AC impedance (ZK),

indicates the knee characteristics of the devices.



VT - Test Voltage: Voltage at which IP and ZT are specified.

ZK - Knee AC impedance at Testing Voltage:To test

for ZK, a 90 Hz signal (V

K), with RMS value

equal to 10% of the test voltage, VK, is superimposed

on VK: ZK = VK /iK, where iK is the

resultant AC current due to VK To provide the

most constant current, ZK should be as

high as possible.

ZT - AC Impedance at Test Voltage: Specified as a

minimum value. To test for ZT, a 90 Hz signal

with RMS value equal to 10% of Test Voltage

(VT), is superimposed on VT.


In operation the CLD regulates the amount of current that can flow over a

voltage range of about 1 to 100 volts. As shown by the characteristic curve and

the figure above, the CLD begins to conduct when a reverse biased voltage is

applied from the cathode to the anode or PN junction. As the reverse biased

voltage is increased to VL, the current increases due to the bulk resistance of the N region. As the current approaches the knee section of the curve, a

depletion region develops between the N region and the P-type gate. This

depletion region decreases the current path in the N region slowing the increase

of current flow. Eventually, the depletion region meets the P-type gate and

pinch-off occurs, allowing current flow to become constant and almost



independent of applied voltage until PN junction breakdown occurs somewhere

above POV. When the polarity of the applied voltage is reversed and a forward

bias is applied to the PN junction, the CLD exhibits characteristics similar to

those of a forward biased diode or rectifier.


One application for a constant-current diode is to automatically limit current

through an LED or laser diode over a wide range of power supply voltages.

Of course, the constant-current diode's regulation point should be chosen to

match the LED or laser diodes optimum forward current. This is especially

important for the laser diode, not so much for the LED, as regular LEDs tend to

be more tolerant of forward current variations.

Another application is in the charging of small secondary-cell batteries,

where a constant charging current leads to predictable charging times. Of

course, large secondary-cell battery banks might also benefit from constant-

current charging, but constant-current diodes tend to be very small devices,

limited to regulating currents in the milliamp range.

different types of diodes-main

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