TYPES OF DIODE

PN junction diode Diode types overview Diode specifications Gunn diode IMPATT diode Laser diode Photo diode PIN diode Schottky barrier diode Step recovery diode Tunnel diode Varactar diode Zener diode Light emitting diode, LED

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.

Leaded Semiconductor DiodesDiodes 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.

SMT Diodes on a PCB

Types of diodes

It is sometimes useful to summarise the different types of diode 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.

Backward diode:   This type of diode is sometimes also called the back diode. Although not widely used, it is a form of PN junction diode that is very similar to the tunnel diode in its operation. It finds a few specialist applications where its particular properties can be used. Read more about the Backward diode

BARITT diode:   This form of diode gains its name from the words Barrier Injection Transit Time diode. It is used in microwave applications and bears many similarities to the more widely used IMPATT diode. Read more about the Baritt diode

Gunn Diode:   Although not a diode in the form of a PN junction, this type of diode is a semiconductor device that has two terminals. It is generally used for generating microwave signals. Read more about the Gunn diode

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. Read more about the laser diode

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. Read more about the light emitting diode

Photodiode:   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 photodetectors. Read more about the photo-diode

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 photodiodes, etc. Read more about the PIN diode

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. Read more about the diode

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. Read more about the Schottky diode

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. Read more about the Step recovery diode

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. Read more about the Tunnel diode

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. Read more about the varactor diode

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.Read more about the Zener / voltage reference diode

Semiconductor diodes are widely used throughout all areas of the electronics industry from electronics design through to production and repair. The semiconductor diode is very versatile, and there are very many variants and different types of diode that enable all the variety of different applications to be met.

The different diode types of types of diodes include those for small signal applications, high current and voltage as well as different types of diodes for light emission and detection as well as types for low forward voltage drops, and types to give variable capacitance. In addition to this there are a number of diode types that are used for microwave applications.

Diode specifications characteristics and parameters

The list below provides details of the various diode characteristics, and diode parameters found in the datasheets and specifications for diodes.

Semiconductor material:   The semiconductor material used in the PN junction diode is of paramount importance because the material used affects many of the major diode characteristics and properties. Silicon is the most widely used material as if offers high levels of performance for most applications and it offers low manufacturing costs. The other material that is used is germanium. Other materials are generally reserved for more specialist diodes. The semiconductor material choice is of particular importance as it governs the turn on voltage for the diode - around 0.6volts for silicon and 0.3 volts for germanium, etc..

Forward voltage drop (Vf):   Any electronics device passing current will develop a resulting voltage across it and this diode characteristic is of great importance, especially for power rectification where power losses will be higher for a high forward voltage drop. Also RF diodes often need a small forward voltage drop as signals may be small but still need to overcome it.

The voltage across a PN junction diode arise for two reasons. The first of the nature of the semiconductor PN junction and results from the turn-on voltage mentioned above. This voltage enables the depletion layer to be overcome and for current to flow. The second arises from the normal resistive losses in the device. As a result a figure for the forward voltage drop are a specified current level will be given. This figure is particularly important for rectifier diodes where significant levels of current may be passed.

Peak Inverse Voltage (PIV):   This diode characteristics is the maximum voltage a diode can withstand in the reverse direction. This voltage must not be exceeded otherwise the device may fail. This voltage is not simply the RMS voltage of the incoming waveform. Each circuit needs to be considered on its own merits, but for a simple single diode half wave rectifier with some form of smoothing capacitor afterwards, it should be remembered that the capacitor will hold a voltage equal to the peak of the incoming voltage waveform. The diode will then also see the peak of the incoming waveform in the reverse direction and therefore under these circumstances it will see a peak inverse voltage equal to the peak to peak value of the waveform.

Maximum forward current:   When designing a circuit that passes any levels of current it is necessary to ensure that the maximum current levels for the diode are not exceeded. As the current levels rise, so additional heat is dissipated and this needs to be removed.

Leakage current:   If a perfect diode were available, then no current would flow when it was reverse biased. It is found that for a real PN junction diode, a very small amount of current flow in the reverse direction as a result of the minority carriers in the semiconductor. The level of leakage current is dependent upon three main factors. The reverse voltage is obviously significant. It is also temperature dependent, rising appreciably with temperature. It is also found that it is very dependent upon the type of semiconductor material used - silicon is very much better than germanium.

The leakage current characteristic or specification for a PN junction diode is specified at a certain reverse voltage and particular temperature. The specification is normally defined in terms of in microamps, μA or picoamps, pA.

Junction capacitance:   All PN junction diodes exhibit a junction capacitance. The depletion region is the dielectric spacing between the two plates which are effectively formed at the edge of the depletion region and the area with majority carriers. The actual value of capacitance being dependent upon the reverse voltage which causes the depletion region to change (increasing reverse voltage increases the size of the depletion region and hence decreases the capacitance). This fact is used in varactor or varicap diodes to good effect, but for many other applications, especially RF applications this needs to be minimised. As the capacitance is of importance it is specified. The parameter is normally detailed as a given capacitance (in pF) at a given voltage or voltages. Also special low capacitance diodes are available for many RF applications.

Package type:   Diodes can be mounted in a variety of packages according to their applications, and in some circumstances, especially RF applications, the package is a key element in defining the overall RF diode characteristics. Also for power applications where heat dissipation is important, the package can define many of the overall diode parameters because high power diodes may require packages that can be bolted to heatsinks, whereas small signal diodes may be available in leaded formats or as surface mount devices.

Gunn diodes are also known as transferred electron devices, TED, are widely used in microwave RF applications for frequencies between 1 and 100 GHz.

The Gunn diode is most commonly used for generating microwave RF signals - these circuits may also be called a transferred electron oscillator or TEO. The Gunn diode may also be used for an amplifier in what may be known as a transferred electron amplifier or TEA.

As Gunn diodes are easy to use, they form a relatively low cost method for generating microwave RF signals.

Gunn diode basicsThe Gunn diode is a unique component - even though it is called a diode, it does 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 mechanism behind the transferred electron effect was first published by Ridley and Watkins in a paper in 1961. Further work was published by Hilsum in 1962, and then in 1963 John Battiscombe (J. B.) Gunn independently observed the first transferred electron oscillation using Gallium Arsenide, GaAs semiconductor.

Gunn diode symbol for circuit diagramsThe 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.

Gunn diode symbol

Gunn diode constructionGunn 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 is necessary 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.

A discrete Gunn diode with the active layer mountedonto a heatsink for efficient heat transfer

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 1014cm-3 and 1016cm-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.

Devices are normally mounted on a conducting base to which a wire connection 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.

During manufacture there are a number of mandatory requirements for the devices to be successful - the material must be defect free and it must also have a very uniform level of doping.

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 semiconductor where electrons are the majority carriers.

The Gunn diode operation depends upon the very thin active region for its operation, it forms an ideal low power microwave RF oscillator, although it may also be used as an RF amplifier as well.

Gunn diode operation basics

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 characteristic

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.

Gunn diode operation at microwave frequencies

At microwave frequencies, it is found that the dynamic action of the diode incorporates elements resulting from the thickness of the active region. When the voltage across the active region reaches a certain point a current is initiated and travels across the active region. During the time when the current pulse is moving across the active region the potential gradient falls preventing any further pulses from forming. Only when the pulse has reached the far side of the active region will the potential gradient rise, allowing the next pulse to be created.

It can be seen that the time taken for the current pulse to traverse the active region largely determines the rate at which current pulses are generated, and hence it determines the frequency of operation.

To see how this occurs, it is necessary to look at the electron concentration across the active region. Under normal conditions the concentration of free electrons would be the same regardless of the distance across the active diode region. However a small perturbation may occur resulting from noise from the current flow, or even external noise - this form of noise will always be present and acts as the seed for the oscillation. This grows as it passes across the active region of the Gunn diode.

Gunn diode operation

The increase in free electrons in one area cause the free electrons in another area to decrease forming a form of wave. It also results in a higher field for the electrons in this region. This higher field slows down these electrons relative to the remainder. As a result the region of excess electrons will grow because the electrons in the trailing path arrive with a higher velocity. Similarly the area depleted of electrons will also grow because the electrons slightly ahead of the area with excess electrons can move faster. In this way, more electrons enter the region of excess making it larger, and more electrons leave the depleted region because they too can move faster. In this way the perturbation increases.

Gunn diode operation - electrons in the peak move more slowly

The peak will traverse across the diode under the action of the potential across the diode, and growing as it traverses the diode as a result of the negative resistance.

A clue to the reason for this unusual action can be seen if the voltage and current curves are plotted for a normal diode and a Gunn diode. For a normal diode the current increases with voltage, although the relationship is not linear. On the other hand the current for a Gunn

diode starts to increase, and once a certain voltage has been reached, it starts to fall before rising again. The region where it falls is known as a negative resistance region, and this is the reason why it oscillates.

IMPATT DIODES

The IMPATT diode or to give it the complete name, IMPact ionisation Avalanche Transit Time diode is an RF semiconductor device that is used for generating microwave radio frequency signals.

With the ability to operate at frequencies between about 3 and 100 GHz or more, one of the main advantages of this microwave diode is the relatively high power capability (often ten watts and more) when compared to other forms of microwave diode.

The IMPATT diode has many advantages, although with highly stable frequency synthesizers offering high levels of performance and other semiconductor technologies improving, in recent years it has been used less widely.

ApplicationsIMPATT diodes are ideal where small cost effective microwave radio sources are needed. The main drawback of generators using IMPATT diodes is the high level of phase noise they generate. This results from the statistical nature of the avalanche process that is key to their operation. Nevertheless these diodes make excellent signal sources for many RF microwave applications.

Typically the device is used in a number of applications including:

Alarms

Radar

Detectors using RF technology

Discovery & developmentThe original idea for the diode was put forward by Shockley in 1954. He thought of the idea of creating negative resistance using a transit time delay mechanism. The method of injection for the carriers was a forward biased PN junction. He published this in the Bell Systems Technical Journal in 1954 in an item entitled: 'Negative resistance arising from transit time in semiconductor diodes.'

However it was not until 1958 that W.T. Read of Bell Laboratories proposed the p+ n i n+ diode structure which was later called the Read diode. This diode used the avalanche multiplication as the

injection mechanism. Again this was published in the Bell Systems Technical Journal in 1958 under the title: A proposed high-frequency, negative resistance diode.'

Although the injection mechanism and diode had been postulated, it was not until 1965 that the first practical operating diodes were made that enabled oscillations to be observed. The diode used for this demonstration was fabricated using silicon and had a p+ n structure.

After this, operation of the Read diode was demonstrated and then in 1966 a p i n diode was also demonstrated to work.

IMPATT basicsIn many respects the IMPATT diode is an unusual diode in that it is able to provide high power RF signals at microwave frequencies using a structure that is not that far different from the basic PN junction. However it has been developed to enable its different mode of operation to be utilised properly.

Theory &operation:   The IMPATT diode relies upon a negative resistance effect caused by the transit time of the carriers. This negative resistance enables the diode to act as an oscillator, creating signals at microwave frequencies. Read more about the IMPATT theory & operation

Fabrication & structure:   There are a number of structures and fabrication methods used for IMPATT diodes. Each one has its own advantages and disadvantages. Read more about the IMPATT structure & fabrication

Practical operationThe main application for IMPATT diodes is in microwave generators. An alternating signal is generated simply by applying a DC supply when a suitable tuned circuit is applied.

The output is reliable and relatively high when compared to other forms of microwave diode. In view of its high levels of phase noise it is used in transmitters more frequently than as a local oscillator in receivers where the phase noise performance is generally more important. It is also used in applications where phase noise performance is unlikely to be of importance.

To run an IMPATT diode, a relatively high voltage, often as high as 70 volts or higher may be required. This often limits their application as voltages of this order are not always easy to use in some pieces of equipment. Nevertheless IMPATT diodes are particularly attractive option for microwave diodes for many areas.

While much of the structure of the IMPATT diode is similar to a standard Schottky or PIN diode in its basic outline, the theory and method of operation are very different.

The diode utilises avalanche breakdown combined with the transit times of the charge carriers to enable it to provide a negative resistance region and therefore act as an oscillator.

As the nature of the avalanche breakdown is very noisy, and signals created by an IMPATT diode have high levels of phase noise.

IMPATT diode theory basicsLike any other diode, an IMPATT has a relatively standard IV characteristic. In the forward direction it will conduct after it has reached the forward conduction point. In the reverse direction it will block current.

However at a certain voltage the diode will break down and current will flow in the reverse direction.

Graphical representation of the IMPATT diode IV characteristic

The IMPATT diode is operated under reverse bias conditions. These are set so that avalanche breakdown occurs. This occurs in the region very close to the P+ (i.e. heavily doped P region). The electric field at the p-n junction is very high because the voltage appears across a very narrow gap creating a high potential gradient. Under these circumstances any carriers are accelerated very quickly.

As a result they collide with the crystal lattice and free other carriers. These newly freed carriers are similarly accelerated and collide with the crystal lattice freeing more carriers. This process gives rise to what is termed avalanche breakdown as the number of carriers multiplies very quickly. For this type of breakdown only occurs when a certain voltage is applied to the junction. Below this the potential does not accelerate the carriers sufficiently.

In terms of its operation the IMPATT diode can be considered to consist of two areas, namely the avalanche region or injection region, and secondly the drift region.

These two areas provide different functions. The avalanche or injection region creates the carriers which may be either holes of electrons, and the drift region is where the carriers move across the diode taking a certain amount of time dependent upon its thickness.

The two types of carrier drift in opposite directions.

Charge carrier movement within an IMPATT diode

IMPATT diode operationOnce the carriers have been generated the device relies on negative resistance to generate and sustain an oscillation. The effect does not occur in the device at DC, but instead, here it is an AC effect that is brought about by phase differences that are seen at the frequency of operation. When an AC signal is applied the current peaks are found to be 180° out of phase with the voltage. This results from two delays which occur in the device: injection delay, and a transit time delay as the current carriers migrate or drift across the device.

IMPATT diode voltage & current waveforms

The voltage applied to the IMPATT diode has a mean value where it is on the verge of avalanche breakdown. The voltage varies as a sine wave, but the generation of carriers

does not occur in unison with the voltage variations. It might be expected that it would occur at the peak voltage. This arises because the generation of carriers is not only a function of the electric field but also the number of carriers already in existence.

As the electric field increases so does the number of carriers. Then even after the field has reached its peak the number of carriers still continues to grow as a result of the number of carriers already in existence. This continues until the field falls to below a critical value when the number of carriers starts to fall. As a result of this effect there is a phase lag so that the current is about 90° behind the voltage. This is known as the injection phase delay.

When the electrons move across the N+ region an external current is seen, and this occurs in peaks, resulting in a repetitive waveform.

IMPATT circuitsIMPATT diodes are generally used at frequencies above around 3 GHz. It is found that when a tuned circuit is applied along with a voltage around the breakdown voltage to the IMPATT, and oscillation will occur.

Compared to other devices that use negative resistance and are available for operation at these frequencies, the IMPATT is able to produce much higher levels of power. Typically figures of ten or more watts may be obtained, dependent upon the device.

One of the main drawbacks of the IMPATT diode in its operation is the generation of high levels of phase noise as a result of the avalanche breakdown mechanism. It is found the devices based around Gallium Arsenide technology are much better than those using Silicon. This results from the much closer ionisation coefficients for holes and electrons.

The TRAPATT or TRApped, Plasma Avalanche Triggered Transit diode belongs to the same basic family as the IMPATT diode.

However the TRAPATT has a number of advantages and as a result it is used in a number of applications.

Essentially the TRAPATT normally used as a microwave oscillator, but has the advantage of a greater level of efficiency - typically the DC to RF signal conversion efficiency may be in the region of . 20 to 60%.

TRAPATT basicsThe TRAPATT diode is based around the initial concept of the IMPATT. However for the TRAPATT, the doping level between the junction and the anode.

Typically the construction of the device consists of a p+ n n+, although where for higher power levels an n+ p p+ structure is better.

For operation the TRAPATT is excited using a current pulse which causes the electric field to increase to a critical value where avalanche multiplication occurs. At this point the field collapses locally due to the generated plasma.

The separation and drift of the electrons and holes are then driven by a very much smaller field. It virtually appears that they have been 'trapped' behind with a velocity smaller than the saturation velocity. After the plasma spreads across the whole active region, the holes and electrons begin to drift to the opposite terminals and then the electric field begins to rise again.

Diagrammatic TRAPATT diode structure

The criterion for operation in TRAPATT operation is that the avalanche front advances fasrer than the saturation velocity of the carriers. In general it exceeds the saturation value by a factor of around three.

The TRAPATT mode does not depend upon the injection phase delay.

Although the TRAPATT diode provides a much higher level of efficiency than the IMPATT, its major disadvantage is that the noise levels on the signal are even higher than they are when using an IMPATT. A balance needs to be made according to the application required.

SCHOTTKY BARRIER DIODE

Schottky Barrier Diode: Tutorial

- the Schottky Barrier Diode is used for its low turn-on voltage; fast recover time; and low junction capacitance in applications from power rectification to RF signal applications, and logic.

IN THIS SECTION

Schottky diode tutorial

Schottky diode technology & structure

Schottky diode characteristics & specs

Schottky diode power rectifier

Other diodes

The Schottky diode or Schottky Barrier diode is an electronics component that is widely used for radio frequency, RF applications as a mixer or detector diode.

The diode is also used in power applications as a rectifier, again because of its low forward voltage drop leading to lower levels of power loss compared to ordinary PN junction diodes.

Although normally called the Schottky diode these days, named after Schottky, it is also sometimes referred to as the surface barrier diode, hot carrier diode or even hot electron diode.

Discover & introduction

Despite the fact that Schottky barrier diodes have many applications in today's high tech electronics scene, it is actually one of the oldest semiconductor devices in existence. As a metal-semiconductor devices, its applications can be traced back to before 1900 where crystal detectors, cat's whisker detectors and the like were all effectively Schottky barrier diodes.

Circuit symbol

The Schottky circuit symbol used in many circuit schematic diagrams may be that of an ordinary diode symbol. However it is often necessary to use a specific Schottky diode symbol to signify that a Schottky diode rather than another one must be used because it is essential to the operation of the circuit. Accordingly a specific Schottky diode symbol has been accepted for use. The circuit symbol is shown below:

Schottky diode symbol

It can be seen from the circuit symbol that it is based on the normal diode one, but with additional elements to the bar across the triangle shape.

Advantages

Schottky diodes are used in many applications where other types of diode will not perform as well. They offer a number of advantages:

Low turn on voltage:   The turn on voltage for the diode is between 0.2 and 0.3 volts for a silicon diode against 0.6 to 0.7 volts for a standard silicon diode. This makes it have very much the same turn on voltage as a germanium diode.

Fast recovery time:   The fast recovery time because of the small amount of stored charge means that it can be used for high speed switching applications.

Low junction capacitance:   In view of the very small active area, often as a result of using a wire point contact onto the silicon, the capacitance levels are very small.

The advantages of the Schottky diode, mean that its performance can far exceed that of other diodes in many areas.

Applications

The Schottky barrier diodes are widely used in the electronics industry finding many uses as diode rectifier. Its unique properties enable it to be used in a number of applications where other diodes would not be able to provide the same level of performance. In particular it is used in areas including:

RF mixer and detector diode:   The Schottky diode has come into its own for radio frequency applications because of its high switching speed and high frequency capability. In view of this Schottky barrier diodes are used in many high performance diode ring mixers. In addition to this their low turn on voltage and high frequency capability and low capacitance make them ideal as RF detectors.

Power rectifier:   Schottky barrier diodes are also used in high power applications, as rectifiers. Their high current density and low forward voltage drop mean that less power is wasted than if ordinary PN junction diodes were used. This increase in efficiency means that less heat has to be dissipated, and smaller heat sinks may be able to be incorporated in the design.

Power OR circuits:   Schottky diodes can be used in applications where a load is driven by two separate power supplies. One example may be a mains power supply and a battery supply. In these instances it is necessary that the power from one supply does not enter the other. This can be achieved using diodes. However it is important that any voltage drop across the diodes is minimised to ensure maximum efficiency. As in many other applications, this diode is ideal for this in view of its low forward voltage drop.

Schottky diodes tend to have a high reverse leakage current. This can lead to problems with any sensing circuits that may be in use. Leakage paths into high impedance circuits can give rise to false readings. This must therefore be accommodated in the circuit design.

Solar cell applications:   Solar cells are typically connected to rechargeable batteries, often lead acid batteries because power may be required 24 hours a day and the Sun is not always available. Solar cells do not like the reverse charge applied and therefore a diode is required in series with the solar cells. Any voltage drop will result in a reduction in efficiency and therefore a low voltage drop diode is needed. As in other applications, the low voltage drop of the Schottky diode is particularly useful, and as a result they are the favoured form of diode in this application.

Clamp diode - especially with its use in LS TTL:   Schottky barrier diodes may also be used as a clamp diode in a transistor circuit to speed the operation when used as a switch. They were used in this role in the 74LS (low power Schottky) and 74S (Schottky) families of logic circuits. In these chips the diodes are inserted between the collector and base of the driver transistor to act as a clamp. To produce a low or logic "0" output the transistor is driven hard on, and in this situation the base collector junction in the diode is forward biased. When the Schottky diode is present this takes most of the current and allows the turn off time of the transistor to be greatly reduced, thereby improving the speed of the circuit.

An NPN transistor with Schottky diode clamp

In view of its properties, the Schottky diode finds uses in applications right through from power rectification to uses in clamp diodes in high speed logic devices and then on to high frequency RF applications as signal rectifiers and in mixers.

Their properties span many different types of circuit making them almost unique in the variety of areas and circuits in which they can be used.

Although it may appear a rather straightforward form of component when compared to others, Schottky diode technology has much to offer.

The Schottky diode structure while appearing straightforward in its most simple version is still able to offer performance that no other form of diode can provide.

As a result, Schottky diode technology has developed to enable it to be used in areas that would otherwise not have been possible. Many diodes incorporating elements including guard rings enhance the performance in some areas by considerable degrees.

Basic Schottky diode structureThe Schottky barrier diode can be manufactured in a variety of forms. The most simple is the point contact diode where a metal wire is pressed against a clean semiconductor surface. This was how the early Cat's Whisker detectors were made, and they were found to be very unreliable, requiring frequent repositioning of the wire to ensure satisfactory operation. In fact the diode that is formed may either be a Schottky barrier diode or a standard PN junction dependent upon the way in which the wire and semiconductor meet and the resulting forming process.

Point contact Schottky diode structureAlthough some diodes still use this very simple format, any diode requiring a long term reliability needs to be fabricated in a more reliable way.

Vacuum deposited Schottky diode structureAlthough point contact diodes were manufactured many years later, these diodes were also unreliable and they were subsequently replaced by a fabrication technique in which metal was vacuum deposited.

Deposited metal Schottky barrier diode structureThis format for a Schottky diode is very basic and is more diagrammatic than actually practical. However it does show the basic metal-on-semiconductor format that is key to its operation.

Schottky diode structure with guard ring

One of the problems with the simple deposited metal diode is that breakdown effects are noticed around the edge of the metallised area. This arises from the high electric fields that are present around the edge of the plate. Leakage effects are also noticed.

To overcome these problems a guard ring of P+ semiconductor fabricated using a diffusion process is used along with an oxide layer around the edge. In some instances metallic silicides may be used in place of the metal.

The guard ring in this form of Schottky diode structure operates by driving this region into avalanche breakdown before the Schottky junction is damaged by large levels of reverse current flow during transient events.

Schottky diode rectifier structure showing with guard ringThis form of Schottky diode structure is used particularly in rectifier diodes where the voltages may be high and breakdown is more of a problem.

Schottky diode structure notesThere are a number of points of interest from the fabrication process.

The most critical element in the manufacturing process is to ensure a clean surface for an intimate contact of the metal with the semiconductor surface, and this is achieved chemically. The metal is normally deposited in a vacuum either by the use of evaporation or sputtering techniques. However in some instances chemical deposition is gaining some favour, and actual plating has been used although it is not generally controllable to the degree required.

When silicides are to be used instead of a pure metal contact, this is normally achieved by depositing the metal and then heat treating to give the silicide. This process has the advantage that the reaction uses the surface silicon, and the actual junction propagates below the surface, where the silicon will not have been exposed to any contaminants. A further advantage of the whole Schottky structure is that it can be fabricated using relatively low temperature techniques, and does not generally need the high temperature steps needed in impurity diffusion.

The Schottky diode is used in a variety of forms for many different applications. Obviously those used for signal applications are in much smaller packages, often in SMT ones these days. Those devices used for power applications are in much larger packages, often ones which can be bolted to a heat-sink.

The Schottky diode is a very useful form of diode. It is widely used within electronics circuits because it has some particularly useful characteristics.

Its characteristics mean that it can be used where other forms of diode do not perform so successfully.

Schottky diode characteristicsThe Schottky diode is what is called a majority carrier device. This gives it tremendous advantages in terms of speed because it does not rely on holes or electrons recombining when they enter the opposite type of region as in the case of a conventional diode. By making the devices small the normal RC type time constants can be reduced, making these diodes an order of magnitude faster than the conventional PN diodes. This factor is the prime reason why they are so popular in radio frequency applications.

The diode also has a much higher current density than an ordinary PN junction. This means that forward voltage drops are lower making the diode ideal for use in power rectification applications.

Its main drawback is found in the level of its reverse current which is relatively high. For many uses this may not be a problem, but it is a factor which is worth watching when using it in more exacting applications.

The overall I-V characteristic is shown below. It can be seen that the Schottky diode has the typical forward semiconductor diode characteristic, but with a much lower turn on voltage. At high current levels it levels off and is limited by the series resistance or the maximum level of current injection. In the reverse direction breakdown occurs above a certain level. The mechanism is similar to the impact ionisation breakdown in a PN junction.

Schottky diode IV characteristicThe IV characteristic is generally that shown below. In the forward direction the current rises exponentially, having a knee or turn on voltage of around 0.2 V. In the reverse direction, there is a greater level of reverse current than that experienced using a more conventional PN junction diode.

Schottky diode IV characteristicThe use of a guard ring in the fabrication of the diode has an effect on its performance in both forward and reverse directions. [see page on structure and fabrication]. Both forward and reverse characteristics show a better level of performance.

However the main advantage of incorporating a guard ring into the structure is to improve the reverse breakdown characteristic. There is around a 4 : 1 difference in breakdown voltage between the two - the guard ring providing a distinct improvement in reverse breakdown. Some small signal diodes without a guard ring may have a reverse breakdown of only 5 to 10 V.

Key specification parametersIn view of the particular properties of the Schottky diode there are several parameters that are of key importance when determining the operation of one of these diodes against the more normal PN junction diodes.

Forward voltage drop:   In view of the low forward voltage drop across the diode, this is a parameter that is of particular concern. As can be seen from the Schottky diode IV characteristic, the voltage across the diode varies according to the current being carried. Accordingly any specification given provides the forward voltage drop for a given current. Typically the turn-on voltage is assumed to be around 0.2 V.

Reverse breakdown:   Schottky diodes do not have a high breakdown voltage. Figures relating to this include the maximum Peak Reverse Voltage, maximum Blocking DC Voltage and other similar parameter names. If these figures are exceeded then there is a possibility the diode will enter reverse breakdown. It should be noted that the RMS value for any voltage will be 1/√2 times the constant value. The upper limit for reverse breakdown is not high when compared to normal PN junction diodes. Maximum figures, even for rectifier diodes only reach around 100 V. Schottky diode rectifiers seldom exceed this value because devices that would operate above this value even by moderate amounts would exhibit forward voltages equal to or greater than equivalent PN junction rectifiers.

Capacitance:   The capacitance parameter is one of great importance for small signal RF applications. Normally the junctions areas of Schottky diodes are small and therefore the capacitance is small. Typical values of a few picofarads are normal. As the

capacitance is dependent upon any depletion areas, etc, the capacitance must be specified at a given voltage.

Reverse recovery time:   This parameter is important when a diode is used in a switching application. It is the time taken to switch the diode from its forward conducting or 'ON' state to the reverse 'OFF' state. The charge that flows within this time is referred to as the 'reverse recovery charge'. The time for this parameter for a Schottky diode is normally measured in nanoseconds, ns. Some exhibit times of 100 ps. In fact what little recovery time is required mainly arises from the capacitance rather than the majority carrier recombination. As a result there is very little reverse current overshoot when switching from the forward conducting state to the reverse blocking state.

Working temperature:   The maximum working temperature of the junction, Tj is normally limited to between 125 to 175°C. This is less than that which can be sued with ordinary silicon diodes. Care should be taken to ensure heatsinking of power diodes does not allow this figure to be exceeded.

Reverse leakage current:   The reverse leakage parameter can be an issue with Schottky diodes. It is found that increasing temperature significantly increases the reverse leakage current parameter. Typically for every 25°C increase in the diode junction temperature there is an increase in reverse current of an order of magnitude for the same level of reverse bias.

Schottky diode characteristics summaryThe Schottky diode is used in many applications as a result of its characteristics that differ appreciable from several aspects of the more widely used standard PN junction diode.

COMPARISON OF CHARACTERISTICS OF SCHOTTKY DIODE AND PN DIODE

CHARACTERISTIC SCHOTTKY DIODE PN JUNCTION DIODE

Forward current mechanism

Majority carrier transport. Due to diffusion currents, i.e. minority carrier transport.

Reverse current Results from majority carriers that overcome the barrier. This is less temperature dependent than for standard PN junction.

Results from the minority carriers diffusing through the depletion layer. It has a strong temperature dependence.

Turn on voltage Small - around 0.2 V. Comparatively large - around 0.7 V.

Switching speed Fast - as a result of the use of majority carriers because no recombination is required.

Limited by the recombination time of the injected minority carriers.

Example Schottky diode characteristics

To give some idea of the characteristics to be expected from Schottky diodes a couple of real examples are provided below. These summarise the main specifications and give an idea of their performance

1N5711 Schottky barrier switching diode   This diode is described as an ultra-fast switching diode with high reverse breakdown, low forward drop voltage and a guard ring for junction protection.

TYPICAL 1N5711 CHARACTERISTICS / SPECIFICATIONS

CHARACTERISTIC TYPICAL VALUE

UNIT DETAILS

Max DC Blocking Voltage, Vr 70 V

Max forward continuous current, Ifm 15 mA

Reverse breakdown voltage, V(BR)R 70 V @ reverse current of 10µA

Reverse leakage current, IR 200 µA At VR=50V

Forward voltage drop, VF 0.41

1.00

V at IF = 1.0 mA

IF=15mA

Junction capacitance, Cj 2.0 pF VR = 0V, f=1MHz

Reverse recovery time, trr 1 nS

1N5828 Schottky barrier power rectifier diode   This diode is described as a Schottky diode, stud type, i.e. for power rectification.

TYPICAL 1N5258 SHOTTKY DIODE CHARACTERISTICS / SPECIFICATIONS

CHARACTERISTIC TYPICAL VALUE

UNIT DETAILS

Maximum recurrent peak reverse voltage 40 V

Maxim DC blocking voltage 40 V

Average forward current, IF (AV) 15 A T = 100°C

Peak forward surge current, IFSM 500 A

Maximum instantaneous forward voltage, VF

0.5 V At IFM = 15A and Tj = 25°C

Maximum instantaneous reverse current at rated blocking voltage, IR

10 

250

mA Tj= 25°C

Tj = 125°C

Even though the example here gives a reverse voltage characteristic of 40 V which is fairly typical, the maximum that can normally be obtained is around 100 V.

It should be noted that even though these figures are given as examples of the figures that may be expected for typical Schottky diodes, figures even for a given device number will also vary slightly between different manufacturers.

The Schottky diode finds many uses as a high voltage or power rectifier. The Schottky diode rectifier has many advantages over other types of diode and as a such can be utilised to advantage.

The Schottky diode has been used as a rectifier for many years in the power supply industry where its use is essential to many designs.

Advantages of using a Schottky diode rectifierThe Schottky diode rectifier offers many advantages in power rectifier and power supply circuits. There are a number of aspects of the Schottky diode rectifier that makes them ideal components in many power supply applications:

Low forward voltage drop:   The low forward voltage drop offered by Scottky diode power rectifiers is a significant advantage in many applications. It reduces the power losses normally incurred in the rectifier and other diodes used within the power supply. With standard silicon diodes offering the main alternative, their turn on voltage is around 0.6 to 0.7 volts. With Schottky diode rectifiers having a turn on voltage of around 0.2 to 0.3 volts, there is a significant power saving to be gained. However it is necessary to remember that there will also be losses introduced by the resistance of the material, and the voltage drop across the diode will increase with current. The losses of the Schottky diode rectifier will be less than that of the equivalent silicon rectifier.

Fast switching speeds:   The very fast switch speeds of the Schottky diode rectifier mean that this diode lends itself to use in switching regulator circuits.

Schottky diode rectifier design considerationsSchottky diode rectifiers offer many advantages, but when they are used, there are a number of design considerations to account for. These should be acknowledged in the circuit design being undertaken.

Some of the points to be taken into account include the following:

High reverse leakage current:   Schottky diode rectifiers have a much higher reverse leakage current than standard PN junction silicon diodes. Although this may not be a problem in some designs it may have an impact on others.

Limited junction temperature:   The maximum junction temperature of a Schottky diode rectifier is normally limited to the range 125°C to 175°C but check the manufacturers ratings for the given component. This compares to temperatures of around 200°C for silicon diode rectifiers.

Limited reverse voltage:   As a result of its structure, Schottky diode rectifiers have a limited reverse voltage capability. The maximum figures are normally around 100 volts. If devices were manufactured with figures above this, it would be found that the forward voltages would rise and be equal to or greater than their equivalent silicon diodes for reasonable levels of current.

VARACTAR DIODE

Varactor diodes or varicap diodes are semiconductor devices that are widely used in the electronics industry and are used in many applications where a voltage controlled variable capacitance is required. Although the terms varactor diode and varicap diode can be used interchangeably, the more common term these days is the varactor diode.

Although ordinary PN junction diodes exhibit the variable capacitance effect and these diodes can be used for this applications, special diodes optimised to give the required changes in capacitance. Varactor diodes or varicap diodes normally enable much higher ranges of capacitance change to be gained as a result of the way in which they are manufactured. There are a variety of types of varactor diode ranging from relatively standard varieties to those that are described as abrupt or hyperabrupt varactor diodes.

Varactor diode applicationsVaractor diodes are widely used within the RF design arena. They provide a method of varying he capacitance within a circuit by the application of a control voltage. This gives them an almost unique capability and as a result varactor diodes are widely used within the RF industry.

Although varactor diodes can be used within many types of circuit, they find applications within two main areas:

Voltage controlled oscillators, VCOs:   Voltage controlled oscillators are used for a variety of applications. One major area is for the oscillator within a phase locked loop - this are used in almost all radio, cellular and wireless receivers. A varactor diode is a key component within a VCO.

RF filters:   Using varactor diodes it is possible to tune filters. Tracking filters may be needed in receiver front end circuits where they enable the filters to track the incoming received signal frequency. Again this can be controlled using a control voltage. Typically this might be provided under microprocessor control via a digital to analogue converter.

Varactor diode basicsThe varactor diode or varicap diode consists of a standard PN junction, although it is obviously optimised for its function as a variable capacitor. In fact ordinary PN junction diodes can be used as varactor diodes, even if their performance is not to the same standard as specially manufactured varactors.

The basis of operation of the varactor diode is quite simple. The diode is operated under reverse bias conditions and this gives rise to three regions. At either end of the diode are the P and N regions where current can be conducted. However around the junction is the depletion region

where no current carriers are available. As a result, current can be carried in the P and N regions, but the depletion region is an insulator.

This is exactly the same construction as a capacitor. It has conductive plates separated by an insulating dielectric.

The capacitance of a capacitor is dependent on a number of factors including the plate area, the dielectric constant of the insulator between the plates and the distance between the two plates. In the case of the varactor diode, it is possible to increase and decrease the width of the depletion region by changing the level of the reverse bias. This has the effect of changing the distance between the plates of the capacitor.

Varactor diode symbolAs the primary function of a varactor diode is as a variable capacitor, its circuit symbol represents this. Sometimes they may be shown as ordinary diodes, whereas more usually the varactor diode circuit symbol shows the bar as a capacitor, i.e. two lines.

Varactor diode circuit symbolVaractor diodes are always operated under reverse bias conditions, and in this way there is no conduction. They are effectively voltage controlled capacitors, and indeed they are sometimes called varicap diodes, although the term varactor is more widely used these days.

Varactor diodes, or as they are sometimes called, varicap diodes are a particularly useful form of semiconductor diode. Finding uses in many applications where electronically controlled tuning of resonant circuits is required, for items such as oscillators and filters, varactor diodes are an essential component within the portfolio of the electronics design engineer. However to be able to use varactor diodes to their best advantage it is necessary to understand features of varactor diodes including the capacitance ratio, Q, gamma, reverse voltage and the like. If used correctly, varactor diodes provide very reliable service particularly as they are a solid state device and have no mechanical or moving elements as in their mechanical variable capacitor counterparts.