Light Emitting Diodes

What is LED?

A “Light Emitting Diode” or LED as it is more commonly called, is basically just a specialised type of PN junction diode, made from a very thin layer of fairly heavily doped semiconductor material.

When the diode is forward biased, electrons from the semiconductors conduction band recombine with holes from the valence band releasing sufficient energy to produce photons which emit a monochromatic (single colour) of light. Because of this thin layer a reasonable number of these photons can leave the junction and radiate away producing a coloured light output.

LED Construction

Then we can say that when operated in a forward biased direction Light Emitting Diodes are semiconductor devices that convert electrical energy into light energy.

The construction of a Light Emitting Diode

Light-Emitting Diodes

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 is very different from that of a normal signal diode. The PN junction of an LED is surrounded by a transparent, hard plastic epoxy resin hemispherical shaped shell or body which protects the LED from both vibration and shock.

Surprisingly, an LED junction does not actually emit that much light so the epoxy resin body is constructed in such a way that the photons of light emitted by the junction are reflected away from the surrounding substrate base to which the diode is attached and are focused upwards through the domed top of the LED, which itself acts like a lens concentrating the amount of light. This is why the emitted light appears to be brightest at the top of the LED.

However, not all LEDs are made with a hemispherical shaped dome for their epoxy shell. Some indication LEDs have a rectangular or cylindrical shaped construction that has a flat surface on top or their body is shaped into a bar or arrow. Also, nearly all LEDs have their cathode, ( K ) terminal identified by either a notch or flat spot on the body, or by one of the leads being shorter than the other, ( the Anode, A ).

Unlike normal incandescent lamps and bulbs which generate large amounts of heat when illuminated, the light emitting diode produces a “cold” generation of light which leads to high efficiencies than the normal “light bulb” because most of the generated energy radiates away within the visible spectrum. Because LEDs are solid-state devices, they can be extremely small and durable and provide much longer lamp life than normal light sources.

LED operation

The LED is a specialised form of PN junction that uses a compound junction. The semiconductor material used for the junction must be a compound semiconductor. The commonly used semiconductor materials including silicon and germanium are simple elements and junction made from these materials do not emit light. Instead compound semiconductors including gallium arsenide, gallium phosphide and indium phosphide are compound semiconductors and junctions made from these materials do emit light.

These compound semiconductors are classified by the valence bands their constituents occupy. For gallium arsenide, gallium has a valency of three and arsenic a valency of five and this is what is termed a group III-V semiconductor and there are a number of other semiconductors that fit this category. It is also possible to have semiconductors that are formed from group III-V materials.

The diode emits light when it is forward biased. When a voltage is applied across the junction to make it forward biased, current flows as in the case of any PN junction. Holes from the p-type region and electrons from the n-type region enter the junction and recombine like a normal diode to enable the current to flow. When this occurs energy is released, some of which is in the form of light photons.

It is found that the majority of the light is produced from the area of the junction nearer to the P-type region. As a result the design of the diodes is made such that this area is kept as close to the surface of the device as possible to ensure that the minimum amount of light is absorbed in the structure.

To produce light which can be seen the junction must be optimised and the correct materials must be chosen. Pure gallium arsenide releases energy in the infra read portion of the spectrum. To bring the light emission into the visible red end of the spectrum aluminium is added to the semiconductor to give aluminium gallium arsenide (AlGaAs). Phosphorus can also be added to give red light. For other colours other materials are used. For example galium phoshide gives green light and aluminium indium gallium phosphide is used for yellow and orange light. Most LEDs are based on gallium semiconductors.

Light Emitting Diode Colours

So how does a light emitting diode get its colour. Unlike normal signal diodes which are made for detection or power rectification, and which are made from either Germanium or Silicon semiconductor materials, Light Emitting Diodes are made from exotic semiconductor compounds such as Gallium Arsenide (GaAs), Gallium Phosphide (GaP), Gallium Arsenide Phosphide (GaAsP), Silicon Carbide (SiC) or Gallium Indium Nitride (GaInN) all mixed together at different ratios to produce a distinct wavelength of colour.

Different LED compounds emit light in specific regions of the visible light spectrum and therefore produce different intensity levels. The exact choice of the semiconductor material used will determine the overall wavelength of the photon light emissions and therefore the resulting colour of the light emitted.

Light Emitting Diode Colours

Typical LED Characteristics
Semiconductor Material	Wavelength	Colour	VF @ 20mA
GaAs	850-940nm	Infra-Red	1.2v
GaAsP	630-660nm	Red	1.8v
GaAsP	605-620nm	Amber	2.0v
GaAsP:N	585-595nm	Yellow	2.2v
AlGaP	550-570nm	Green	3.5v
SiC	430-505nm	Blue	3.6v
GaInN	450nm	White	4.0v

Thus, the actual colour of a light emitting diode is determined by the wavelength of the light emitted, which in turn is determined by the actual semiconductor compound used in forming the PN junction during manufacture.

Therefore the colour of the light emitted by an LED is NOT determined by the colouring of the LED’s plastic body although these are slightly coloured to both enhance the light output and to indicate its colour when its not being illuminated by an electrical supply.

Light emitting diodes are available in a wide range of colours with the most common being RED,AMBER,  YELLOW  and GREEN and are thus widely used as visual indicators and as moving light displays.

Recently developed blue and white coloured LEDs are also available but these tend to be much more expensive than the normal standard colours due to the production costs of mixing together two or more complementary colours at an exact ratio within the semiconductor compound and also by injecting nitrogen atoms into the crystal structure during the doping process.

From the table above we can see that the main P-type dopant used in the manufacture of Light Emitting Diodes is Gallium (Ga, atomic number 31) and that the main N-type dopant used is Arsenic (As, atomic number 33) giving the resulting compound of Gallium Arsenide (GaAs) crystalline structure.

The problem with using Gallium Arsenide on its own as the semiconductor compound is that it radiates large amounts of low brightness infra-red radiation (850nm-940nm approx.) from its junction when a forward current is flowing through it.

The amount of infra-red light it produces is okay for television remote controls but not very useful if we want to use the LED as an indicating light. But by adding Phosphorus (P, atomic number 15), as a third dopant the overall wavelength of the emitted radiation is reduced to below 680nm giving visible red light to the human eye. Further refinements in the doping process of the PN junction have resulted in a range of colours spanning the spectrum of visible light as we have seen above as well as infra-red and ultra-violet wavelengths.

By mixing together a variety of semiconductor, metal and gas compounds the following list of LEDs can be produced.

Types of Light Emitting Diode

• • Gallium Arsenide (GaAs) - infra-red
• • Gallium Arsenide Phosphide (GaAsP) - red to infra-red, orange
• • Aluminium Gallium Arsenide Phosphide (AlGaAsP) - high-brightness red, orange-red, orange, and yellow
• • Gallium Phosphide (GaP) - red, yellow and green
• • Aluminium Gallium Phosphide (AlGaP) - green
• • Gallium Nitride (GaN) - green, emerald green
• • Gallium Indium Nitride (GaInN) - near ultraviolet, bluish-green and blue
• • Silicon Carbide (SiC) - blue as a substrate
• • Zinc Selenide (ZnSe) - blue
• • Aluminium Gallium Nitride (AlGaN) - ultraviolet

Like conventional PN junction diodes, light emitting diodes are current-dependent devices with its forward voltage drop VF, depending on the semiconductor compound (its light colour) and on the forward biased LED current. The point where conduction begins and light is produced is about 1.2V for a standard red LED to about 3.6V for a blue LED.

The exact voltage drop will of course depend on the manufacturer because of the different dopant materials and wavelengths used. The voltage drop across the LED at a particular current value, for example 20mA, will also depend on the initial conduction VF point. As an LED is effectively a diode, its forward current to voltage characteristics curves can be plotted for each diode colour as shown below.

Light Emitting Diodes I-V Characteristics.

Light Emitting Diode (LED) Schematic symbol and I-V Characteristics Curves
showing the different colours available.

 

Before a light emitting diode can “emit” any form of light it needs a current to flow through it, as it is a current dependant device with their light output intensity being directly proportional to the forward current flowing through the LED.

As the LED is to be connected in a forward bias condition across a power supply it should becurrent limited using a series resistor to protect it from excessive current flow. Never connect an LED directly to a battery or power supply as it will be destroyed almost instantly because too much current will pass through and burn it out.

From the table above we can see that each LED has its own forward voltage drop across the PN junction and this parameter which is determined by the semiconductor material used, is the forward voltage drop for a specified amount of forward conduction current, typically for a forward current of 20mA.

In most cases LEDs are operated from a low voltage DC supply, with a series resistor, RS used to limit the forward current to a safe value from say 5mA for a simple LED indicator to 30mA or more where a high brightness light output is needed.

LED Series Resistance.

The series resistor value RS is calculated by simply using Ohm´s Law, by knowing the required forward current IF of the LED, the supply voltage VS across the combination and the expected forward voltage drop of the LED, VF at the required current level, the current limiting resistor is calculated as:

LED Series Resistor Circuit

Light Emitting Diode Example No1

An amber coloured LED with a forward volt drop of 2 volts is to be connected to a 5.0v stabilised DC power supply. Using the circuit above calculate the value of the series resistor required to limit the forward current to less than 10mA. Also calculate the current flowing through the diode if a 100Ω series resistor is used instead of the calculated first.

1). series resistor required at 10mA.

 

2). with a 100Ω series resistor.

 

We remember from the Resistors tutorials, that resistors come in standard preferred values. Our first calculation above shows that to limit the current flowing through the LED to 10mA exactly, we would require a 300Ω resistor. In the E12 series of resistors there is no 300Ω resistor so we would need to choose the next highest value, which is 330Ω. A quick re-calculation shows the new forward current value is now 9.1mA, and this is ok.

Connecting LEDs Together in Series

We can connect LED’s together in series to increase the number required or to increase the light level when used in displays. As with series resistors, LED’s connected in series all have the same forward current, IF flowing through them as just one. As all the LEDs connected in series pass the same current it is generally best if they are all of the same colour or type.

LED’s in Series

 

Although the LED series chain has the same current flowing through it, the series voltage drop across them needs to be considered when calculating the required resistance of the current limiting resistor, RS. If we assume that each LED has a voltage drop across it when illuminated of 1.2 volts, then the voltage drop across all three will be 3 x 1.2v = 3.6 volts.

If we also assume that the three LEDs are to be illuminated from the same 5 volt logic device or supply with a forward current of about 10mA, the same as above. Then the voltage drop across the resistor, RS and its resistance value will be calculated as:

Again, in the E12 (10% tolerance) series of resistors there is no 140Ω resistor so we would need to choose the next highest value, which is 150Ω.

Schottky Diode Technology & Structure

Basic Schottky diode structure

The 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 structure

Although 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 structure

Although 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 structure

This 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 ring

This 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 notes

There 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 characteristics

The 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 characteristic

The 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 characteristic

The 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 parameters

In 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 summary

The 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.

Schottky diode/ hot carrier diode

A Schottky diode, also known as a hot carrier diode, is a semiconductor diode which has a low forward voltage drop and a very fast switching action. There is a small voltage drop across the diode terminals when current flows through a diode. A normal diode will have a voltage drop between 0.6 to 1.7 volts, while a Schottky diode voltage drop is usually between 0.15 and 0.45 volts. This lower voltage drop provides better system efficiency and higher switching speed. In a Schottky diode, a semiconductor–metal junction is formed between a semiconductor and a metal, thus creating a Schottky barrier. The N-type semiconductor acts as the cathode and the metal side acts as the anode of the diode. This Schottky barrier results in both a low forward voltage drop and very fast switching.



The Schottky diode (named after German physicist Walter H. Schottky), also known as hot carrier diode, is a semiconductor diode with a low forward voltage drop and a very fast switching action. The cat's-whisker detectors used in the early days of wireless and metal rectifiers used in early power applications can be considered primitive Schottky diodes.



Schottky diodes are constructed of a metal-to-N junction rather than a P-N semiconductor junction. Also known as hot-carrier diodes, Schottky diodes are characterized by fast switching times (low reverse-recovery time), low forward voltage drop (typically 0.25 to 0.4 volts for a metal-silicon junction), and low junction capacitance.

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.

Varactor diode/ Varicap diode/ Tuning diode

In electronics, a varicap diode, varactor diode, variable capacitance diode, variable reactance diode or tuning diode is a type of diode designed to exploit the voltage-dependent capacitance of a reversed-biased p–n junction.

A tuning diode, also known as a varactor diode, variable capacitance diode, varicap diode or

 variable reactance 

diode, is a diode that has a variable capacitance which is a function of the voltage that is

 impressed on its  terminals. Tuning / varactor diodes are operated reverse-biased, and 

therefore no current flows.

However, since the thickness of the depletion zone varies with the applied bias 

voltage, the capacitance of the diode can be made to vary. Usually, the capacitance is 

inversely proportional to the depletion region thickness and the depletion region thickness 

is proportional to the square root of the applied voltage. Therefore, the 

capacitance is inversely proportional to the square root of the voltage applied to the diode.

Varactor diode applications

Varactor 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 basics

The 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 symbol

As 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.

Internal structure of a varicap
Type	Passive
Pin configuration	anode and cathode
Electronic symbol

 

Varactor diode circuit symbol

Varactor 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.

Operation

Operation of a varicap

Varactors are operated in a reverse-biased state. No current flows, but since the thickness of the depletion zone varies with the applied bias voltage, the capacitance of the diode can be made to vary. Generally, the depletion region thickness is proportional to the square rootof the applied voltage; capacitance is inversely proportional to the depletion region thickness. Thus, the capacitance is inversely proportional to the square root of applied voltage.

All diodes exhibit this phenomenon to some degree, but varactor diodes are manufactured specifically to exploit this effect and increase the capacitance (and thus the range of variability), whereas most ordinary diode fabrication strives to minimize the capacitance.

The figure shows an example of a cross section of a varactor with the depletion layer formed of a PN junction. This depletion layer can also be made of a MOS or a Schottky diode. This is very important in CMOS and MMIC technology.