PIN / PN photodiode structure

Although an ordinary p-n junction can be used as the basis of a photodiode, the p-i-n junction provides a far more satisfactory photodiode structure. In the photodiode fabrication process a thick intrinsic layer is inserted between the p-type and n-type layers. The middle layer may be either completely instrinsic, or very lightly doped to make it and n- layer. In some instances it may be grown as an epitaxial layer onto the substrate, or alternatively it may be contained within the substrate itself.

PIN photodiode structure

One of the main requirements of the photodiode is to ensure that the maximum amount of light reaches the intrinsic layer. One of the most efficient ways of achieving this is to place the electrical contacts at the side of the device as shown. This enables the maximum amount of light to reach the active area. It is found that as the substrate is heavily doped, there is very little loss of light due to the fact that this is not the active area.

As light is mostly absorbed within a certain distance, the thickness of the intrinsic layer is normally made to match this. Any increase in thickness over this will tend to reduce the speed of operation - a vital factor in many applications, and it will not improve the efficiency greatly.

It is also possible to have the light enter the photo diode from the side of the junction. By operating the photo diode in this fashion the intrinsic layer can be made much less to increase the speed of operation, although the efficiency is reduced.

In some instances a heterojunction may be used. This form of structure has the additional flexibility that light can be received from the substrate and this has a larger energy gap which makes it transparent to light.

PIN photodiode heterojunction structure

The heterojunction format for a PIN photodiode uses less standard technology often using materials such as the InGaAs and InP depicted in the diagram. Being a less standard process, it is more expensive to implement and as a result tends to be used for more specialist products.

PN / PIN photodiode materials

The materials used within a photodiode determine many of its critical properties. The wavelength of light to which it responds and the level of noise are both critical parameters that are dependent upon the material used in the photodiode.

The wavelength sensitivity of the different materials occurs because only photons with sufficient energy to excite an electron across the bandgap of the material will produce significant energy to develop the current from the photodiode.

MATERIAL	WAVELENGTH SENSITIVITY (NM)
Germanium	800 - 1700
Indium gallium arsenide	800 - 2600
Lead sulphide	~1000 - 3500
Silicon	190 - 1100

Wavelength ranges for commonly used photodiode materials

While the wavelength sensitivity of the material is very important, another parameter that can have a major impact on the performance of the photodiode is the level of noise that is produced. Because of their greater bandgap, silicon photodiodes generate less noise than germanium photodiodes. However it is also necessary to consider the wavelengths for which the photodiode is required and germanium photodiodes must be used for wavelengths longer than approximately 1000 nm.

Avalanche photodiode structure

The avalanche photodiode structure is relatively similar to that of the more commonly used PN photodiode structure or the structure of the PIN photodiode. However as the avalanche photodiode is operated under a high level of reverse bias a guard ring is placed around the perimeter of the diode junction. This prevents surface breakdown mechanisms.

Avalanche PIN photodiode structure

Avalanche photodiode materials

Like the standard PN or PIN photodiodes, the materials used have a major effect on determining the characteristics of the avalanche diode.

MATERIAL	PROPERTIES
Germanium	Can be used for wavelengths in the region 800 - 1700 nm. Has a high level of multiplication noise.
Silicon	Can be used for wavelengths in the region between 190 - 1100 nm. Diodes exhibit a comparatively low level of multiplication noise when compared to those using other materials, and in particular germanium.
Indium gallium arsenide	Can be used for wavelengths to 1600 nm and has a lower level of multiplication noise than germanium.

Commonly used avalanche photodiode materials

For optimum noise performance the large difference in the ionisation coefficients for electrons and holes is needed. Silicon provides a good noise performance with a ratio between the different coefficients of 50. Germanium and many group III-V compounds only have ratios of less than 2. While the noise performance of these materials is much inferior, they need to be used for longer wavelengths that require the smaller energy gap offered.

Photodiode Operation & Theory

Although there are several different types of photodiode, they all utilise the same basic principles, although some are enhanced by other effects.

different types of photodiode work in slightly different ways, the basis of operation of all photodiodes remains the same.

Light energy can be considered in terms of photons or packets of light. When a photon of sufficient energy enters the depletion region of a semiconductor diode, it may strike an atom with sufficient energy to release the electron from the atomic structure. This creates a free electron and a hole (i.e. an atom with a space for an electron). The electron is negatively charged, while the hole is positively charged.

The electrons and holes may remain free, or other electrons may combine with holes to form complete atoms again in the crystal lattice. However it is possible that the electrons and holes may remain free and be pulled away from the depletion region by an external field. In this way the current through the diode will change and a photocurrent is produced.

PIN / PN photodiode Operation

The photodiode is operated under a moderate reverse bias. This keeps the depletion layer free of any carriers and normally no current will flow. However when a light photon enters the intrinsic region it can strike an atom in the crystal lattice and dislodge an electron. In this way a hole-electron pair is generated. The hole and electron will then migrate in opposite directions under the action of the electric field across the intrinsic region and a small current can be seen to flow. It is found that the size of the current is proportional to the amount of light entering the intrinsic region. The more light, the greater the numbers of hole electron pairs that are generated and the greater the current flowing.

Operating diodes under reverse bias increases the sensitivity as it widens the depletion layer where the photo action occurs. In this way increasing the reverse bias has the effect of increasing the active area of the photodiode and strengthens what may be termed as the photocurrent.

It is also possible to operate photodiodes under zero bias conditions in what is termed as a photovoltaic mode. In zero bias, light falling on the diode causes a current across the device, leading to forward bias which in turn induces "dark current" in the opposite direction to the photocurrent. This is called the photovoltaic effect, and is the basis for solar cells. It is therefore possible to construct a solar cell using a large number of individual photodiodes. Also when photodiodes are used in a solar cell, the diodes are made larger so that there is a larger active area, and they are able to handle higher currents. For those used for data applications, speed is normally very important and the diode junctions are smaller to reduce the effects of capacitance.

When not exposed to light the photo diode follows a normal V-I characteristic expected of a diode. In the reverse direction virtually no current flows, but in the forward direction it steadily increases, especially after the knee or turn on voltage is reached. This is modified in the presence of light. When used as a photo-diode it can be seen that the greatest effect is seen in the reverse direction. Here the largest changes are noticed, and the normal forward current does not mask the effects due to the light.

Avalanche photodiode diode operation

Light enters the un-doped region of the avalanche photodiode and causes the generation of hole-electron pairs. Under the action of the electric field the electrons migrate towards the avalanche region. Here the electric field causes their velocity to increase to the extent that collisions with the crystal lattice create further hole electron pairs. In turn these electrons may collide with the crystal lattice to create even more hole electron pairs. In this way a single electron created by light in the un-doped region may result in many more being created.

The avalanche photodiode has a number of differences when compared to the ordinary p-i-n diode. The avalanche process means that a single electron produced by light in the un-doped region is multiplied several times by the avalanche process. As a result the avalanche photo diode is far more sensitive. However it is found that it is not nearly as linear, and additionally the avalanche process means that the resultant signal is far noisier than one from a p-i-n diode. The structure of the avalanche diode is also more complicated. An n-type guard ring is required around the p-n junction to minimise the electric field around the edge of the junction. It is also found that the current gain is dependent not only on the bias applied, but also thermal fluctuations. As a result it is necessary to ensure the devices are placed on an adequate heat sink.

Avalanche photodiode & Schottky Photodiode

There is a variety of types of photodiode that are available. The p-n and p-i-n photodiodes are the most widely used, but avalanche photodiodes are also available.

Avalanche photodiodes have advantages in some applications although their use may be more specialised.

Avalanche photodiode basics

The avalanche photodiode possesses a similar structure to that of the PIN or PN photodiode. A structure similar to that of a Schottky photodiode can also be used but this is less common. However the structure is optimised for avalanche operation.

The main difference of the avalanche photodiode operates under a slightly different scenario to that of the more standard photodiodes. It operates under a high reverse bias condition to enable avalanche multiplication of the holes and electrons created by the initial hole electron pairs created by the photon / light impact.

The avalanche action enables the gain of the diode to be increased many times, providing a much greater level of sensitivity.

Avalanche photodiode advantages and disadvantages

The avalanche photodiode has a number of different characteristics to the normal p-n or p-i-n photodiodes, making them more suitable for use in some applications. In view of this it is worth summarising their advantages and disadvantages..

The main advantages of the avalanche photodiode include:

• Greater level of sensitivity

The disadvantages of the avalanche photodiode include:

• Much higher operating voltage may be required.
• Avalanche photodiode produces a much higher level of noise than a p-n photodiode
• Avalanche process means that the output is not linear

Circuit conditions

Avalanche photodiodes require a high reverse bias for their operation. For silicon, a diode will typically require between 100 and 200 volts, and with this voltage they will provide a current gain effect of around 100 resulting from the avalanche effect. Some diodes that utilise specialised manufacturing processes enable much higher bias voltages of up to 1500 volts to be applied. As it is found that the gain levels increase when higher voltages are applied, the gain of these avalanche diodes can rise to the order of 1000. This can provide a distinct advantage where sensitivity is of paramount importance.

The avalanche photodiodes are not as widely used as their p-i-n counterparts. They are used primarily where the level of gain is of paramount importance, because the high voltages required, combined with a lower reliability means that they are often less convenient to use.

Schottky photodiode

The Schottky photodiode is based around the Schottky barrier diode and it may also be called the metal-semiconductor diode as a result of its structure.

The Schottky photodiode provides additional capabilities over other forms of photodiode in terms of speed and long wavelength detection capability. As a result the Schottky photodiode has a unique niche in amongst the other forms of photodiode that are available.

Schottky photodiode basics

As the name indicates, the Schottky photodiode uses the Schottky diode (or Schottky barrier diode as it is sometimes called) as its basis of the photodetector.

The Schottky photodiode is unique as a photodetector as it is able to operate in two photo-detection modes:

• Electron pair generation:   This occurs from band to band or energy gap excitation in the semiconductor.
• Emission of carriers:   The emission of carriers occurs from the metal to the semiconductor over the Schottky barrier. This often referred to as internal photoemission.

The metal-semiconductor junction provides a similar action to that of the intrinsic layer of the PIN photodiode. Accordingly is provides a larger areas for capture of the photon energy.

Schottky photodiode applications

The Schottky photodiode is particularly compatible with mature silicon and silicide technology. As a result these photodiodes have been widely used in CCD - charge coupled device - as the image sensing photodetector.

The Schottky photodiode can be integrated into a single chip along with CCD transfer gate - these are also compatible with silicon technology. The CCD itself forms a shift register to allow the data from the array of Schottky photodiode detectors to transferred out of the overall chip in a managed way - providing method of extracting the huge data files from the large array of photodetectors with a sensible number of leads on the integrated circuit.

As a result of these advantages the Schottky photodiode detector has been the most widely used technology for focal plane arrays.

Photodiode (PIN Photo Diode)

Types of photodiode

Although the term photodiode is widely used, there are actually a number of different types of photodiode technology that can be used. As they offer different properties, the different photodiode technologies are used in different areas.

• PIN photodiode:   This type of photodiode is one of the most widely used forms of photodiode today. Although the PIN or p-i-n photodiode was not the first type of photodiode to be used, it collects the light photons more efficiently than the more standard PN photodiode, and also offers a lower capacitance.
• PN photodiode:   The PN photodiode was the first form of photodiode to be developed and used. Nowadays, it is not as widely used as other types which are able to offer better performance parameters. Nevertheless it is still used in some instances.
• Avalanche photodiode:   Avalanche photodiode technology is used in areas of low light. The avalanche photodiode offers very high levels of gain, but against this it has high levels of noise. Accordingly this photodiode technology is not suitable for all applications and it tends to be used
• Schottky photodiode:   As the name indicates, Schottky photodiode technology is based upon the Schottky diode. In view of the small diode capacitance it offers a very high speed capability and is used in high bandwidth communication systems.

Photodiode basics

Although the different types of photodiode work in slightly different ways, the basis of operation of all photodiodes remains the same.

Light energy can be considered in terms of photons or packets of light. When a photon of sufficient energy enters the depletion region of a semiconductor diode, it may strike an atom with sufficient energy to release the electron from the atomic structure. This creates a free electron and a hole (i.e. an atom with a space for an electron). The electron is negatively charged, while the hole is positively charged.

The electrons and holes may remain free, or other electrons may combine with holes to form complete atoms again in the crystal lattice. However it is possible that the electrons and holes may remain free and be pulled away from the depletion region by an external field. In this way the current through the diode will change and a photocurrent is produced.

Photodiode symbol

The photodiode symbol shows the basic format for a diode. However the photodiode symbol also shows the light in the form of arrows striking the diode junction - the arrows are in the opposite direct to that of a light emitting diode where they emanate from the device.

Photodiode symbol used for circuit schematics

There are several forms of photodiode that are available. Each type of photodiode has its own advantages and disadvantages, thereby allowing a choice of photodiode technology to be made to gain the best results. Factors including noise, reverse bias constraints, gain, wavelength, and more all play a part. With PIN, PN, avalanche and Schottky photodiodes all available, an informed choice can be made to ensure the optimum photodiode technology is used.

The two most common forms of photodiode are the PIN or p-i-n photodiode and the PN photodiode.

Both the PIN photodiode and PN photodiodes are widely used for a variety of photo-detection applications. Both the PIN photodiode and the PN photodiode have their advantages and disadvantages.

PIN photodiode basics

One of the key requirements for any photodetector is a sufficiently large area in which the light photons can be collected and converted. This is achieved by creating a large depletion region - the region where the light conversion takes place - by adding an intrinsic area into the PN junction to create a PIN junction.

One of the key parameters within the design of the PIN photodiode is to enable the light to enter the intrinsic region. The physical design of the photodiode needs to take account of this so that the light collection is optimised.

Photodiodes in general and in this case the PIN photodiode will respond differently to different light wavelengths. It is generally the thickness of the top p type region or layer that is one of the key parameters in determining the response sensitivity.

PIN photodiode applications

The PIN photo-diode does not have any gain, and for some applications this may be a disadvantage. Despite this it is still the most widely used form of diode, finding applications in audio CD players, DVD players as well as computer CD drives. In addition to this they are used in optical communication systems.

PIN photodiode are also used as nuclear radiation detectors. There are several types of nuclear radiation. The radiation may be in the form of high energy charged or uncharged particles, or it may also be electromagnetic radiation. The diode can detect all these forms of radiation. The electromagnetic radiation, of which light is a form, generates the hole-electron pairs as already mentioned. The particles have exactly the same effect. However as only a small amount of energy is required to generate a hole-electron pair a single high-energy particle may generate several hole-electron pairs.

PN photodiode

While the PIN photodiode is the most widely used, the PN photodiode is also used in some circumstances. It is essentially the same as the PN photodiode, except that it does not have an intrinsic layer within the depletion region.

PN / PIN photodiode comparison

Both PN photodiodes and PIN photodiodes are available on the market. When designing circuit it is necessary to choose the correct type. Both PN photodiodes and PIN photodiodes have their advantages and disadvantages:

• A PIN photodiode needs reverse bias for its operation as a result of the presence of the intrinsic region. This reverse bias has several consequences:
  
  
  • Reverse bias introduces a noise current which reduces signal to noise ratio
  • Reverse bias offers better performance for high bandwidth applications
  • Reveres bias offers better performance for high dynamic range applications
• A PN photodiode does not require a reverse bias and as a result is more suitable for low light applications.

Photodiodes

A photodiode is a diode optimized to produce an electron current flow in response to irradiation by ultraviolet, visible, or infrared light. Silicon is most often used to fabricate photodiodes; though, germanium and gallium arsenide can be used. The junction through which light enters the semiconductor must be thin enough to pass most of the light on to the active region (depletion region) where light is converted to electron hole pairs.

In Figure below a shallow P-type diffusion into an N-type wafer produces a PN junction near the surface of the wafer. The P-type layer needs to be thin to pass as much light as possible. A heavy N+ diffusion on the back of the wafer makes contact with metalization. The top metalization may be a fine grid of metallic fingers on the top of the wafer for large cells. In small photodiodes, the top contact might be a sole bond wire contacting the bare P-type silicon top.

Photodiode: Schematic symbol and cross section.

The intensity of the light entering the top of the photodiode stack falls off exponentially as a function of depth. A thin top P-type layer allows most photons to pass into the depletion region where electron-hole pairs are formed. The electric field across the depletion region due to the built in diode potential causes electrons to be swept into the N-layer, holes into the P-layer. Actually electron-hole pairs may be formed in any of the semiconductor regions. However, those formed in the depletion region are most likely to be separated into the respective N and P-regions. Many of the electron-hole pairs formed in the P and N-regions recombine. Only a few do so in the depletion region. Thus, a few electron-hole pairs in the N and P-regions, and most in the depletion region contribute to photocurrent, that current resulting from light falling on the photodiode.

The voltage out of a photodiode may be observed. Operation in this photovoltaic (PV) mode is not linear over a large dynamic range, though it is sensitive and has low noise at frequencies less than 100 kHz. The preferred mode of operation is often photocurrent (PC) mode because the current is linearly proportional to light flux over several decades of intensity, and higher frequency response can be achieved. PC mode is achieved with reverse bias or zero bias on the photodiode. A current amplifier (transimpedance amplifier) should be used with a photodiode in PC mode. Linearity and PC mode are achieved as long as the diode does not become forward biased.

High speed operation is often required of photodiodes, as opposed to solar cells. Speed is a function of diode capacitance, which can be minimized by decreasing cell area. Thus, a sensor for a high speed fiber optic link will use an area no larger than necessary, say 1 mm2. Capacitance may also be decreased by increasing the thickness of the depletion region, in the manufacturing process or by increasing the reverse bias on the diode.

PIN diode The p-i-n diode or PIN diode is a photodiode with an intrinsic layer between the P and N-regions as in Figure below. The P-Intrinsic-N structure increases the distance between the P and N conductive layers, decreasing capacitance, increasing speed. The volume of the photo sensitive region also increases, enhancing conversion efficiency. The bandwidth can extend to 10’s of gHz. PIN photodiodes are the preferred for high sensitivity, and high speed at moderate cost.

PIN photodiode: The intrinsic region increases the thickness of the depletion region.

Avalanche photo diode:An avalanche photodiode (APD)designed to operate at high reverse bias exhibits an electron multiplier effect analogous to a photomultiplier tube. The reverse bias can run from 10’s of volts to nearly 2000 V. The high level of reverse bias accelerates photon created electron-hole pairs in the intrinsic region to a high enough velocity to free additional carriers from collisions with the crystal lattice. Thus, many electrons per photon result. The motivation for the APD is to achieve amplification within the photodiode to overcome noise in external amplifiers. This works to some extent. However, the APD creates noise of its own. At high speed the APD is superior to a PIN diode amplifier combination, though not for low speed applications. APD’s are expensive, roughly the price of a photomultiplier tube. So, they are only competitive with PIN photodiodes for niche applications. One such application is single photon counting as applied to nuclear physics.

Laser Diode

                            Laser Diode

The laser diode is a further development upon the regular light-emitting diode, or LED. The term “laser” itself is actually an acronym, despite the fact its often written in lower-case letters. “Laser” stands for Light Amplification by Stimulated Emission of Radiation, and refers to another strange quantum process whereby characteristic light emitted by electrons falling from high-level to low-level energy states in a material stimulate other electrons in a substance to make similar “jumps,” the result being a synchronized output of light from the material. This synchronization extends to the actual phase of the emitted light, so that all light waves emitted from a “lasing” material are not just the same frequency (color), but also the same phase as each other, so that they reinforce one another and are able to travel in a very tightly-confined, nondispersing beam. This is why laser light stays so remarkably focused over long distances: each and every light wave coming from the laser is in step with each other.

Laser diode overview

Laser diodes are used in all areas of electronics from domestic equipment, through commercial applications to hash industrial environments. In all these applications laser diodes are able to provide a cost effective solution while being rugged and reliable and offering a high level of performance.

Laser diode technology has a number of advantages:

• Power capability:   Laser diodes are able to provide power levels from a few milliwatts right up to a few hundreds of watts.
• Efficiency:   Laser diode efficiency levels can exceed 30%, making laser diodes a particularly efficient method of generating coherent light.
• Coherent light:   The very nature of a laser is that it generates coherent light. This can be focussed to a diffraction limited spot for high density optical storage applications.
• Rugged construction:   Laser diodes are completely solid state and do not require fragile glass elements or critical set-up procedures. Accordingly they are able to operate under harsh conditions.
• Compact:   Laser diodes can be quite small allowing for laser diode technology to provide a very compact solution.
• Variety of wavelengths:   Using the latest technology and a variety of materials, laser diode technology is able to generate light over a wide spectrum. The use of blue light having a short wavelength allows for tighter focussing of the image for higher density storage.
• Modulation:   It is easy to modulate a laser diode, and this makes laser diode technology ideal for many high data rate communications applications. The modulation is achieved by directly modulating the drive current to the laser diode. This enables frequencies up to several GHz to be achieved for applications such as high-speed data communications.

Laser diode symbol

the laser diode symbol used for circuit diagrams is often the same one used for light emitting diodes. This laser diode circuit symbol uses the basic semiconductor diode symbol with arrows indicating the generation and emanation of light.

Laser diode circuit symbol

When used within a circuit, they are often denoted as being a laser diode to distinguish them from other forms of light emitting diode.

Laser diode basics

There are two maintypes of semiconductor laser diodes. They operate in quite different ways, although many of the concepts used within them are very similar.

• Injection Laser Diode:   The Injection laser diode, ILD, has many factors in common with light emitting diodes. They are manufactured using very similar processes. The main difference is that laser diodes are manufactured having a long narrow channel with reflective ends. This acts as a waveguide for the light.
  
  In operation, current flows through the PN junction and light is generated using the same process that generates light in a light emitting diode. However the light is confined within the waveguide formed in the diode itself. Here the light is reflected and then amplified before exiting though one end of the laser diode.
• Optically Pumped Semiconductor Laser:   Optically pumped semiconductor laser, OPSL uses a III-V semiconductor chip as its basis. This acts as an optical gain medium, and another laser which may be an ILD is used as the pump source. The OPSL approach offers several advantages, particularly in wavelength selection and lack of interference from internal electrode structures.

A more complete explanation of laser diode theory and operation can be found in another page within this tutorial.

The laser diode is now well established, and used in a wide variety of applications. Although not nearly as cheap as many other forms of diode, laser diodes are still produced in vast quantities and at a relatively low cost, as demonstrated by the fact that laser diodes are even used in the light pencils used for illustrating overhead projector slide presentations. At the other end of the market, laser diodes for use in optical communications systems have been shown with data rates in excess of 20 Gbits per second. With performance levels in this region, they are being used increasingly in many communications applications.

Laser diode major categories

There are two major categories of semiconductor laser diodes. They operate in quite different ways, although many of the concepts used within them are very similar.

• Injection Laser Diode:   The Injection laser diode, ILD, has many factors in common with light emitting diodes. They are manufactured using very similar processes. The main difference is that laser diodes are manufactured having a long narrow channel with reflective ends. This acts as a waveguide for the light.
  
  In operation, current flows through the PN junction and light is generated using the same process that generates light in a light emitting diode. However the light is confined within the waveguide formed in the diode itself. Here the light is reflected and then amplified as a result of stimulated emission before exiting though one end of the laser diode as the external beam.
• Optically Pumped Semiconductor Laser:   Optically pumped semiconductor laser, OPSL uses a III-V semiconductor chip as its basis. This acts as an optical gain medium, and another laser which may be an ILD is used as the pump source. The optical gain is provided by stimulated emission. The OPSL approach offers several advantages, particularly in wavelength selection and lack of interference from internal electrode structures.

Laser diode theory basics

There are three main processes in semiconductors that are associated with light:

• Light absorption:   Absorption occurs when light enters a semiconductor and its energy is transferred to the semiconductor to generate additional free electrons and holes. This effect is widely used and enables devices like to photo-detectors and solar cells to operate.
• Spontaneous emission:   The second effect known as spontaneous emission occurs in LEDs. The light produced in this manner is what is termed incoherent. In other words the frequency and phase are random, although the light is situated in a given part of the spectrum.
• Stimulated emission:   Stimulated emission is different. A light photon entering the semiconductor lattice will strike an electron and release energy in the form of another light photon. The way in which this occurs releases this new photon of identical wavelength and phase. In this way the light that is generated is said to be coherent.

The key to the laser diode operation occurs at the junction of the highly doped p and n type regions. In a normal p-n junction current flows across the p-n junction. This action can occur because the holes from the p-type region and the electrons from the n-type region combine. With an electromagnetic wave (in this instance light) in passing through the laser diode junction diode junction it is found that the photo-emission process occurs. Here the photons release further photons of light occurs when they strike electrons during the recombination of holes and electrons occurs.

Naturally there is some absorption of the light, resulting in the generation of holes and electrons but there is an overall gain in level.

The structure of the laser diode creates an optical cavity in which the light photons have multiple reflections. When the photons are generated only a small number are able to leave the cavity. In this way when one photon strikes an electron and enables another photon to be generated the process repeats itself and the photon density or light level starts to build up. It is in the design of better optical cavities that much of the current work on lasers is being undertaken. Ensuring the light is properly reflected is the key to the operation of the device.

PIN diodes

A PIN diode is a fast low capacitance switching diode. Do not confuse a PIN switching diode with a PIN photo diode . A PIN diode is manufactured like a silicon switching diode with an intrinsic region added between the PN junction layers. This yields a thicker depletion region, the insulating layer at the junction of a reverse biased diode. This results in lower capacitance than a reverse biased switching diode.


A PIN diode is a diode with a wide, undoped intrinsic semiconductor region between a p-type semiconductor and an n-type semiconductor region. The p-type and n-type regions are typically heavily doped because they are used for ohmic contacts.

The PIN diode differs from the basic PN junction diode in that the PIN diode includes a layer of intrinsic material between the P and N layers. As a result of the intrinsic layer, PIN diodes have a high breakdown voltage and they also exhibit a low level of junction capacitance. In addition to this the larger deletion region of the PIN diode is ideal for applications as a photodiode.

PIN diode basics and operation

The PIN diode can be shown diagrammatically as being a PN junction, but with an intrinsic layer between the PN and layers. The intrinsic layer of the PIN diode is a layer without doping, and as a result this increases the size of the depletion region - the region between the P and N layers where there are no majority carriers. This change in the structure gives the PIN diode its unique properties.

Basic PIN diode structure

The PIN diode operates in exactly the same way as a normal diode. The only real difference is that the depletion region, that normally exists between the P and N regions in an unbiased or reverse biased diode is larger.

In any PN junction, the P region contains holes as it has been doped to ensure that it has a predominance of holes. Similarly the N region has been doped to contain excess electrons. The region between the P and N regions contains no charge carriers as any holes or electrons combine As the depletion region has no charge carriers it acts as an insulator.

Within a PIN diode the depletion region exists, but if the diode is forward biased, the carriers enter the depletion region (including the intrinsic region) and as the two carrier types meet, current starts to flow.

When the diode is forward biased, the carrier concentration, i.e. holes and electrons is very much higher than the intrinsic level carrier concentration. Due to this high level injection level, the electric field extends deeply (almost the entire length) into the region. This electric field helps in speeding up of the transport of charge carriers from p to n region, which results in faster operation of the diode, making it a suitable device for high frequency operations.

PIN diode structure

The PIN diode consists of a semiconductor diode with three layers. The usual P and N regions are present, but between them is a layer of intrinsic material a very low level of doping. This may be either N-type or P-type, but with a concentration of the order of 13^13 cm^-3 which gives it a resistivity of the order of one k-ohm cm.

The thickness of the intrinsic layer is normally very narrow, typically ranging from 10 to 200 microns. The outer P and N-type regions are then heavily doped.

There are two ways in which the PIN diode can be realised. One is to fabricate the p-i-n diode in a planar structure, and the other is to use a mesa structure. When the planar structure is fabricated an epitaxial film is grown onto the substrate material and the P+ region is introduced either by diffusion or ion implantation. The mesa structure has layers grown onto the substrate. These layers have the dopants incorporated. In this way it is possible to control the thickness of the layers and the level of dopants more accurately and a very thin intrinsic layer can be fabricated if required. This is ideal for high frequency operation. A further advantage of the mesa structure is that it provides a reduced level of fringing capacitance and inductance as well as an improved level of surface breakdown.

PIN diode with a planar construction

PIN diodes are widely made of silicon, and this was the semiconductor material that was used exclusively until the 1980s when gallium arsenide was introduced.

PIN diode uses and advantages

The PIN diode is used in a number of areas as a result of its structure proving some properties which are of particular use.

• High voltage rectifier:   The PIN diode can be used as a high voltage rectifier. The intrinsic region provides a greater separation between the PN and N regions, allowing higher reverse voltages to be tolerated.
• RF switch:   The PIN diode makes an ideal RF switch. The intrinsic layer between the P and N regions increases the distance between them. This also decreases the capacitance between them, thereby increasing he level of isolation when the diode is reverse biased.
• Photodetector:   As the conversion of light into current takes place within the depletion region of a photdiode, increasing the depletion region by adding the intrinsic layer improves the performance by increasing he volume in which light conversion occurs.

These are three of the main applications for PIN diodes, although they can also be used in some other areas as well.

The PIN diode is an ideal component to provide electronics switching in many areas of electronics. It is particularly useful for RF design applications and for providing the switching, or attenuating element in RF switches and RF attenuators. The PIN diode is able to provide much higher levels of reliability than RF relays that are often the only other alternative.

Key PIN diode characteristics

There are a number of PIN diode characteristics that set this diode apart from other forms of diode. These key PIN diode characteristics include the following:

• High breakdown voltage:   The wide depletion layer provided by the intrinsic layer ensures that PIN diodes have a high reverse breakdown characteristic.
• Low capacitance:   Again the intrinsic layer increases the depletion region width. As the capacitance of a capacitor reduces with increasing separation, this means that a PIN diode will have a lower capacitance as the depletion region will be wider than a conventional diode. This PIN diode characteristic can have significant advantages in a number of RF applications - for example when a PIN diode is used as an RF switch.
• Carrier storage:   Carrier storage gives a most useful PIN diode characteristic. For small signals at high frequencies the stored carriers within the intrinsic layer are not completely swept by the RF signal or recombination. At these frequencies there is no rectification or distortion and the PIN diode characteristic is that of a linear resistor which introduces no distortion or rectification. The PIN diode resistance is governed by the DC bias applied. In this way it is possible to use the device as an effective RF switch or variable resistor for an attenuator producing far less distortion than ordinary PN junction diodes.
• Sensitive photodetection:   The sensitive area of a photodiode is the depletion region. Light striking the crystal lattice can release holes and electrons which are drawn away out of the depletion region by the reverse bias on the diode. By having a larger depletion region - as in the case of a PIN diode - the volume for light reception is increased. This makes PIN diodes ideal for use as photodetectors.

PIN Diode Applications

PIN diodes are useful as RF switches, attenuators, photodetectors, and phase shifters.

Although the p-i-n diode finds many applications in the high voltage arena, it is probably for radio frequency applications where it is best known. The fact that when it is forward biased, the diode is linear, behaving like a resistor, can be put to good use in a variety of applications. It can be used as a variable resistor in a variable attenuator, a function that few other components can achieve as effectively. The PIN diode can also be used as an RF switch. In the forward direction it can be biased sufficiently to ensure it has a low resistance to the RF that needs to be passed, and when a reverse bias is applied it acts as an open circuit, with only a relatively small level of capacitance.

PIN diode attenuator and switch circuit