TYPES OF DIODES

VARICAP (VARACTOR diode)

In electronics, a Vari-cap diode, varactor diode, variable capacitance diode, variable reactance diode or tuning diode is a type of diode which has a variable capacitance that is a function of the voltage impressed on its terminals.

Applications

Varactor diodes are used as voltage-controlled capacitors. They are commonly used in parametric amplifiers, parametric oscillators and voltage-controlled oscillators as part of phase-locked loops and frequency synthesizers. For example, varactor diodes are used in the tuners of television sets to electronically tune the receiver to different stations.

Operation of a vari-cap

Internal structure of a vari-cap

Varactors are operated reverse-biased so 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 root of the applied voltage; and 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 specially made varactor diodes exploit the effect to boost the capacitance and variability range achieved - most diode fabrication attempts to achieve the opposite.

In the figure we can see an example of a cross-section of a varactor with the depletion layer formed of a p-n-junction. But the depletion layer can also be made of a MOS-diode or a Schottky diode. This is very important in CMOS and MMIC technology.

Tuning circuits

Generally the use of a vari-cap diode in a circuit requires connecting it to a tuned circuit, usually in parallel with any existing capacitance or inductance. Because a D.C. voltage must be applied reverse bias across the vari-cap to alter its capacitance, this must be blocked from entering the tuned circuit. This is accomplished by placing a D.C. blocking capacitor with a capacitance about 100 times greater than the maximum capacitance of the vari-cap diode in series with it and applying the D.C. from a high impedance source to the node between the vari-cap cathode and the blocking capacitor as shown in the upper left hand diagram, right.

Since no current flows in the vari-cap, the value of the resistor connecting its cathode back to the D.C. control voltage can somewhere in the range of 22 to 150K Ohms and the blocking capacitor somewhere in the range of 5-100nF. Sometimes, with very high Q tuned circuits an inductor is placed in series with the resistor to increase the source impedance of the control voltage so as not to load the tuned circuit and decrease its Q.

A second circuit, lower left in the image, using two back-to-back, (cathode to cathode) series connected vari-cap diodes is another common configuration. Effectively the second vari-cap replaces the blocking capacitor in the first circuit. This reduces the overall capacitance by half and the change in capacitance to half also, but possesses the advantage of reducing the A.C. component of voltage across each device and symmetrical distortion should the A.C. component possess enough amplitude to bias the vari-caps into forward conduction.

When designing tuning circuits with vari-caps it is usually good practice to maintain the A.C. component of voltage across the vari-cap at a minimal level, usually less than 100mV peak to peak, to prevent this changing the capacitance of the diode too much and thus distorting the signal and adding harmonics to it.

One remaining circuit, right, depicts two series connected vari-caps being used in a circuit with separate D.C. and A.C. signal 'ground' connections. The DC ground is being depicted as the traditional 'ground' symbol, and the A.C. ground being depicted as a triangle. Separation of 'grounds' is often done to prevent high frequency radiation from the low frequency ground node or D.C. currents in the A.C. ground node upsetting biasing and operating points of active devices such as vari-caps and transistors.

These circuit configurations are quite common in television tuners and electronically tuned broadcast A.M. and F.M. receivers as well as other communications equipment and industrial equipment. Early vari-cap diodes usually required a reverse voltage range from 0-33v to obtain maximum change in capacitance which was quite small, from 1-10pF or so. These types were and are still extensively used in television tuners where only small capacitance changes are required at the carrier high frequencies of more than 50 MHz used by television. In time vari-cap diodes were developed which exhibited very large changes in capacitance, 100-500pF, with relatively small changes in reverse bias of 0-12 or 0-5v. These types allowed electronically tuned A.M. broadcast receivers to be realized as well as a multitude of other functions requiring large capacitance changes at lower frequencies; generally below 10 MHz some of designs of electronic security tag readers used in retail outlets require these high capacitance vari-caps in their voltage-controlled oscillators.

The three leaded devices depicted at the top of the page are generally two common cathode connected vari-caps in a single package. In the consumer A.M.-F.M. tuner depicted at the right, a single dual package vari-cap diode adjusts both the pass band of the tank circuit, (the main station selector) and the local oscillator with a single vari-cap for each. This is done to keep costs down, two dual packages could have been used, and one for the tank and one for the oscillator, four diodes in all, and this was what were depicted in the application data for the LA1851N A.M. radio chip. Two lower capacitance dual varactors are used in the F.M. section which operates at a frequency about one hundred times greater and are highlighted by red arrows. In this case four diodes are used, one dual package each for the tank/band pass filter and the local oscillator.

Switching

Special types of vari-cap diode exhibiting an abrupt change in capacitance can often be found in consumer equipment such as television tuners, which are used to switch radio frequency signal paths. When in the high capacitance state, usually with low or no bias, they present a low impedance path to R.F., whereas when reverse biased their capacitance abruptly decreases and their R.F. impedance increases. Although they are still slightly conductive to the R.F. path, the attenuation they introduce decreases the unwanted signal to an acceptably low level. They are often used in pairs to switch between two different R.F. sources such as the V.H.F. and U.H.F. bands in a television tuner by supplying them with complimentary bias voltages. The fourth device from the left in the picture at the head of this page is one such device.

LED

A light-emitting diode (LED) is a semiconductor light source. LEDs are used as indicator lamps in many devices and are increasingly used for other lighting. Appearing as practical electronic components in 1962, early LEDs emitted low-intensity red light, but modern versions are available across the visible, ultraviolet, and infrared wavelengths, with very high brightness.

When a light-emitting diode is forward-biased (switched on), electrons are able to recombine with electron holes within the device, releasing energy in the form of photons. This effect is called electroluminescence and the color of the light (corresponding to the energy of the photon) is determined by the energy gap of the semiconductor. An LED is often small in area (less than 1 mm2), and integrated optical components may be used to shape its radiation pattern. LEDs present many advantages over incandescent light sources including lower energy consumption, longer lifetime, improved physical robustness, smaller size, and faster switching. LEDs powerful enough for room lighting are relatively expensive and require more precise current and heat management than compact fluorescent lamp sources of comparable output.

Light-emitting diodes are used in applications as diverse as aviation lighting, automotive lighting, advertising, general lighting, and traffic signals. LEDs have allowed new text, video displays, and sensors to be developed, while their high switching rates are also useful in advanced communications technology. Infrared LEDs are also used in the remote control units of many commercial products including televisions, DVD players, and other domestic appliances.

Technology

Inner workings of LED

I-V diagram for a diode

An LED will begin to emit light when the on-voltage is exceeded. Typical on voltages are 23 volts.

Physics

The LED consists of a chip of semiconducting material doped with impurities to create a p-n junction. As in other diodes, current flows easily from the p-side, or anode, to the n-side, or cathode, but not in the reverse direction. Charge-carriers electrons and holes flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon.

The wavelength of the light emitted, and thus its color depends on the band gap energy of the materials forming the p-n junction. In silicon or germanium diodes, the electrons and holes recombine by a non-radiative transition, which produces no optical emission, because these are

indirect band gap materials. The materials used for the LED have a direct band gap with energies corresponding to near-infrared, visible, or near-ultraviolet light.

LED development began with infrared and red devices made with gallium arsenide. Advances in materials science have enabled making devices with ever-shorter wavelengths, emitting light in a variety of colors.

LEDs are usually built on an n-type substrate, with an electrode attached to the p-type layer deposited on its surface. P-type substrates, while less common, occur as well. Many commercial LEDs, especially GaN/InGaN, also use sapphire substrate.

Most materials used for LED production have very high refractive indices. This means that much light will be reflected back into the material at the material/air surface interface. Thus, light extraction in LEDs is an important aspect of LED production, subject to much research and development.

Colors and materials

Following table shows the available colors with wavelength range, voltage drop and material:

Color Wavelength [nm] Voltage drop [V] Semiconductor material

Infrared > 760 V < 1.63 Gallium arsenide (GaAs) Aluminium gallium arsenide (AlGaAs)

Red 610 < < 760 1.63 < V < 2.03

Aluminium gallium arsenide (AlGaAs) Gallium arsenide phosphide (GaAsP) Aluminium gallium indium phosphide (AlGaInP) Gallium(III) phosphide (GaP)

Orange 590 < < 610 2.03 < V < 2.10

Gallium arsenide phosphide (GaAsP) Aluminium gallium indium phosphide (AlGaInP) Gallium(III) phosphide (GaP)

Yellow 570 < < 590 2.10 < V < 2.18 Gallium arsenide phosphide (GaAsP) Aluminium gallium indium phosphide

(AlGaInP) Gallium(III) phosphide (GaP)

Green 500 < < 570 1.9[56] < V < 4.0

Indium gallium nitride (InGaN) / Gallium(III) nitride (GaN) Gallium(III) phosphide (GaP) Aluminium gallium indium phosphide (AlGaInP) Aluminium gallium phosphide (AlGaP)

Blue 450 < < 500 2.48 < V < 3.7

Zinc selenide (ZnSe) Indium gallium nitride (InGaN) Silicon carbide (SiC) as substrate Silicon (Si) as substrate under development

Violet 400 < < 450 2.76 < V < 4.0 Indium gallium nitride (InGaN)

Purple multiple types 2.48 < V < 3.7

Dual blue/red LEDs, blue with red phosphor, or white with purple plastic

Ultraviolet < 400 3.1 < V < 4.4

Diamond (235 nm)[57] Boron nitride (215 nm)[58][59] Aluminium nitride (AlN) (210 nm)[60] Aluminium gallium nitride (AlGaN) Aluminium gallium indium nitride (AlGaInN) down to 210 nm[61]

Pink multiple types V ~ 3.3[62]

Blue with one or two phosphor layers: yellow with red, orange or pink phosphor added afterwards,

or white with pink pigment or dye.[63]

White Broad spectrum V = 3.5 Blue/UV diode with yellow phosphor

LEDs are produced in a variety of shapes and sizes. The color of the plastic lens is often the same as the actual color of light emitted, but not always. For instance, purple plastic is often used for infrared LEDs, and most blue devices have clear housings. Modern high power LEDs such as those used for lighting and backlighting are generally found in surface-mount technology (SMT) packages, (not shown).

Application-specific variations

Flashing LEDs are used as attention seeking indicators without requiring external electronics. Flashing LEDs resemble standard LEDs but they contain an integrated multivibrator circuit that causes the LED to flash with a typical period of one second. In diffused lens LEDs this is visible as a small black dot. Most flashing LEDs emit light of one color, but more sophisticated devices can flash between multiple colors and even fade through a color sequence using RGB color mixing.

Bi-color LEDs are two different LED emitters in one case. There are two types of these. One type consists of two dies connected to the same two leads anti parallel to each other. Current flow in one direction emits one color, and current in the opposite direction emits the other color. The other type consists of two dies with separate leads for both dies and another lead for common anode or cathode, so that they can be controlled independently.

Tri-color LEDs are three different LED emitters in one case. Each emitter is connected to a separate lead so they can be controlled independently. A four-lead arrangement is typical with one common lead (anode or cathode) and an additional lead for each color.

RGB LEDs are Tri-color LEDs with red, green, and blue emitters, in general using a four-wire connection with one common lead (anode or cathode). These LEDs can have either

common positive or common negative leads. Others however, have only two leads (positive and negative) and have a built in tiny electronic control unit.

Calculator LED display, 1970s

Alphanumeric LED displays are available in seven-segment and starburst format. Seven-segment displays handle all numbers and a limited set of letters. Starburst displays can display all letters. Seven-segment LED displays were in widespread use in the 1970s and 1980s, but rising use of liquid crystal displays, with their lower power needs and greater display flexibility, has reduced the popularity of numeric and alphanumeric LED displays.

Advantages

Efficiency: LEDs emit more light per watt than incandescent light bulbs. Their efficiency is not affected by shape and size, unlike fluorescent light bulbs or tubes.

Color: LEDs can emit light of an intended color without using any color filters as traditional lighting methods need. This is more efficient and can lower initial costs.

Size: LEDs can be very small (smaller than 2 mm2) and are easily populated onto printed circuit boards.

On/Off time: LEDs light up very quickly. A typical red indicator LED will achieve full brightness in under a microsecond. LEDs used in communications devices can have even faster response times.

Cycling: LEDs are ideal for uses subject to frequent on-off cycling, unlike fluorescent lamps that fail faster when cycled often, or HID lamps that require a long time before restarting.

Dimming: LEDs can very easily be dimmed either by pulse-width modulation or lowering the forward current.

Cool light: In contrast to most light sources, LEDs radiate very little heat in the form of IR that can cause damage to sensitive objects or fabrics. Wasted energy is dispersed as heat through the base of the LED.

Slow failure: LEDs mostly fail by dimming over time, rather than the abrupt failure of incandescent bulbs.

Lifetime: LEDs can have a relatively long useful life. One report estimates 35,000 to 50,000 hours of useful life, though time to complete failure may be longer. Fluorescent tubes typically are rated at about 10,000 to 15,000 hours, depending partly on the conditions of use, and incandescent light bulbs at 1,000 to 2,000 hours. Several DOE demonstrations have shown that reduced maintenance costs from this extended lifetime, rather than energy savings, is the primary factor in determining the payback period for an LED product.

Shock resistance: LEDs, being solid-state components, are difficult to damage with external shock, unlike fluorescent and incandescent bulbs, which are fragile.

Focus: The solid package of the LED can be designed to focus its light. Incandescent and fluorescent sources often require an external reflector to collect light and direct it in a usable manner.

Disadvantages

High initial price: LEDs are currently more expensive, price per lumen, on an initial capital cost basis, than most conventional lighting technologies. As of 2010, the cost per thousand lumens (kilolumen) was about $18. The price is expected to reach $2/kilolumen by 2015. The additional expense partially stems from the relatively low lumen output and the drive circuitry and power supplies needed.

Temperature dependence: LED performance largely depends on the ambient temperature of the operating environment. Over-driving an LED in high ambient temperatures may result in overheating the LED package, eventually leading to device failure. An adequate heat sink is needed to maintain long life. This is especially important in automotive, medical, and military uses where devices must operate over a wide range of temperatures, and need low failure rates.

Voltage sensitivity: LEDs must be supplied with the voltage above the threshold and a current below the rating. This can involve series resistors or current-regulated power supplies.

Light quality: Most cool-white LEDs have spectra that differ significantly from a black body radiator like the sun or an incandescent light. The spike at 460 nm and dip at 500 nm can cause the color of objects to be perceived differently under cool-white LED illumination than sunlight or incandescent sources, due to metamerism, red surfaces being rendered particularly badly by typical phosphor-based cool-white LEDs. However, the color rendering properties of common fluorescent lamps are often inferior to what is now available in state-of-art white LEDs.

Area light source: Single LEDs do not approximate a point source of light giving a spherical light distribution, but rather a lambertian distribution. So LEDs are difficult to apply to uses needing a spherical light field. LEDs cannot provide divergence below a few degrees. In contrast, lasers can emit beams with divergences of 0.2 degrees or less.

Electrical polarity: Unlike incandescent light bulbs, which illuminate regardless of the electrical polarity, LEDs will only light with correct electrical polarity.

Blue hazard: There is a concern that blue LEDs and cool-white LEDs are now capable of exceeding safe limits of the so-called blue-light hazard as defined in eye safety specifications such as ANSI/IESNA RP-27.105: Recommended Practice for Photo biological Safety for Lamp and Lamp Systems.

Blue pollution: Because cool-white LEDs with high color temperature emit proportionally more blue light than conventional outdoor light sources such as high-pressure sodium vapor lamps, the strong wavelength dependence of Rayleigh scattering means that cool-white LEDs can cause more light pollution than other light sources. The International Dark-Sky Association discourages using white light sources with correlated color temperature above 3,000 K.

Droop: The efficiency of LEDs tends to decrease as current increases.

Applications

In general, all the LED products can be divided into two major parts, the public lighting and indoor lighting. LED uses fall into four major categories:

Visual signals where light goes more or less directly from the source to the human eye, to convey a message or meaning.

Illumination where light is reflected from objects to give visual response of these objects. Measuring and interacting with processes involving no human vision. Narrow band light sensors where LEDs operate in a reverse-bias mode and respond to

incident light, instead of emitting light.

For more than 70 years, until the LED, practically all lighting was incandescent and fluorescent with the first fluorescent light only being commercially available after the 1939 World's Fair.

In electronics, the basic LED circuit is an electric power circuit used to power a light-emitting diode or LED. The simplest such circuit consists of a voltage source and two components connected in series: a current-limiting resistor (sometimes called the ballast resistor), and an LED. Optionally, a switch may be introduced to open and close the circuit. The switch may be replaced with another component or circuit to form a continuity tester.

(Although simple, this circuit is not necessarily the most energy efficient circuit to drive an LED, since energy is lost in the resistor. More complicated circuits may be used to improve energy efficiency).

The LED used will have a voltage drop, specified at the intended operating current. Ohm's law and Kirchhoff's circuit laws are used to calculate the resistor that is used to attain the correct current. The resistor value is computed by subtracting the LED voltage drop from the supply voltage, and then dividing by the desired LED operating current. If the supply voltage is equal to the LED's voltage drop, no resistor is needed.

Series resistor

Series resistors are a simple way to stabilize the LED current, but energy is wasted in the resistor.

Miniature indicator LEDs are normally driven from low voltage DC via a current limiting resistor. Currents of 2 mA, 10 mA and 20 mA are common. Sub-mA indicators may be made by driving ultrabright LEDs at very low current. Efficiency tends to reduce at low currents, but indicators running on 100 A are still practical. The cost of ultrabright LEDs is higher than that of 2 mA indicator LEDs.

In coin cell powered keyring type LED lights, the resistance of the cell itself is usually the only current limiting device. The cell should not therefore be replaced with a lower resistance type.

LEDs can be purchased with built-in series resistors. These can save printed circuit board space and are especially useful when building prototypes or populating a PCB in a way other than its designers intended. However, the resistor value is set at the time of manufacture, removing one of the key methods of setting the LED's intensity. Alphanumeric LEDs use the same drive strategy as indicator LEDs, the only difference being the larger number of channels, each with its own resistor. Seven-segment and starburst LED arrays are available in both common-anode and common-cathode form.

Series resistor calculation

The formula to calculate the correct resistance to use is

Where power supply voltage (Vs) is the voltage of the power supply, e.g. a 9 volt battery, LED voltage drop (Vf) is the forward voltage drop across the LED, and LED current (I) is the desired current of the LED. The above formula requires the current in amperes, although this value is usually given by the manufacturer in mA, such as 20 mA.

Typically, a LED forward voltage is about 1.83.3 volts; it varies by the color of the LED. A red LED typically drops 1.8 volts, but voltage drop normally rises as the light frequency increases, so a blue LED may drop around 3.3 volts.

The formula can be explained considering the LED as a resistance, and applying Kirchhoff's voltage law (KVL) (R is the unknown quantity):

Polarity

Unlike incandescent light bulbs, which illuminate regardless of the electrical polarity, LEDs will only light with correct electrical polarity. When the voltage across the p-n junction is in the correct direction, a significant current flows and the device is said to be forward-biased. If the voltage is of the wrong polarity, the device is said to be reverse biased and very little current flows, and no light is emitted. LEDs can be operated on an alternating current voltage, but they will only light with positive voltage, causing the LED to turn on and off at the frequency of the AC supply.

Most LEDs have low reverse breakdown voltage ratings, so they will also be damaged by an applied reverse voltage above this threshold. The cause of damage is over current resulting from the diode breakdown, not the voltage itself. LEDs driven directly from an AC supply of more than the reverse breakdown voltage may be protected by placing a diode (or another LED) in inverse parallel.

PHOTODIODE ( P-N-)

A photodiode is a type of photodetector capable of converting light into either current or voltage, depending upon the mode of operation. The common, traditional solar cell used to generate electric solar power is a large area photodiode. It is PN junction and the junction is coated with one of the photo sensitive material (CdS, Se, Zns, PbS etc).

Photodiodes are similar to regular semiconductor diodes except that they may be either exposed (to detect vacuum UV or X-rays) or packaged with a window or optical fiber connection to allow light to reach the photo sensitive part of the device. Many diodes designed for use specifically as a photodiode use a PIN junction rather than a p-n junction, to increase the speed of response. A photodiode is designed to operate in reverse bias.

A photodiode is a p-n junction or PIN structure. When a photon of sufficient energy strikes the diode (at junction), it excites an electron, thereby creating a free electron (and a positively charged electron hole). This mechanism is also known as the inner photoelectric effect. If the absorption occurs in the junction's depletion region, or one diffusion length away from it, these

carriers are swept from the junction by the built-in field of the depletion region. Thus holes move toward the anode, and electrons toward the cathode, and a photocurrent is produced. This photocurrent is the sum of both the dark current (without light) and the light current, so the dark current must be minimized to enhance the sensitivity of the device.

Photovoltaic mode

When used in zero bias or photovoltaic mode, the flow of photocurrent out of the device is restricted and a voltage builds up. This mode exploits the photovoltaic effect, which is the basis for solar cells a traditional solar cell is just a large area photodiode.

Photoconductive mode

In this mode the diode is often reverse biased (with the cathode positive), dramatically reducing the response time at the expense of increased noise. This increases the width of the depletion layer, which decreases the junction's capacitance resulting in faster response times. The reverse bias induces only a small amount of current (known as saturation or back current) along its direction while the photocurrent remains virtually the same. For a given spectral distribution, the photocurrent is linearly proportional to the illuminance (and to the irradiance).

Although this mode is faster, the photoconductive mode tends to exhibit more electronic noise. The leakage current of a good PIN diode is so low (

Other modes of operation

Avalanche photodiodes have a similar structure to regular photodiodes, but they are operated with much higher reverse bias. They are fabricated only with Si. This allows each photo-generated carrier to be multiplied by avalanche breakdown, resulting in internal gain within the photodiode, which increases the effective responsivity of the device. It can handle large signal power when compared to a photodiode.

A phototransistor is in essence a bipolar transistor encased in a transparent case so that light can reach the base-collector junction. It was invented by Dr. John N. Shive (more famous for his wave machine) at Bell Labs in 1948, but it wasn't announced until 1950. The electrons that are generated by photons in the base-collector junction are injected into the base, and this photodiode current is amplified by the transistor's current gain (or hfe). If the emitter is left unconnected, the phototransistor becomes a photodiode. While phototransistors have a higher responsivity for light they are not able to detect low levels of light any better than photodiodes. Phototransistors also have significantly longer response times.

Materials

The material used to make a photodiode is critical to defining its properties, because only photons with sufficient energy to excite electrons across the material's band gap will produce significant photocurrents. Materials commonly used to produce photodiodes include:

Material Electromagnetic spectrum wavelength range (nm)

Silicon 1901100 Germanium 4001700 Indium gallium arsenide 8002600 Lead(II) sulfide

Critical performance parameters of a photodiode include:

Responsivity

The ratio of generated photocurrent to incident light power is known as responsivity, typically expressed in A/W when used in photoconductive mode. The responsivity may also be expressed as Quantum efficiency, or the ratio of the number of photogenerated carriers to incident photons and thus a unitless quantity.

Dark current

The current through the photodiode in the absence of light, when it is operated in photoconductive mode is called Dark current. The dark current includes photocurrent generated by background radiation and the saturation current of the semiconductor junction. Dark current must be accounted for by calibration if a photodiode is used to make an accurate optical power measurement, and it is also a source of noise when a photodiode is used in an optical communication system.

Noise-equivalent power (NEP)

The minimum input optical power to generate photocurrent, equal to the rms noise current in a 1 hertz bandwidth. NEP is essentially the minimum detectable power. The related characteristic "detectivity" (D) is the inverse of NEP, 1/NEP.

There is also the "specific detectivity" ( ) which is the detectivity multiplied by the square root of the area (A) of the photodetector, ( ) for a 1 Hz bandwidth. The specific detectivity allows different systems to be compared independent of sensor area and system bandwidth; a higher detectivity value indicates a low-noise device or system.[8] Although it is traditional to give ( ) in many catalogues as a measure of the diode's quality, in practice, it is hardly ever the key parameter.

When a photodiode is used in an optical communication system, these parameters contribute to the sensitivity of the optical receiver, which is the minimum input power required for the receiver to achieve a specified bit error rate.

Applications

P-N photodiodes are used in similar applications to other photodetectors, such as photoconductors, charge-coupled devices, and photomultiplier tubes. They may be used to generate an output which is dependent upon the illumination (analog; for measurement and the like), or to change the state of circuitry (digital; either for control and switching, or digital signal processing).

Photodiodes are used in consumer electronics devices such as compact disc players, smoke detectors, and the receivers for infrared remote control devices used to control equipment from televisions to air conditioners. For many applications either photodiodes or photoconductors may be used. Either type of photosensor may be used for light measurement, as in camera light meters, or to respond to light levels, as in switching on street lighting after dark.

Photosensors of all types may be used to respond to incident light, or to a source of light which is part of the same circuit or system. A photodiode is often combined into a single component with an emitter of light, usually a light-emitting diode (LED), either to detect the presence of a mechanical obstruction to the beam (slotted optical switch), or to couple two digital or analog circuits while maintaining extremely high electrical isolation between them, often for safety (optocoupler).

Photodiodes are often used for accurate measurement of light intensity in science and industry. They generally have a more linear response than photoconductors.

They are also widely used in various medical applications, such as detectors for computed tomography (coupled with scintillators), instruments to analyze samples (immunoassay), and pulse oximeters.

PIN diodes are much faster and more sensitive than p-n junction diodes, and hence are often used for optical communications and in lighting regulation.

P-N photodiodes are not used to measure extremely low light intensities. Instead, if high sensitivity is needed, avalanche photodiodes, intensified charge-coupled devices or photomultiplier tubes are used for applications such as astronomy, spectroscopy, night vision equipment and laser range finding.

TUNNEL DIODE

A tunnel diode or Esaki diode is a type of semiconductor diode which is capable of very fast operation, well into the microwave frequency region, by using the quantum mechanical effect called tunneling.

It was invented in August 1957 by Leo Esaki when he was with Tokyo Tsushin Kogyo, now known as Sony. In 1973 he received the Nobel Prize in Physics, jointly with Brian Josephson, for discovering the electron tunneling effect used in these diodes. Robert Noyce independently came

up with the idea of a tunnel diode while working for William Shockley, but was discouraged from pursuing it.

These diodes have a heavily doped pn junction only some 10 nm (100 ) wide. The heavy doping results in a broken band gap, where conduction band electron states on the n-side are more or less aligned with valence band hole states on the p-side.

Tunnel diodes were first manufactured by Sony in 1957 followed by General Electric and other companies from about 1960, and are still made in low volume today. Tunnel diodes are usually made from germanium, but can also be made in gallium arsenide and silicon materials. They are used in frequency converters and detectors. They have negative differential resistance in part of their operating range, and therefore are also used as oscillators, amplifiers, and in switching circuits using hysteresis.

Forward bias operation

Under normal forward bias operation, as voltage begins to increase, electrons at first tunnel through the very narrow pn junction barrier because filled electron states in the conduction band on the n-side become aligned with empty valence band hole states on the p-side of the p-n junction. As voltage increases further these states become more misaligned and the current drops this is called negative resistance because current decreases with increasing voltage. As voltage increases yet further, the diode begins to operate as a normal diode, where electrons travel by conduction across the pn junction, and no longer by tunneling through the pn junction barrier. Thus the most important operating region for a tunnel diode is the negative resistance region.

Reverse bias operation

When used in the reverse direction they are called back diodes and can act as fast rectifiers with zero offset voltage and extreme linearity for power signals (they have an accurate square law characteristic in the reverse direction). Under reverse bias filled states on the p-side become increasingly aligned with empty states on the n-side and electrons now tunnel through the pn junction barrier in reverse direction.

I-V curve similar to a tunnel diode characteristic curve

It has negative resistance in the shaded voltage region, between v1 and v2.

In a conventional semiconductor diode, conduction takes place while the pn junction is forward biased and blocks current flow when the junction is reverse biased. This occurs up to a point known as the reverse breakdown voltage when conduction begins (often accompanied by destruction of the device). In the tunnel diode, the dopant concentration in the p and n layers are increased to the point where the reverse breakdown voltage becomes zero and the diode conducts in the reverse direction. However, when forward-biased, an odd effect occurs called quantum mechanical tunneling which gives rise to a region where an increase in forward voltage is accompanied by a decrease in forward current. This negative resistance region can be exploited in a solid state version of the dynatron oscillator which normally uses a tetrode thermionic valve (or tube).

The tunnel diode showed great promise as an oscillator and high-frequency threshold (trigger) device since it would operate at frequencies far greater than the tetrode would, well into the microwave bands. Applications for tunnel diodes included local oscillators for UHF television tuners, trigger circuits in oscilloscopes, high speed counter circuits, and very fast-rise time pulse generator circuits. The tunnel diode can also be used as low-noise microwave amplifier.[5] However, since its discovery, more conventional semiconductor devices have surpassed its performance using conventional oscillator techniques. For many purposes, a three-terminal device, such as a field-effect transistor, is more flexible than a device with only two terminals. Practical tunnel diodes operate at a few milliamperes and a few tenths of a volt, making them low-power devices. The Gunn diode has similar high frequency capability and can handle more power.

Tunnel diodes are also relatively resistant to nuclear radiation, as compared to other diodes. This makes them well suited to higher radiation environments, such as those found in space applications.

Longevity

Esaki diodes are notable for their longevity; devices made in the 1960s still function. Writing in Nature, Esaki and coauthors state that semiconductor devices in general are extremely stable, and suggest that their shelf life should be "infinite" if kept at room temperature. They go on to report that a small-scale test of 50-year-old devices revealed a "gratifying confirmation of the diode's longevity". As noticed on some samples of Esaki diodes, the gold plated iron pins can in fact corrode and short out to the case. This can usually be diagnosed, and the diode inside normally still works.

A Tunnel Diode is s pn junction that exhibits negative resistance between two values of forward voltage.

The tunnel diode s basically a pn junction with heavy doping of p type and n type semiconductor materials .tunnel diode is doped 1000 times as heavily as a conventional diode Heavy doping results in large no of majority carriers. Because this large no of carriers, most are not used during

initial recombination that produces depletion layer. It is very narrow. Depletion layer of tunnel diode is 100 times narrower. Operation of tunnel diode depends on the tunneling effect.

TUNNELING

The movement of valence electrons from the valence energy band to the conduction band with little or no applied forward voltage is called tunneling.

VI CHARACTERISTICS

As the forward voltage is first increased, the tunnel diode is increased from zero, electrons from the n region tunnel through the potential barrier to the potential barrier to the p region. As the forward voltage increases the diode current also increases until the peak to peak is reached. Ip = 2.2 mA. Peak point voltage =0.07V

As the voltage is increased beyond Vp the tunneling action starts decreasing and the diode current decreases as the forward voltage is increased until valley point V is reached at valley point voltage Vv= 0.7V between V and P the diode exhibits negative resistance i.e., as the forward bias is increased , the current decreases. When operated in the negative region used as oscillator.

SCHOTTKY DIODE

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 can be considered primitive Schottky diodes.

When current flows through a diode, there is small voltage drop across the diode terminals. A normal silicon diode has a voltage drop between 0.61.7 volts, while a Schottky diode voltage drop is between approximately 0.150.45 volts. This lower voltage drop can provide higher switching speed and better system efficiency.

Construction

A metalsemiconductor junction is formed between a metal and a semiconductor, creating a Schottky barrier (instead of a semiconductorsemiconductor junction as in conventional diodes). Typical metals used are molybdenum, platinum, chromium or tungsten; and the semiconductor would typically be N-type silicon.[1] The metal side acts as the anode and N-type semiconductor acts as the cathode of the diode. This Schottky barrier results in both very fast switching and low forward voltage drop.

Reverse recovery time

The most important difference between the p-n and Schottky diode is reverse recovery time, when the diode switches from conducting to non-conducting state. Where in a p-n diode the reverse recovery time can be in the order of hundreds of nanoseconds and less than 100 ns for fast diodes, Schottky diodes do not have a recovery time, as there is nothing to recover from (i.e. no charge carrier depletion region at the junction). The switching time is ~100 ps for the small signal diodes, and up to tens of nanoseconds for special high-capacity power diodes. With p-n junction switching, there is also a reverse recovery current, which in high-power semiconductors brings increased EMI noise. With Schottky diodes switching essentially instantly with only slight capacitive loading, this is much less of a concern.

It is often said that the Schottky diode is a "majority carrier" semiconductor device. This means that if the semiconductor body is doped n-type, only the n-type carriers (mobile electrons) play a significant role in normal operation of the device. The majority carriers are quickly injected into the conduction band of the metal contact on the other side of the diode to become free moving electrons. Therefore no slow, random recombination of n- and p- type carriers is involved, so that this diode can cease conduction faster than an ordinary p-n rectifier diode. This property in turn allows a smaller device area, which also makes for a faster transition. This is another reason why Schottky diodes are useful in switch-mode power converters; the high speed of the diode means that the circuit can operate at frequencies in the range 200 kHz to 2 MHz, allowing the use of small inductors and capacitors with greater efficiency than would be possible with other diode types. Small-area Schottky diodes are the heart of RF detectors and mixers, which often operate up to 50 GHz.

Limitations

The most evident limitations of Schottky diodes are the relatively low reverse voltage ratings for silicon-metal Schottky diodes, typically 50 V and below, and a relatively high reverse leakage current. Some higher-voltage designs are available; 200V is considered a high reverse voltage. Reverse leakage current, because it increases with temperature, leads to a thermal instability issue. This often limits the useful reverse voltage to well below the actual rating.

While higher reverse voltages are achievable, they would be accompanied by higher forward voltage drops, comparable to other types; such a Schottky diode would have no advantage.[2]

Silicon carbide Schottky diode

Schottky diodes constructed from silicon carbide have a much lower reverse leakage current than silicon Schottky diodes, and higher reverse voltage. As of 2011 they were available from manufacturers in variants up to 1700 V.[3]

Silicon carbide has a high thermal conductivity, and temperature has little influence on its switching and thermal characteristics. With special packaging silicon carbide Schottky diodes can operate at junction temperatures of over 500 K (about 200 C), which allows passive radiative cooling in aerospace applications.[3]

Applications

Voltage clamping

While standard silicon diodes have a forward voltage drop of about 0.7 volts and germanium diodes 0.3 volts, Schottky diodes voltage drop at forward biases of around 1 mA is in the range 0.15 V to 0.46 V (see the 1N5817[4] and 1N5711[5] datasheets found online at manufacturer's websites), which makes them useful in voltage clamping applications and prevention of transistor saturation. This is due to the higher current density in the Schottky diode.

Reverse current and discharge protection

Because of a Schottky diodes low forward voltage drop, less energy is wasted as heat making them the most efficient choice for applications sensitive to efficiency. For instance, they are used in stand-alone ("off-grid") photovoltaic (PV) systems to prevent batteries from discharging through the solar panels at night, and in grid-connected systems with multiple strings connected in parallel, in order to prevent reverse current flowing from adjacent strings through shaded strings if the bypass diodes have failed.

Power supply

They are also used as rectifiers in switched-mode power supplies; the low forward voltage and fast recovery time leads to increased efficiency.

Schottky diodes can be used in power supply "OR"ing circuits in products that have both an internal battery and a mains adapter input, or similar. However, the high reverse leakage current presents a problem in this case, as any high-impedance voltage sensing circuit (e.g. monitoring the battery voltage or detecting whether a mains adaptor is present) will see the voltage from the other power source through the diode leakage.

PIN DIODE

A PIN diode is a diode with a wide, lightly doped 'near' 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 wide intrinsic region is in contrast to an ordinary PN diode. The wide intrinsic region makes the PIN diode an inferior rectifier (one typical function of a diode), but it makes the PIN diode suitable for attenuators, fast switches, photodetectors, and high voltage power electronics applications.

Operation

A PIN diode operates under what is known as high-level injection. In other words, the intrinsic "i" region is flooded with charge carriers from the "p" and "n" regions. Its function can be likened to filling up a water bucket with a hole on the side. Once the water reaches the hole's level it will begin to pour out. Similarly, the diode will conduct current once the flooded electrons and holes reach an equilibrium point, where the number of electrons is equal to the number of holes in the intrinsic region. When the diode is forward biased, the injected carrier concentration is typically several orders of magnitude higher than the intrinsic level carrier concentration. Due to this high level injection, which in turn is due to the depletion process, 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.

Characteristics

A PIN diode obeys the standard diode equation for low frequency signals. At higher frequencies, the diode looks like an almost perfect (very linear, even for large signals) resistor. There is a lot of stored charge in the intrinsic region. At low frequencies, the charge can be removed and the diode turns off. At higher frequencies, there is not enough time to remove the charge, so the diode never turns off. The PIN diode has a poor reverse recovery time.

The high-frequency resistance is inversely proportional to the DC bias current through the diode. A PIN diode, suitably biased, therefore acts as a variable resistor. This high-frequency resistance may vary over a wide range (from 0.1 ohm to 10 k in some cases; the useful range is smaller, though).

The wide intrinsic region also means the diode will have a low capacitance when reverse biased.

In a PIN diode, the depletion region exists almost completely within the intrinsic region. This depletion region is much larger than in a PN diode, and almost constant-size, independent of the

reverse bias applied to the diode. This increases the volume where electron-hole pairs can be generated by an incident photon. Some photodetector devices, such as PIN photodiodes and phototransistors (in which the base-collector junction is a PIN diode), use a PIN junction in their construction.

The diode design has some design tradeoffs. Increasing the dimensions of the intrinsic region (and its stored charge) allows the diode to look like a resistor at lower frequencies. It adversely affects the time needed to turn off the diode and its shunt capacitance. PIN diodes will be tailored for a particular use.

Applications

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

RF and Microwave Switches

Under zero or reverse bias, a PIN diode has a low capacitance. The low capacitance will not pass much of an RF signal. Under a forward bias of 1 mA, a typical PIN diode will have an RF resistance of about 1 ohm, making it a good RF conductor. Consequently, the PIN diode makes a good RF switch.

Although RF relays can be used as switches, they switch very slowly (on the order of 10 milliseconds). A PIN diode switch can switch much more quickly (e.g., 1 microsecond).

The capacitance of an off discrete PIN diode might be 1 pF. At 320 MHz, the reactance of 1 pF is about 500 ohms. In a 50 ohm system, the off state attenuation would be about 20 dB -- which may not be enough attenuation. In applications that need higher isolation, switches are cascaded to improve the isolation. Cascading three of the above switches would give 60 dB of attenuation.

PIN diode switches are used not only for signal selection, but they are also used for component selection. For example, some low phase noise oscillators use PIN diodes to range switch inductors.

RF and Microwave Variable Attenuators

By changing the bias current through a PIN diode, it's possible to quickly change the RF resistance.

At high frequencies, the PIN diode appears as a resistor whose resistance is an inverse function of its forward current. Consequently, PIN diode can be used in some variable attenuator designs as amplitude modulators or output leveling circuits.

PIN diodes might be used, for example, as the bridge and shunt resistors in a bridged-T attenuator.

Limiters

PIN diodes are sometimes used as input protection devices for high frequency test probes. If the input signal is within range, the PIN diode has little impact as a small capacitance. If the signal is large, then the PIN diode starts to conduct and becomes a resistor that shunts most of the signal to ground.

Photodetector and photovoltaic cell

The PIN photodiode was invented by Jun-ichi Nishizawa and his colleagues in 1950.

PIN photodiodes are used in fibre optic network cards and switches. As a photodetector, the PIN diode is reverse biased. Under reverse bias, the diode ordinarily does not conduct (save a small dark current or Is leakage). A photon entering the intrinsic region frees a carrier. The reverse bias field sweeps the carrier out of the region and creates a current. Some detectors can use avalanche multiplication.

The PIN photovoltaic cell works in the same mechanism. In this case, the advantage of using a PIN structure over conventional semiconductor junction is the better long wavelength response of the former. In case of long wavelength irradiation, photons penetrate deep into the cell. But only those electron-hole pairs generated in and near the depletion region contribute to current generation. The depletion region of a PIN structure extends across the intrinsic region, deep into the device. This wider depletion width enables electron-hole pair generation deep within the device. This increases the quantum efficiency of the cell.

Typically, amorphous silicon thin-film cells use PIN structures. On the other hand, CdTe cells use NIP structure, a variation of the PIN structure. In a NIP structure, an intrinsic CdTe layer is sandwiched by n-doped CdS and p-doped ZnTe. The photons are incident on the n-doped layer unlike a PIN diode.

A PIN photodiode can also detect X-ray and gamma ray photons.