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Home , Schottky diode, Varicap

Self-study guidelines for the discipline “English for Specific purposes”, Module “Basics of ”

Goals and objectives

The purpose of the individual student work is formation of competences, special knowledge and skills. This work teaches students to work individually and in a team, to show the skills required for professional and personal development.

Self-study organization

All students are divided into groups of 2 or 3 students in each. Each group studies the materials on one type of . Using the materials students compose mind map on given type of diode. Each group presents the mind map using whiteboard or flipchart. All other groups prepare and ask the questions on the topic.

Example questions: 1. What is the normal operating region for a ? 2. Draw the symbols of a . Is it true that a photodiode is used in a reverse-bias position, and it will increase conduction as the light intensity increases? 3. Draw the Zener diode characteristic (V/A). Mark the Zener voltage VZ. 4. Why the Schottky are used in very fast-switching circuits? 5. What do we call the process of emitting photons from a semiconductive material? 6. What diode is used in seven-segment displays: Zener, LED or Schottky? 7. Draw the symbol of a bidirectional Zener diode. Where can we use this device?

Special-Purpose Diodes

Diodes are known for their unidirectional current property. Basically, diodes are used for rectifying waveforms, and can be used within power supplies or within radio detectors. They can also be used in circuits where ‘one way’ effect of diode is required. Diodes transmit electric currents in one direction, however, the manner in which they do so can vary. Several types of diodes are available for the use in the electronics design. Some of diode types are: Zener Diode: This type of the diode provides a stable reference voltage. It is a very useful type and has a wide application. The diode runs in reverse bias, and breaks down at a certain voltage. A stable voltage is produced, if the current flowing through the is limited. Schottky Diodes: These diodes feature a lower forward voltage drop if compared to the ordinary p-n junction diodes. The voltage drop may be in the range from 0.15 to 0.4 volts at a low current, if compared to the 0.6 volts for a silicon diode. Photodiode: are usually used to detect light. Generally, these diodes operate in reverse bias, wherein even a small current flow, resulting from the light, can be detected. Photodiodes can be used to generate electricity, used as solar cells and even in photometry. Light Emitting Diode (LED): It is one of the most popular types of diodes. When this diode permits the transfer of between the electrodes, light is produced. The color of light depends on the energy gap of the . : This type of a diode operates in the reverse bias, and uses avalanche effect. The avalanche breakdown takes place across the entire p-n junction, when the voltage drop is constant and is independent of the current. Generally, the avalanche diode is used for photo-detection, wherein high levels of sensitivity can be obtained by the avalanche process. : This type of a diode is different from the LED type, as it produces coherent light. These diodes are used in DVD and CD drives, laser pointers, etc. Laser diodes are more expensive than LEDs. However, they are cheaper than other laser generators. Diode or Varactor Diode: This type of a diode uses a reverse bias placed upon it, which varies the width of the depletion layer depending on the voltage applied to the diode. This diode acts as a . By altering the bias on the diode, the width of the changes, thereby varying the capacitance. Diode: These diodes are used to rectify alternating power inputs in power supplies. They can rectify current levels that range from an amp upwards. If low voltage drops are required, Schottky diodes can be used, however, generally they are p-n junction diodes.

(a) (b) (c) (d) (e) (f) (g) (h)

Figure 1. Diode symbols: (a) diode; (b) Zener diode; (c) bidirectional Zener diode; (d) ; (e) ; (f) varicap diode; (g) photodiode; (h) light emitting diode

Diodes are used widely in electronics, from design to production. Besides the above mentioned types, other diodes are PIN diodes, point contact diodes, signal diodes, step recovery diodes, tunnel diodes and gold doped diodes. The diode type to transfer the electric current depends on the type and amount of the transmission, as well as on specific applications.

Zener Diode

A Zener diode is a special kind of a diode which allows the current to flow in the forward direction in the same manner as an ideal diode, but it also permits it to flow in the reverse direction when the voltage is above a certain value known as the breakdown voltage, ‘Zener knee voltage’ or ‘Zener voltage’. The device was named after Clarence Zener, who discovered this electrical property. A conventional solid-state diode does not allow the significant current if it is reverse-biased below its reverse breakdown voltage. When the reverse bias breakdown voltage is exceeded, a conventional diode is subject to the high current due to the avalanche breakdown. Unless this current is limited by the circuitry, the diode will be permanently damaged due to the overheating. In the case of a large forward bias, the diode exhibits a voltage drop due to its junction built-in voltage and internal resistance. The amount of the voltage drop depends on the semiconductor material and concentrations. A Zener diode exhibits almost the same properties, except the device is specially designed to have a greatly reduced breakdown voltage, the so-called Zener voltage. The volt-ampere characteristic of a Zener diode showing the breakdown region is shown in Figure 4.6. By contrast with the conventional device, a reverse-biased Zener diode exhibits a controlled breakdown and allows the current to keep the voltage across the Zener diode close to the Zener breakdown voltage. For example, a diode with a Zener breakdown voltage of 4.7 V exhibits a voltage drop of very nearly 4.7 V across a wide range of reverse currents. The Zener diode is therefore ideal for various applications such as the generation of a reference voltage (e.g. for an stage), or as a voltage stabilizer for low-current applications.

+IF Forward K A Current Forward Bias Region

Zener -VZ Reverse Bias Voltage +V -VR F I Forward Bias Z(min) VF

“Zener” Breakdown Region

IZ(max) Reverse Current -IR

Figure 2. Zener diode symbol and I-V characteristics

The Zener diode’s operation depends on the heavy doping of its p-n junction allowing to tunnel from the valence band of the p-type material to the conduction band of the n-type material. In the atomic scale, this tunneling corresponds to the transport of valence band electrons into the empty conduction band states. This occurs as a result of the reduced barrier between these bands and high electric fields that are induced due to the relatively high levels of dopings on both sides. The breakdown voltage can be controlled quite accurately in the doping process. While tolerances within 0.05% are available, the most widely used tolerances are 5% and 10%. Breakdown voltage for commonly available zener diodes can vary widely from 1.2 volts to 200 volts. Another mechanism that produces a similar effect is the avalanche effect as in the avalanche diode. These two types of the diode are in fact constructed in the same way and both effects are present in diodes of this type. In silicon diodes up to 5.6 volts, the Zener effect is predominant and shows a marked negative temperature coefficient. Above 5.6 volts, the avalanche effect becomes predominant and exhibits a positive temperature coefficient. In a 5.6 V diode, the two effects occur together and their temperature coefficients nearly cancel each other out, thus a 5.6 V diode is the component of choice in temperature-critical applications. Modern manufacturing techniques have produced devices with the voltage lower than 5.6 V with negligible temperature coefficients, but as higher voltage devices are encountered, the temperature coefficient rises dramatically. A 75 V diode has 10 times the coefficient of a 12 V diode. All such diodes are usually marketed under the term of ‘Zener diode’.

The Zener Diode Regulator

Zener diodes can be used to produce a stabilized voltage output with low ripple under varying load current conditions (Fig. 4.7). A small current passes through the diode from a voltage source via a suitable current limiting resistor (RS). Then the Zener diode will conduct sufficient current to maintain a voltage drop of Vout. We remember from the previous chapters that the d.c. output voltage from the half or full-wave contains ripple superimposed onto the d.c. voltage. By connecting a simple zener stabilizer circuit as shown below across the output of the rectifier, a more stable output voltage can be produced. The resistor, RS is connected in series with the Zener diode to limit the current flow through the diode with the voltage source, VS being connected across the combination. The stabilised output voltage Vout is taken from across the Zener diode. The Zener diode is connected with its cathode terminal connected to the positive rail of the d.c. supply so it is reverse biased and will be operating in its breakdown condition. Resistor RS is selected to limit the maximum current flowing in the circuit.

IS +V DC input voltage from rectifier or RS smoothing circuit

Vin (VS) IL

IZ Vout VD RL 0V

Figure 3. Zener diode regulator

With no load connected to the circuit, the load current is zero, (IL = 0), and all the circuit current passes through the Zener diode which in turn dissipates its maximum power. Also a small value of the series resistor RS results in a greater diode current when the load resistance RL is connected. In this case, the power dissipation of the diode increases. So care must be taken when selecting the appropriate value of series resistance so that the zener’s maximum power rating is not exceeded under this no-load or high-impedance condition. The load is connected in parallel with the Zener diode, so the voltage across RL is always the same as the zener voltage, (VR = VZ). There is a minimum zener current for which the voltage stabilization is effective and the zener current must stay above this value operating under the load within its breakdown region all the time. The upper limit of the current is, of course, dependent upon the power rating of the device. The supply voltage VS must be greater than VZ. One small problem with zener diode stabiliser circuits is that the diode can sometimes generate electrical noise on top of the d.c. supply as it tries to stabilise the voltage. Normally this is not a problem for most applications but the addition of a large value decoupling capacitor across the Zener’s output may be required to give additional smoothing. To summarize, a Zener diode is always operated in its reverse biased condition. A circuit can be designed using a Zener diode to maintain a constant d.c. output voltage across the load in spite of variations in the input voltage or changes in the load current. The zener voltage regulator consists of a current limiting resistor RS connected in series with the input voltage VS with the Zener diode connected in parallel with the load RL in this reverse biased condition. The stabilized output voltage is always selected to be the same as the breakdown voltage VZ of the diode.

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 the can be considered primitive Schottky diodes. When the current flows through a diode there is a small voltage drop across the diode terminals. A normal silicon diode has a voltage drop between 0.6–1.7 volts, while a Schottky diode voltage drop is between approximately 0.15–0.45 volts. This lower voltage drop can provide higher switching speed and better system efficiency. A metal-semiconductor junction is formed between a metal and a semiconductor, creating a (instead of a semiconductor– semiconductor junction as in conventional diodes). Typical metals used are molybdenum, platinum, chromium or tungsten; and the semiconductor would typically be n-type silicon. 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 p-n and Schottky diodes is reverse recovery time, when the diode from non-conducting to conducting state and vice versa. Where in a p-n diode the reverse recovery time can be hundreds of nanoseconds and less than 100 ns for fast diodes, Schottky diodes do not have recovery time, as there is nothing to recover from (i.e. no 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 brings increased EMI noise in high-power . This is not so important with Schottky diodes switching instantly with only slight capacitive loading. It is often said that the Schottky diode is a ‘majority carrier’ . 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 the conduction faster than an ordinary p-n rectifier diode. This property, in turn, allows a smaller device area, which also makes the transition faster. This is another reason why Schottky diodes are useful in -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 and with greater efficiency than would be possible with other diode types. Small-area Schottky diodes are used in RF detectors and mixers, which often operate up to 50 GHz. The most evident limitations of Schottky diodes are the relatively low reverse voltage rating for silicon-metal Schottky diodes, 50 V and below, and a relatively high reverse leakage current. Diode designs have been improving over time. Nowadays voltage ratings reach 200 V. The reverse leakage current, as it increases with temperature, leads to a thermal instability issue. This often limits the useful reverse voltage.

Silicon Carbide Schottky Diode

Since 2001 another important invention was presented by CREE (NC, USA): a (SiC) Schottky diode. SiC Schottky diodes have about 40 times lower reverse leakage current compared to silicon Schottky diodes. In 2011 they were available from several manufacturers in variants up to 1700 V. Silicon carbide has a high thermal conductivity and temperature has little influence on its switching and thermal characteristics. With special packaging it is possible to have operating junction temperatures of over 500 K, which allows passive radiation cooling in aerospace applications.

Applications

Voltage Clamping While standard silicon diodes have a forward voltage drop of about 0.6 volts (voltage drop of diodes is about 0.3 volts), Schottky diodes’ voltage drop at forward biases of around 1 mA is in the range from 0.15 V to 0.46 V, which makes them useful in voltage clamping applications and in the prevention of the saturation. This is due to the higher in the Schottky diode.

Reverse Current Protection Schottky diodes are used in photovoltaic (PV) systems to prevent a reverse current flowing through the PV modules. For instance, they are used in stand-alone (‘off-grid’) systems to prevent batteries from discharging through the solar cells at night. They are used 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 ‘OR’ing circuits in products that have both an internal battery and a mains adapter input. However, the high reverse leakage current presents a problem in this case, as any high-impedance voltage sensing circuit detects the voltage from another power source through the diode leakage. For example, ST Microelectronics offers Schottky and ultrafast rectifier solutions for all market requirements. ST’s latest developments include ULVF™ ultra-low VF diodes, improved avalanche rating, and the integration of higher currents in low-profile PowerFLAT™ packages. The range of signal Schottky diodes with the new flip-chip and SOD-923 devices helps meet the most precise space-saving requirements, especially for portable communication equipment. For power converter applications where silicon diodes reach the limits of their operating temperature and power density, ST silicon carbide (SiC) devices take over with optimal reliability.

Photodiode

A photodiode is a type of capable of converting light into either the current or the voltage, depending upon the mode of operation. 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 fibre connection to allow light to reach the sensitive part of the device. Many diodes designed for use as a photodiode also use a PIN junction rather than the typical p-n junction.

Principle of Operation

A photodiode is a p-n junction or PIN structure. When a photon of sufficient energy strikes the diode, it excites an thereby creating a mobile electron and a positively charged . 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. Fundamentally a photodiode is a current generator.

R RP S i ID IPH C S RL Light intensity Ideal IR E4>E3>E2>E1 diode

Dark current Load line ID  Dark current E1=0 v IPH  Photocurrent E2 CS  Diode capacitance RP  Parallel resistance E 3 IR  Noise current R  Series resistance E S 4 Photovoltaic RS  Load resistance Photoconductive mode (solar cell) mode

Figure 4. Photodiode operation

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. The diode becomes forward biased and ‘dark current’ begins to flow across the junction in the direction opposite to the photocurrent. This mode is responsible for the photovoltaic effect (Fig. 4.8), which is the basis for solar cells. In fact, a solar cell is just an array of large photodiodes.

Photoconductive Mode In this mode the diode is often (but not always) reverse biased. This increases the width of the depletion layer, which decreases the junction capacitance resulting in faster response time. 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. Although this mode is faster, the photovoltaic mode tends to exhibit less electronic noise. (The leakage current of a good PIN diode is so low – less than 1nA – that the Johnson-Nyquist noise of the load resistance in a typical circuit often dominates).

Light-Emitting Diode

The first known report of a light-emitting solid-state diode was made in 1907 by the British experimenter H.J. Round. However, no practical use was made of the discovery for several decades. Independently, Oleg Vladimirovich Losev published ‘Luminous carborundum [silicon carbide] and detection with crystals’ in the Russian journal Telegrafiya i Telefoniya bez Provodov (Wireless Telegraphy and Telephony). A light-emitting diode, usually called an LED, is a semiconductor diode that emits incoherent narrow-spectrum light when the p-n junction is electrically biased in the forward direction, as in the common LED circuit. This effect is a form of electroluminescence. Blue, green, and red LEDs; these can be combined to produce any color, including white. Infrared and ultraviolet (UV) LEDs are also available. A LED is usually a small area light source, often with extra optics added to the chip that shapes its radiation pattern. LEDs are often used as small indicator lights on electronic devices and increasingly in higher power applications such as flashlights and area lighting. The color of the emitted light depends on the composition and condition of the semiconducting material used, and can be infrared, visible, or ultraviolet. LEDs can also be used as a regular household light source. Besides lighting, interesting applications include water sterilization and disinfection of devices.

LED Panels, Flat Panel LED TV and Others A combination of red, green and blue LEDs can produce the impression of white light, though white LEDs today rarely use this principle. Most ‘white’ LEDs in production today are modified blue LEDs: GaN-based, InGaN-active-layer LEDs emit blue light of wavelengths between 450 nm and 470 nm. If the emitting layer material of the LED is an organic compound, it is known as an Organic Light Emitting Diode (OLED).

Physical Function

Like a normal diode, the LED consists of a chip of semiconducting material doped with impurities to create a p-n junction. As in other diodes, the 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 therefore 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

Figure 5. A light-emitting diode construction 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 made possible the production of devices with ever-shorter wavelengths, producing 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. Less common p-type substrates occur as well. Many commercial LEDs, especially GaN/InGaN, also use sapphire substrate. Substrates that are transparent to the emitted wavelength, and backed by a reflective layer, increase the LED efficiency. The refractive index of the package material should match the index of the semiconductor, otherwise the produced light gets partially reflected back into the semiconductor, where it may be absorbed and turned into additional heat, thus lowering the efficiency. This type of reflection also occurs at the surface of the package if the LED is coupled to a medium with a different refractive index such as a glass fiber or air. The refractive index of most LED semiconductors is rather high, so almost in all cases the LED is coupled into a much lower-index medium. The large index difference makes the reflection quite substantial (per the Fresnel coefficients), and this is usually one of the dominant causes of LED inefficiency. Often more than half of the emitted light is reflected back at the LED-package and package-air interfaces. The reflection is most commonly reduced by using a dome-shaped (half-sphere) package with the diode in the center so that the outgoing light rays strike the surface perpendicularly, at which angle the reflection is minimized. An anti-reflection coating may be added as well. The package may be cheap plastic, which may be colored, but this is only for cosmetic reasons or to improve the contrast ratio; the color of the packaging does not substantially affect the color of the light emitted. Other strategies for reducing the impact of the interface reflections include designing the LED to reabsorb and reemit the reflected light (called photon recycling) and manipulating the microscopic structure of the surface to reduce the reflectance, either by introducing random roughness or by creating programmed special surface patterns. Conventional LEDs are made from a variety of inorganic semiconductor materials, producing the following colors: Aluminium gallium arsenide (AlGaAs) – red and infrared Gallium phosphide (GaP) – red, yellow and green Indium (InGaN) – 450 nm - 470 nm – near ultraviolet, bluish-green and blue Silicon carbide (SiC) as substrate – blue Zinc selenide (ZnSe) – blue Diamond (C) – ultraviolet and others. With this wide variety of colors, arrays of multicolor LEDs can be designed to produce unconventional color patterns.

Types of LEDs

The main types of LEDs are miniature, high power devices and custom designs such as alphanumeric or multi-color.

Miniature LEDs These are mostly single die LEDs used as indicators, and come in various size packages: surface mount; 2 mm; 3 mm; 5 mm. Other sizes are also available, but less common. Common package shapes: round, dome top; round, flat top; rectangular, flat top (often seen in LED bargraph displays); triangular or square, flat top. The encapsulation may also be clear or semi opaque to improve the contrast and the viewing angle. There are 3 main categories of miniature single die LEDs: Low current – typically rated for 2 mA at around 2 V (approximately 4 mW consumption). Standard – 20 mA LEDs at around 2 V (approximately 40 mW) for red, orange, yellow & green, and 20 mA at 4-5 V (approximately 0.1 W) for blue, violet and white. Ultra high output – 20 mA at approximately 2 V or 4-5 V, designed for viewing in direct sunlight.

Multicolor LEDs Bicolor LEDs contain 2 dice of different colors connected back to back, and can produce any of 3 colors. Current flow in one direction produces one color, current in the other direction produces the other color, and bidirectional current produces both colors mixed together. Tricolor LEDs contain 2 dice of different colors with a 3 connection, available in common anode or common cathode configurations. The most common form of both the bicolor and tricolor LEDs is red/green, producing orange when both colors are powered. RGB LEDs contain red, green and blue emitters, generally using a 4 wire connection with one common (anode or cathode).

Alphanumeric LEDs LED displays are available in 7 segment and starburst format. 7 segment displays handle all numbers and a limited set of letters. Starburst displays can display all letters. 7 segment LED displays were in widespread use in the 1970s and 1980s, but the increasing use of LCD displays with their lower power consumption and greater display flexibility has reduced the popularity of numeric and alphanumeric LED displays.

High-power LEDs (HPLED) can be driven at the current values from hundreds of mA to more than an ampere, compared with tens of mA for other LEDs. Some can emit over a thousand lumens. Since the overheating is destructive, the HPLEDs must be mounted on a heat sink to allow for heat dissipation. If the heat from a HPLED is not removed, the device will fail in seconds. One HPLED can often replace an incandescent bulb in a torch, or be set in an array to form a powerful LED lamp. Some well-known HPLEDs in this category are the Osram Opto Semiconductors Golden Dragon, Cree X-lamp. Some HPLEDs manufactured by Cree Inc. now exceed 105 lm/W and are being sold in lamps intended to replace incandescent, halogen, and even fluorescent lights, as LEDs grow more cost competitive.

Figure 6. High-power light emitting diode

LEDs developed by Seoul Semiconductor can operate on a.c. power without a DC converter. For each half-cycle, a part of the LED emits light and a part is dark, and this is reversed during the next half-cycle. The efficacy of this type of HPLED is typically 40 lm/W. A large number of LED elements in series can operate directly from the line voltage.

REFERENCES

1. Fundamentals of Electric Circuits / Charles K. Alexander, Matthew N.O. Sadiku. – 5th ed., 2013. – 996 p. 2. Electronics: a complete course / Nigel P. Cook. – 2nd ed., 2004. – 1037 p. 3. Electronics: a system approach / Neil Storey. – 3rd ed., 2006. –645 p. 4. The art of electronics / Paul Horowitz, Winfield Hill. – 2nd ed., 1989. – 1105 p.

Internet resource: 1. https://en.wikipedia.org/wiki/Diode 2. https://en.wikipedia.org/wiki/Zener_diode 3. https://en.wikipedia.org/wiki/Varicap 4. https://en.wikipedia.org/wiki/Schottky_diode 5. https://www.microsemi.com/sites/default/files/micnotes/401.pdf 6. https://en.wikipedia.org/wiki/Photodiode 7. http://hyperphysics.phy-astr.gsu.edu/hbase/Electronic/photdet.html 8. http://ecee.colorado.edu/~bart/book/book/chapter4/ch4_7.htm 9. https://en.wikipedia.org/wiki/Tunnel_diode 10. http://www.radio- electronics.com/info/data/semicond/tunneldiode/tunneldiode.php 11. https://en.wikipedia.org/wiki/Light-emitting_diode