Digital Electronics Innovation-01

8/8/2019 Digital Electronics Innovation-01

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SILICON CONTROL RECTIFIER (SCR)Symbol:

SCR

Modes of operation:In the normal "off" state, the device restricts current to the leakage current . When the gate-to-cathode voltage exceeds a certain threshold , the device turns "on" and conducts current.The device will remain in the "on" state even after gate current is removed so long ascurrent through the device remains above the holding current . Once current falls below theholding current for an appropriate period of time, the device will switch "off". If the gateis pulsed and the current through the device is below the holding current, the device willremain in the "off" state.

If the applied voltage increases rapidly enough, capacitive coupling may induce enough

charge into the gate to trigger the device into the "on" state; this is referred to as "dv/dttriggering." This is usually prevented by limiting the rate of voltage rise across the device,

perhaps by using a snubber . "dv/dt triggering" may not switch the SCR into fullconduction rapidly and the partially-triggered SCR may dissipate more power than isusual, possibly harming the device.

SCRs can also be triggered by increasing the forward voltage beyond their rated breakdown voltage (also called as break over voltage), but again, this does not rapidly
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switch the entire device into conduction and so may be harmful so this mode of operationis also usually avoided. Also, the actual breakdown voltage may be substantially higher than the rated breakdown voltage, so the exact trigger point will vary from device todevice.

SCR Characteristics

As already mentioned, the SCR is a four-layer device with three terminals, namely, theanode, the cathode and the gate. When the anode is made positive with respect to thecathode, junctions J 1 and J 3 are forward biased and junction J 2 is reverse-biased and onlythe leakage current will flow through the device. The SCR is then said to be in the forward

blocking state or in the forward mode or off state. But when the cathode is made positivewith respect to the anode, junctions J 1 and J 3 are reverse-biased, a small reverse leakagecurrent will flow through the SCR and the SGR is said to be in the reverse blocking stateor in reverse mode.

When the anode is positive with respect to cathode i.e. when the SCR is in forward mode,the SCR does not conduct unless the forward voltage exceeds certain value, called theforward breakover voltage, V FB0 . In non-conducting state, the current through the SCR isthe leakage current which is very small and is negligible. If a positive gate current issupplied, the SCR can become conducting at a voltage much lesser than forward break-over voltage. The larger the gate current, lower the break-over voltage. With sufficientlylarge gate current, the SCR behaves identical to PN rectifier. Once the SCR is switchedon, the forward voltage drop across it is suddenly reduced to very small value, say about 1volt. In the conducting or on-state, the current through the SCR is limited by the externalimpedance.

When the anode is negative with respect to cathode, that is when the SCR is in reversemode or in blocking state no current flows through the SCR except very small leakagecurrent of the order of few micro-amperes. But if the reverse voltage is increased beyond acertain value, called the reverse break-over voltage, V RB0 avalanche break down takes

place. Forward break-over voltage V FB0 is usually higher than reverse breakover voltage,V RBO.
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From the foregoing discussion, it can be seen that the SCR has two stable and reversibleoperating states. The change over from off-state to on-state, called turn-on, can beachieved by increasing the forward voltage beyond V FB0 . A more convenient and usefulmethod of turn-on the device employs the gate drive. If the forward voltage is less than theforward break-over voltage, V FB0 , it can be turned-on by applying a positive voltage

between the gate and the cathode. This method is called the gate control. Another veryimportant feature of the gate is that once the SCR is triggered to on-state the gate loses itscontrol.

Application of SCRs:

1. Adjustable motor speed controllers.2. Adjustable light dimmers.3. Switching power supplies and battery chargers4. Inverters.

Advantages:1. Switches large levels of current using only a small control current.2. Can switch high voltages.

Disadvantages:1. Cannot be easily turned off.2. For this reason, not suitable for high-power DC circuits.3. In an AC circuit, needs to be turned on each cycle.


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Metaloxidesemiconductor field-effect transistor(MOSFET)

Symbol:

MOSFET construction:

Gate material

The primary criterion for the gate material is that it is a good conductor . Highly-doped polycrystalline silicon is an acceptable but certainly not ideal conductor, and also suffersfrom some more technical deficiencies in its role as the standard gate material.

Nevertheless, there are several reasons favoring use of polysilicon:

1. The threshold voltage (and consequently the drain to source on-current) ismodified by the work function difference between the gate material and channelmaterial. Because polysilicon is a semiconductor, its work function can be

modulated by adjusting the type and level of doping. Furthermore, because polysilicon has the same bandgap as the underlying silicon channel, it is quitestraightforward to tune the work function to achieve low threshold voltages for

both NMOS and PMOS devices. By contrast, the work functions of metals are noteasily modulated, so tuning the work function to obtain low threshold voltages

becomes a significant challenge. Additionally, obtaining low-threshold devices on both PMOS and NMOS devices would likely require the use of different metals for each device type, introducing additional complexity to the fabrication process.

2. The Silicon-SiO 2 interface has been well studied and is known to have relativelyfew defects. By contrast many metalinsulator interfaces contain significant levelsof defects which can lead to Fermi-level pinning, charging, or other phenomena

that ultimately degrade device performance.3. In the MOSFET IC fabrication process, it is preferable to deposit the gate material prior to certain high-temperature steps in order to make better-performingtransistors. Such high temperature steps would melt some metals, limiting thetypes of metal that can be used in a metal-gate-based process.

While polysilicon gates have been the de facto standard for the last twenty years, they dohave some disadvantages which have led to their likely future replacement by metal gates.These disadvantages include:
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1. Polysilicon is not a great conductor (approximately 1000 times more resistive thanmetals) which reduces the signal propagation speed through the material. Theresistivity can be lowered by increasing the level of doping, but even highly doped

polysilicon is not as conductive as most metals. In order to improve conductivityfurther, sometimes a high-temperature metal such as tungsten , titanium , cobalt , andmore recently nickel is alloyed with the top layers of the polysilicon. Such a

blended material is called silicide . The silicide-polysilicon combination has better electrical properties than polysilicon alone and still does not melt in subsequent

processing. Also the threshold voltage is not significantly higher than with polysilicon alone, because the silicide material is not near the channel. The processin which silicide is formed on both the gate electrode and the source and drainregions is sometimes called salicide , self-aligned silicide.

2. When the transistors are extremely scaled down, it is necessary to make the gatedielectric layer very thin, around 1 nm in state-of-the-art technologies. A

phenomenon observed here is the so-called poly depletion , where a depletion layer is formed in the gate polysilicon layer next to the gate dielectric when thetransistor is in the inversion. To avoid this problem, a metal gate is desired. Avariety of metal gates such as tantalum , tungsten , tantalum nitride , and titaniumnitride are used, usually in conjunction with high-k dielectrics. An alternative is touse fully-silicided polysilicon gates, a process known as FUSI .

OPERATION:

The operation of the MOSFET, or IGFET, is basically the same as the operation of theJFET . The current flow between the source and drain can be controlled by using either of two methods or by using a combination of the two methods. In one method the drainvoltage controls the current when the gate potential is at zero volts. A voltage is applied tothe gate in the second method. An electric field is formed by the gate voltage that affectsthe current flow in the channel by either depleting or enhancing the number of currentcarriers available. As previously stated, a reverse bias applied to the gate depletes thecarriers, and a forward bias enhances the carriers. The polarity of the voltages required toforward or reverse bias a MOSFET depends upon whether it is of the p- channel type or the n- channel type.

OUTPUT CHARACTERISTICS
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APPLICATIONS:

IGBTs see application in switching internal combustion engine ignition coils, where fastswitching and voltage blocking capabilities are important.

The most commonly used FET is the MOSFET . The CMOS (complementary-symmetrymetal oxide semiconductor) process technology is the basis for modern digital integrated

circuits . This process technology uses an arrangement where the (usually "enhancement-mode") p-channel MOSFET and n-channel MOSFET are connected in series such thatwhen one is on, the other is off.

The fragile insulating layer of the MOSFET between the gate and channel makes itvulnerable to electrostatic damage during handling. This is not usually a problem after thedevice has been installed in a properly designed circuit.

Advantages of a MOSFET1. Switching time is about 10 times faster than a bipolar transistor 2.Very much smaller switching current 3.

Less affected by temperature

Disadvantages:1. Higher resistance than a bipolar transistor 2. Can be destroyed by high voltages, especially static electricity
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Field-effect transistor

The field-effect transistor (FET) relies on an electric field to control the shapeand hence the conductivity of a channel of one type of charge carrier in asemiconductor material. FETs are sometimes called unipolar transistors tocontrast their single-carrier-type operation with the dual-carrier-type operation of

bipolar (junction) transistors (BJT). The concept of the FET predates the BJT,though it was not physically implemented until after BJTs due to the limitations of semiconductor materials and the relative ease of manufacturing BJTs compared toFETs at the time.

Symbol:

The channel of a FET (explained below) is doped to produce either an N-typesemiconductor or a P-type semiconductor . The drain and source may be doped of oppositetype to the channel, in the case of depletion mode FETs, or doped of similar type to thechannel as in enhancement mode FETs. Field-effect transistors are also distinguished bythe method of insulation between channel and gate. Types of FETs are:

The DEPFET is a FET formed in a fully-depleted substrate and acts as a sensor,amplifier and memory node at the same time. It can be used as an image (photon)sensor.

The DGMOSFET is a MOSFET with dual gates.

The DNAFET is a specialized FET that acts as a biosensor , by using a gate madeof single-strand DNA molecules to detect matching DNA strands.

The FREDFET (Fast Reverse or Fast Recovery Epitaxial Diode FET) is aspecialized FET designed to provide a very fast recovery (turn-off) of the bodydiode.
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The HEMT (High Electron Mobility Transistor), also called an HFET(heterostructure FET), can be made using bandgap engineering in a ternarysemiconductor such as AlGaAs . The fully depleted wide-band-gap material formsthe isolation between gate and body.

The IGBT (Insulated-Gate Bipolar Transistor) is a device for power control. It hasa structure akin to a MOSFET coupled with a bipolar-like main conductionchannel. These are commonly used for the 200-3000 V drain-to-source voltagerange of operation. Power MOSFETs are still the device of choice for drain-to-source voltages of 1 to 200 V.

The ISFET is an Ion-Sensitive Field Effect Transistor used to measure ionconcentrations in a solution; when the ion concentration (such as pH) changes, thecurrent through the transistor will change accordingly.

The JFET (Junction Field-Effect Transistor) uses a reverse biased p-n junction toseparate the gate from the body.

The MESFET (MetalSemiconductor Field-Effect Transistor) substitutes the p-n junction of the JFET with a Schottky barrier ; used in GaAs and other III-Vsemiconductor materials.

The MODFET (Modulation-Doped Field Effect Transistor) uses a quantum well structure formed by graded doping of the active region.

The MOSFET (MetalOxideSemiconductor Field-Effect Transistor) utilizes aninsulator (typically SiO 2) between the gate and the body.

The NOMFET is a Nanoparticle Organic Memory Field-Effect Transistor.

The OFET is an Organic Field-Effect Transistor using an organic semiconductor in its channel.

Fet operation:

IV characteristics and output plot of a JFET n-channel transistor.
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The FET controls the flow of electrons (or electron holes ) from the source to drain byaffecting the size and shape of a "conductive channel" created and influenced by voltage(or lack of voltage) applied across the gate and source terminals (For ease of discussion,this assumes body and source are connected). This conductive channel is the "stream"through which electrons flow from source to drain.

In an n-channel depletion-mode device, a negative gate-to-source voltage causes adepletion region to expand in width and encroach on the channel from the sides,narrowing the channel. If the depletion region expands to completely close the channel,the resistance of the channel from source to drain becomes large, and the FET iseffectively turned off like a switch. Likewise a positive gate-to-source voltage increasesthe channel size and allows electrons to flow easily.

Conversely, in an n-channel enhancement-mode device, a positive gate-to-source voltageis necessary to create a conductive channel, since one does not exist naturally within thetransistor. The positive voltage attracts free-floating electrons within the body towards thegate, forming a conductive channel. But first, enough electrons must be attracted near thegate to counter the dopant ions added to the body of the FET; this forms a region free of mobile carriers called a depletion region , and the phenomenon is referred to as thethreshold voltage of the FET. Further gate-to-source voltage increase will attract evenmore electrons towards the gate which are able to create a conductive channel from sourceto drain; this process is called inversion .

For either enhancement- or depletion-mode devices, at drain-to-source voltages much lessthan gate-to-source voltages, changing the gate voltage will alter the channel resistance,and drain current will be proportional to drain voltage (referenced to source voltage). Inthis mode the FET operates like a variable resistor and the FET is said to be operating in alinear mode or ohmic mode .[2] [3]

If drain-to-source voltage is increased, this creates a significant asymmetrical change inthe shape of the channel due to a gradient of voltage potential from source to drain. Theshape of the inversion region becomes "pinched-off" near the drain end of the channel. If drain-to-source voltage is increased further, the pinch-off point of the channel begins tomove away from the drain towards the source. The FET is said to be in saturation mode ;[4]

some authors refer to it as active mode , for a better analogy with bipolar transistor operating regions .[5][ 6] The saturation mode, or the region between ohmic and saturation, isused when amplification is needed. The in-between region is sometimes considered to be

part of the ohmic or linear region, even where drain current is not approximately linear with drain voltage.

Even though the conductive channel formed by gate-to-source voltage no longer connectssource to drain during saturation mode, carriers are not blocked from flowing. Consideringagain an n-channel device, a depletion region exists in the p-type body, surrounding theconductive channel and drain and source regions. The electrons which comprise thechannel are free to move out of the channel through the depletion region if attracted to thedrain by drain-to-source voltage. The depletion region is free of carriers and has aresistance similar to silicon . Any increase of the drain-to-source voltage will increase thedistance from drain to the pinch-off point, increasing resistance due to the depletion region

proportionally to the applied drain-to-source voltage. This proportional change causes thedrain-to-source current to remain relatively fixed independent of changes to the drain-to-source voltage and quite unlike the linear mode operation.
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Thus in saturation mode, the FET behaves as a constant-current source rather than as aresistor and can be used most effectively as a voltage amplifier. In this case, the gate-to-source voltage determines the level of constant current through the channel.

Applications:

1. IGBTs see application in switching internal combustion engine ignition coils,

where fast switching and voltage blocking capabilities are important.2. The fragile insulating layer of the MOSFET between the gate and channel makes itvulnerable to electrostatic damage during handling. This is not usually a problemafter the device has been installed in a properly designed circuit.
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Bipolar junction transistor Construction:A bipolar (junction) transistor (BJT) is a three-terminal electronic deviceconstructed of doped semiconductor material and may be used in amplifying or

switching applications. Bipolar transistors are so named because their operationinvolves both electrons and holes . Charge flow in a BJT is due to bidirectionaldiffusion of charge carriers across a junction between two regions of differentcharge concentrations. This mode of operation is contrasted with unipolar transistors, such as field-effect transistors , in which only one carrier type isinvolved in charge flow due to drift . By design, most of the BJT collector currentis due to the flow of charges injected from a high-concentration emitter into the

base where they are minority carriers that diffuse toward the collector, and so BJTsare classified as minority-carrier devices.

The symbol of an NPN Bipolar Junction Transistor.

NPN is one of the two types of bipolar transistors, in which the letters "N" (negative) and"P" (positive) refer to the majority charge carriers inside the different regions of thetransistor. Most bipolar transistors used today are NPN, because electron mobility ishigher than hole mobility in semiconductors, allowing greater currents and faster operation.

NPN transistors consist of a layer of P- doped semiconductor (the "base") between two N-doped layers. A small current entering the base in common-emitter mode is amplified inthe collector output. In other terms, an NPN transistor is "on" when its base is pulled highrelative to the emitter.

The arrow in the NPN transistor symbol is on the emitter leg and points in the direction of the conventional current flow when the device is in forward active mode.

One mnemonic device for identifying the symbol for the NPN transistor is "not pointingin, or 'not pointing, no' " [5]

PNP

The other type of BJT is the PNP with the letters "P" and "N" referring to the majoritycharge carriers inside the different regions of the transistor.
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The symbol of a PNP Bipolar Junction Transistor.

PNP transistors consist of a layer of N- doped semiconductor between two layers of P-doped material. A small current leaving the base in common-emitter mode is amplified inthe collector output. In other terms, a PNP transistor is "on" when its base is pulled lowrelative to the emitter.

The arrow in the PNP transistor symbol is on the emitter leg and points in the direction of the conventional current flow when the device is in forward active mode.

One mnemonic device for identifying the symbol for the PNP transistor is "pointing in proudly, or 'pointing.

Regions of operationBipolar transistors have five distinct regions of operation, defined by BJT junction biases.In order to visualize the modes of operation draw a NPN transistor with its collector ontop, base in the middle and emitter on the bottom. Now, there are two voltage differences:

between Collector and base, and between base and emitter. Note two points: Vcb = - Vbc,and 'reverse biased base-collector junction' means Vbc < 0 or Vcb>0. In simple words, itmeans the collector has a higher voltage than the base(if probed). The mechanical analogcan be a pipe and a valve. The valve is base, and two sides of the pipe are collector andemitter. Now the amount of Water ( current) going through depends on how much thevalve is open ( base to emitter) voltage, and how much water you have on top of the pipe (collector to base voltage).

If you write the biases in term of applied voltages (Vcb, Vbe) instead of junction biasingthe modes of operation can be described as:

Forward Active: Base higher than Emitter, Collector higher than Base ( in thismode the collector current is proportional to base current by F ).

Saturation: Base higher than emitter, but collector is not higher than base. Cut-Off: Base lower than emitter, but collector is higher than base. It means the

transistor is not letting conventional current to go through collector to emitter. Reverse-Action: Base lower than emitter, collector lower than base: reverse

conventional current goes through transistor.

In terms of junction biasing: ('reverse biased base-collector junction' means Vbc < 0 or Vcb>0)

Forward-active (or simply, active ): The baseemitter junction is forward biasedand the basecollector junction is reverse biased. Most bipolar transistors aredesigned to afford the greatest common-emitter current gain, F , in forward-activemode. If this is the case, the collectoremitter current is approximately

proportional to the base current, but many times larger, for small base currentvariations.

Reverse-active (or inverse-active or inverted ): By reversing the biasingconditions of the forward-active region, a bipolar transistor goes into reverse-active mode. In this mode, the emitter and collector regions switch roles. Becausemost BJTs are designed to maximize current gain in forward-active mode, the F in
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inverted mode is several (23 for the ordinary germanium transistor) times smaller.This transistor mode is seldom used, for

usually being considered only for failsafe conditions and some types of bipolar logic. The reverse bias breakdown voltage to the base may be an order of magnitude lower in this region.

Saturation : With both junctions forward-biased, a BJT is in saturation mode andfacilitates high current conduction from the emitter to the collector. This modecorresponds to a logical "on", or a closed switch.

Cutoff : In cutoff, biasing conditions opposite of saturation (both junctions reverse biased) are present. There is very little current, which corresponds to a logical"off", or an open switch.

Avalanche breakdown region

Although these regions are well defined for sufficiently large applied voltage, they overlapsomewhat for small (less than a few hundred millivolts) biases. For example, in the typicalgrounded-emitter configuration of an NPN BJT used as a pulldown switch in digital logic,the "off" state never involves a reverse-biased junction because the base voltage never goes below ground; nevertheless the forward bias is close enough to zero that essentiallyno current flows, so this end of the forward active region can be regarded as the cutoff region.

Applications:

1. Surviving the stall condition requires the transistor current gain to hold upat high current densities to ensure the transistor does not drop out of saturation.

2. The bipolar transistor's higher transconductance at these low drive voltagesin combination with its high forward current gain helps achieve therequired motor current, reduce transistor drive losses, and minimize V CE(sat)with a cost-effectively sized chip.
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Advantages & Disadvantages

Linear power suppliesOne advantage of linear power supplies may be familiarity, because they have beenavailable for many years. They are known to be relatively noise-free and reasonablyreliable. They are generally easy to design and fairly inexpensive to manufacture.

Because of the large transformers required, linear power supplies are generally heavy,which may be either an advantage or a disadvantage, depending on the need to balanceweight distribution in a given application. As a general rule of thumb, a 16V-output linear

power supply weighs about one pound per ampere. A possible disadvantage of linear power supplies relates to the power transistor used to regulate the load. Because the power transistor operates in its linear region, and all the output current must pass through it, itrequires large heat sinks to dissipate energy loss. (Recall that the power transistor is inseries with the load and acts as a variable resistor.) Except in rare instances where heat iswanted to warm interior space, the inefficiency of linear power supplies 50% has to beconsidered a disadvantage.

Switching power suppliesAlthough switching power supplies have been available for a number of years, higher

production costs, compared to linear power supplies, have limited their use in someapplications. Early switching power supplies used discrete components to control pulsewidth, and transistors instead of MOSFETs as main switch components. As a result, thedisadvantages of switching power supplies once included uneven reliability and radiatedEMI (electrical noise). Although they were known to be noisy, unreliable and difficult tomass produce, switching power supplies had the advantage of being lighter and smaller than their linear counterparts. In the last few years, big improvements in PWM andMOSFET design have been made. Today, when all design considerations have been taken

into account, switching power supplies are highly reliable and virtually noise-free.Production costs have come down because application-specific components are beingdesigned for use in switching power supplies.


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Junction field effect transistor(JFET)

Symbol:

Construction:The junction gate field-effect transistor (JFET or JUGFET ) is the simplest type of field effect transistor . It can be used as an electronically -controlled switch or as a voltage-controlled resistance . Electric charge flows through a semiconducting channel between"source" and "drain" terminals. By applying a bias voltage to a "gate" terminal, thechannel is "pinched", so that the electric current is impeded or switched off completely.

Structure:

The JFET is a long channel of semiconductor material, doped to contain an abundance of positive charge carriers ( p-type ), or of negative carriers ( n-type ). Contacts at each endform the source(S) and drain(D). The gate(G) (control) terminal has doping opposite tothat of the channel, which it surrounds, so that there is a P-N junction at the interface.Terminals to connect with the outside are usually made ohmic .

Function

JFET operation is like that of a garden hose . The flow of water through a hose can becontrolled by squeezing it to reduce the cross section ; the flow of electric charge through aJFET is controlled by constricting the current-carrying channel. The current depends also

on the electric field between source and drain (analogous to the difference in pressure oneither end of the hose).

Schematic symbols

The JFET gate is sometimes drawn in the middle of the channel (instead of at the drain or source electrode as in these examples). This symmetry suggests that "drain" and "source"are interchangeable, so the symbol should be used only for those JFETs where they areindeed interchangeable (which is not true of all JFETs).
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In every case the arrow head shows the polarity of the P-N junction formed between thechannel and gate. As with an ordinary diode , the arrow points from P to N, the direction of conventional current when forward-biased. An English mnemonic is that the arrow of an

N-channel device "points i n".

To pinch off the channel, it needs a certain reverse bias (V GS) of the junction. This "pinch-off voltage" varies considerably, even among devices of the same type. For example,VGS(off) for the Temic J201 device varies from -0.8V to -4V. [1] Typical values vary from-0.3V to -10V.

To switch off an n -channel device requires a negative gate-source voltage (V GS).Conversely, to switch off a p -channel device requires V GS positive.

In normal operation, the electric field developed by the gate must block conduction between the source and the drain.

Output characteristics

Applications of FET:

1. Low Noise Amplifier. Noise is an undesirable disturbance super-imposed on a usefulsignal. Noise interferes with the information contained in the signal; the greater the noise,the less the information. For instance, the noise in radio-receivers develops crackling andhissing which sometimes completely masks the voice or music. Similarly, the noise in TVreceivers produces small white or black spots on the picture; a severenoise may wipe out the picture. Noise is independent of the signal strength becauseit exists even when the signal is off.

Advantages of JFET

1. It shows a high degree of isolation between input and output.2. The FET is less noisy than a bipolar transistor.
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Disadvantages:

FET has relatively low gain-bandwidth product compared to conventional transistors.

There are two types of FET namely junction field effect transistor (JFET) and insulated-gate field effect transistor (IGFET).

We will look at one example, called an N-channel Junction-FET (J-FET).

UNIJUNCTION TRANSISTOR (UJT )

UJT:

A Unijunction transistor is a three terminal semiconductor switching device.thisdevice has a unique characteristics that when it is triggered , the emitter currentincreases regeneratively until is limited by emitter power supply the unijunctiontransistor can be employed in a variety of applications switching pulse generator sawtooth generator etc.

ConstructionIt consists of an N type silicon bar with an electrical connection on each end the leadsto these connection are called base leads. Base 1 B1 Base 2 B2 the bar between thetwo bases nearer to B2 than B1. A pn junction is formed between a p type emitter andBar.the lead to the junction is called emitter lead E.

Operation


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The device has normally B2 positive w.r.t B1If voltage VBB is applied between B2 and B1 with emitter open. Voltage gradient isestablished along the n type bar since emitter is located nearer to B2 more than half of VBB appears between the emitter and B1. the voltage V1 between emitter and B1establishes a reverse bias on the pn junction and the emitter current is cut off. A smallleakage current flows from B2 to emitter due to minority carriers

If a positive voltage is applied at the emitter the pn junction will remain reverse biased so long as the input voltage is less than V1 if the input voltage to the emitter exceeds V1 the pn junction becomes forward biased.under these conditions holes areinjected from the p type material into the n type bar these holes are repelled by

positive B2 terminal and they are attracted towards B1 terminal of the bar. Thisaccumulation of holes in the emitter to B1 region results in the degrees of resistancein this section of the bar the internal voltage drop from emitter to b1 is decresed henceemitter curret Ie increases as more holes are injected a condition of saturation willeventually be reached at this point a emitter current limited by emitter power supplyonly . the devices is in on state.

If a negative pulse is applied to the emitter , the pn junction is reverse biased and theemitter current is cut off. The device is said to be off state.

Characteristics of UJT


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The curve between Emitter voltage Ve and emitter current Ie of a UJT at a givenvoltage Vbb between the bases this is known as emitter characterstic of UJT

Initially in the cut off region as Ve increases from zero ,slight leakage current flowsfrom terminal B2 to the emitter the current is due to the minority carriers in the reverse

biased diode .

Above a certain value of Ve forward Ie begins to flow , increasing until the peak voltage Vp and current Ip are rreached at point P.

After the peak point P an attempt to increase Ve is followed by a sudden increases inemitter current Ie with decrease in Ve is a neagative resistance portion of the curveThe negative portion of the curve lasts until the valley point V is reached with valley

point voltage Vv.and valley point current Iv after the valley point the device is drivento saturation the difference Vp-Vv is a measure of a switching efficiency of UJT fall of Vbb decreases

Advantages of UJT1. It is a Low cost device2. It has excellent characteristics3. It is a low-power absorbing device under normal operating conditions

Applications: The simplest application of a UJT is as a relaxation oscillator, which is

defined as one in which a capacitor is charged gradually and then dischargedrapidly.


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DIODE AC SWITCH (DIAC )A Diac is two terminal , three layer bi directional device which can be switched

from its off state for either polarity of applied voltage.

Construction:

The diac can be constructed in either npn or pnp form.The two leads are connectedto p-regions of silicon separated by an n region. the structure of diac is similar to that of atransistor differences are

There is no terminal attached to the base layer

The three regions are nearly identical in size. the doping concentrations areidentical to give the device symmetrical properties.

Operation

When a positive or negative voltage is applied across the terminals of Diac only asmall leakage current Ibo will flow through the device as the applied voltage isincreased , the leakage current will continue to flow until the voltage reaches breakover voltage Vbo at this point avalanche breakdown of the reverse biased junction occurs andthe device exhibits negative resistance i.e current through the device increases with thedecreasing values of applied voltage the voltage across the device then drops to breakback voltage Vw

V- I CHARECTERISTICS OF A DIAC


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For applied positive voltage less than + Vbo and Negative voltage less than-Vbo , a small leakage current flows thrugh the device. Under such conditions the diac

blocks flow of current and behaves as an open circuit. the voltage +Vbo and -Vbo are the breakdown voltages and usually have range of 30 to 50 volts.

When the positive or negative applied voltage is equal to or greater than tha breakdown voltage Diac begins to conduct and voltage drop across it becomes a few voltsconduction then continues until the device current drops below its holding current

breakover voltage and holding current values are identical for the forward and reverseregions of operation.

Diacs are used for triggering of triacs in adjustable phase control of a c mains power. Applications are light dimming heat control universal motor speed control

Diac Applications :

The diacs , because of their symmetrical bidirectional switching characteristics, are widely

used as triggering devices in triac phase control circuits employed for lamp dimmer, heatcontrol, universal motor speed control etc.

Although a triac may be fired into the conducting state by a simple resistive triggeringcircuit, but triggering devices are typically placed in series with the gates of SCRs andtriacs as they give reliable and fast triggering. Diac is the most popular triggering devicefor the triac. This is illustrated in the following applications.
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THE TRIAC(TRIODE AC SWITCH)

CONSTRUCTION:Triacs are three terminal devices that are used to switch large a.c. currents with a smalltrigger signal. Triacs are commonly used in dimmer switches, motor speed control circuitsand equipment that automatically controls mains powered equipment including remotecontrol. The triac has many advantages over a relay, which could also be used to control

mains equipment; the triac is cheap, it has no moving parts making it reliable and itoperates very quickly.

OPERATION:

The three terminals on a triac are called Main Terminal 1(MT1), Main Terminal 2 (MT2) and Gate (G). To turn onthe triac there needs to be a small current I GT flowingthrough the gate, this current will only flow when thevoltage between G and MT1 is at least V GT . The signal thatturns on the triac is called the trigger signal. Once the triac is

turned on it will stay on even if there is no gate current untilthe current flowing between MT2 and MT1 fall below thehold current I H.

The triac is always turned fully on or fully off. When the triac is on there is virtually no pd between MT2 and MT1 so the power dissipated in the triac is low so it does not get hot or waste electrical power. When the triac is off no current flows between MT2 and MT1 sothe power dissipated in the triac is low so it does not get hot or waste electrical power.This means that triacs can be small and are very efficient.

Triacs can be used in d.c. circuits in which case

when the triac is triggered it will stay on until power is removed from the triac. It is easy tocalculate the value of gate resistor needed to turn ona triac using the gate characteristics and ohms law.The maximum value of resistor can be found fromthe voltage across the resistor (V S - V GT) divided bythe gate current I GT. So, R = (V S - V GT)/ I GT

In a.c. circuits the triac needs to be repeatedly triggered because the triac turns off whenthe a.c. current goes from positive to negative or negative to positive as the current

become momentarily zero. The triac is used in

mains circuits to control the amount of power byonly turning the triac on for part of the wave a bitlike in pulse width modulation. This can be done

by varying the value of the gate resistor so that thetriac does not turn on until the a.c signal reaches a

particular voltage. The problem with this firstdimmer is that there is a very high voltage acrossthe variable resistor and it will get hot as there is alot of power to dissipate (P=V 2/R).

MT2

MT1

G

VS

0v

R


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Characteristics of triac

Applications of Triac:

Next to SCR, the triac is the most widely used member of the thyristor family. In fact, inmany of control applications, it has replaced SCR by virtue of its bidirectionalconductivity. Motor speed regulation, temperature control, illumination control, liquidlevel control, phase control circuits, power switches etc. are some of its main applications.

However, the triac is less versatile than the SCR when turn-off is considered. Because thetriac can conduct in either direction, forced commutation by reverse-biasing cannot

be employed. So turn-off is either by current starvation, which is usually impracticable,or else by ac line commutation. There are two limitations enforced on the use of triac at

present state of commercially available devices (200 A and 1,000 PRV). The first is thefrequency handling capability produced by the limiting dv/dt at which the triac remains

blocking when no gate signal is applied. This dv/dt value is about 20 Vmicros -1 comparedwith a general figure of 200 Vmicro s -1 for the SCR, so that the limitation of frequency isat the power level of 50 Hz. The same dv/dt limitation means the load to be controlled is

preferably a resistive one. When high frequencies and high dv/dt are involved then the back-to-back SCRs cannot be replaced by the triac.


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Diodes

Construction and operation:

As with transistors, diodes are fabricated from semi-conducting material. So, the first

letter in their identification is A for germanium diode or B for silicon diode. They can beencased in glass, metal or a plastic housing. They have two leads: cathode (k) and ananode (A). The most important property of all diodes is their resistance is very low in onedirection and very large in the opposite direction.When a diode is measured with a multimeter and it reads a low value of ohms, this is notreally the resistance of the diode. It represents the voltage drop across the junction of thediode. This means a multimeter can only be used to detect if the junction is not damaged.If the reading is low in one direction and very high in the other direction, the diode isoperational.

When a diode is placed in a circuit and the voltage on the anode is higher than the cathode,

it acts like a low value resistor and current will flow.If it is connected in the opposite direction it acts like a large value resistor and currentdoes not flow.In the first case the diode is said to be "forward biased" and in the second case it is"reverse biased."

Figure:shows several different diodes:

Fig: Several different types of diodes

The diodes above are all single diodes, however 4 diodes are available in a single package.This is called a BRIDGE or BRIDGE RECTIFIER. Examples of a bridge are shown in thediagram below:


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You must be able to identify each of the 4 leads on a bridge so that it can be inserted into acircuit around the correct way. The surface-mount device above is identified by a cut @45 along one side. The leaded bridge has one leg longer than the others and the top ismarked with AC marks and "+." The high-current bridge has a corner cut off and the other surface-mount device has a cut or notch at one end.

These devices are added to a circuit as shown in the next diagram:

The 4 diodes face the same direction and this means a single diode can be shown on thecircuit diagram:

Symbols in fig show a number of diodes. There are a number of specially-designeddiodes: for high current, high-speed, low voltage-drop, light-detection, and varyingcapacitance as the voltage is altered. Most diodes are made from silicon as it willwithstand high temperature, however germanium is used if a low voltage-drop is required.There is also a light emitting diode called a LED, but this is a completely different type of diode.


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Fig: Diode symbols: a - standard diode, b - LED,c, d - Zener, e - photo, f,g - tunnel, h - Schottky, i - breakdown,

j - capacitative

LEDs (Light Emitting Diodes) are constructed from a crystalline substance that emits lightwhen a current flows through it. Depending on the crystalline material: red, yellow, green,

blue or orange light is produced. The photo below shows some of the colours hat can be produced by LEDs:

LEDs have a cathode and anode lead and must be connected to DC around the correctway. The cathode lead is identified on the body by a flat-spot on the side of the LED. Thecathode lead is the shorter lead.

One of the most important things to remember about a LED is the characteristic voltagethat appears across it when connected to a voltage. This does not change with brightnessand cannot be altered.For a red LED, this voltage is 1.7v and if you supply it with more than this voltage, it will

be damaged.The easy solution is to place a resistor on one lead as shown in the diagram below:


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The LED will allow the exact voltage to appear across it and the brightness will depend onthe value of the resistor.

Zener diodes are designed to stabilize a voltage. Diodes marked as ZPD5.6V or ZPY15Vhave operating voltages of 5.6V and 15V.

Photo diodes (5.2e) are constructed in a way that they allow light to fall on the P-Nconnection. When there is no light, a photo diode acts as a regular diode. It has highresistance in one direction, and low resistance in opposite direction. When there is light,

both resistances are low. Photo diodes and LEDs are the main items in an optocoupler (to be discussed in more detail in chapter 9).

Tunnel diodes (5.2f and 5.2g) are commonly used in oscillators for very high frequencies.

Schottky diodes (5.2h) are used in high frequency circuits and for its low voltage drop inthe forward direction.

Breakdown diodes (5.2i) are actually Zener diodes. They are used in various devices for protection and voltage regulation. It passes current only when voltage rises above a pre-defined value.

A Varicap diode (5.2j) is used instead of a variable capacitor in high frequency circuits.When the voltage across it is changed, the capacitance between cathode and anode ischanged. This diode is commonly used in radio receivers, transceivers and oscillators.

The cathode of a low power diode is marked with a ring painted on the case, but it isworth noting that some manufacturers label the anode this way, so it is best to test it with amultimeter.

Power diodes are marked with a symbol engraved on the housing. If a diode is housed in ametal package, the case is generally the cathode and anode is the lead coming from thehousing.

Diode characteristics:

The most important characteristics when using power diodes is the maximum current inthe forward direction (IFmax), and maximum voltage in the reverse direction (URmax).

The important characteristics for a Zener diode are Zener voltage (UZ), Zener current (IZ)and maximum dissipation power (PD).

When working with capacitive diodes it is important to know their maximum andminimum capacitance, as well as values of DC voltage during which these capacitancesoccur.

With LEDs it is important to know the maximum value of current it is capable of passing.The natural characteristic voltage across a LED depends on the colour and starts at 1.7Vfor red to more than 2.4v for green and blue.


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Current starts at 1mA for a very small glow and goes to about 40mA. High brightnessLEDs and "power LEDs" require up to 1 amp and more. You must know the exact currentrequired by the LED you are using as the wrong dropper resistor will allow too muchcurrent to flow and the LED will be damaged instantly .The value of this resistors will be covered in another chapter.

Beside universal transistors TUN and TUP (mentioned in Chapter 4.4), there are universaldiodes as well. They are marked with DUS (for universal silicon diode) and DUG (for germanium) on circuit diagrams.

DUS = Diode Universal Silicon DUG = Diode Universal Germanium

ADVANTAGES OF DIODES:

1. Rectifiers would convert AC to DC and protect your devices from being swapping +veand -ve power supply.2. LEDs are indicators, comes in various sizes and easy to use than conventional bulbs.3. Varicaps are used in dynamic TV, radio tuning circuits, easy to use and compact.

4. Zeners are used as voltage regulators, would protect the circuits from over-currents.5. Photo-diodes would conduct only under light.

- - - - - - - End- - - - - - -

ByKARTHIK.P

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Digital Electronics Innovation-01