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ELECTRONIC INTERFACE SUBSYSTEMS

TRANSISTOR–TRANSISTOR LOGIC (TTL)

TRANSISTOR–TRANSISTOR LOGIC (TTL) is a class of digital circuits built frombipolar junction transistors (BJT) and resistors. It is called transistor–transistor logic

because both the logic gating function (e.g., AND) and the amplifying function are

performed by transistors

TTL is notable for being a widespread integrated circuit (IC) family used in many

applications such as computers, industrial controls, test equipment and instrumentation,

consumer electronics, synthesizers, etc. The designation TTL is sometimes used to mean

TTL-compatible logic levels, even when not associated directly with TTL integrated

circuits, for example as a label on the inputs and outputs of electronic instruments.

TTL devices consume substantially more power than equivalent CMOS devices at rest,

but power consumption does not increase with clock speed as rapidly as for CMOS

devices. Compared to contemporary ECL circuits, TTL uses less power and has easier

design rules but is substantially slower. Designers can combine ECL and TTL devices in

the same system to achieve best overall performance and economy, but level-shifting

devices are required between the two logic families. TTL is less sensitive to damage from

electrostatic discharge than early CMOS devices.

Due to the output structure of TTL devices, the output impedance is asymmetrical

between the high and low state, making them unsuitable for driving transmission lines.

This drawback is usually overcome by buffering the outputs with special line-driver

devices where signals need to be sent through cables. ECL, by virtue of its symmetric

low-impedance output structure, does not have this drawback.

The TTL "totem-pole" output structure often has a momentary overlap when both the

upper and lower transistors are conducting, resulting in a substantial pulse of current

drawn from the supply. These pulses can couple in unexpected ways between multiple

integrated circuit packages, resulting in reduced noise margin and lower performance.

D e a r t m e n t O

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TTL systems usually have a decoupling capacitor for every one or two IC packages, so

that a current pulse from one chip does not momentarily reduce the supply voltage to the

others.

SUB-TYPES

• Low-power TTL (L), which traded switching speed (33ns) for a reduction in

power consumption (1mW) (now essentially replaced by CMOS logic)

• High-speed TTL (H), with faster switching than standard TTL (6ns) but

significantly higher power dissipation (22mW)

• Schottky TTL (S), introduced in 1969, which used Schottky diode clamps at gate

inputs to prevent charge storage and improve switching time. These gates

operated more quickly (3ns) but had higher power dissipation (19mW)

• Low-power Schottky TTL (LS) — used the higher resistance values of low-power

TTL and the Schottky diodes to provide a good combination of speed (9.5ns) and

reduced power consumption (2mW), and PDP of about 20 pJ. Probably the most

common type of TTL, these were used as glue logic in microcomputers,

essentially replacing the former H, L, and S sub-families.

• Fast (F) and Advanced-Schottky (AS) variants of LS from Fairchild and TI,

respectively, circa 1985, with "Miller-killer" circuits to speed up the low-to-high

transition. These families achieved PDPs of 10 pJ and 4 pJ, respectively, the

lowest of all the TTL families.

• Most manufacturers offer commercial and extended temperature ranges: for

example Texas Instruments 7400 series parts are rated from 0 to 70°C, and 5400

series devices over the military-specification temperature range of −55 to +125°C.

• Radiation-hardened devices are offered for space applications

• Special quality levels and high-reliability parts are available for military and

aerospace applications.

• Low-voltage TTL (LVTTL) for 3.3-volt power supplies and memory interfacing.

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CMOS(COMPLEMENTARY METAL–OXIDE–SEMICONDUCTOR)

CMOS is a major class of integrated circuits. CMOS technology is used in

microprocessors, microcontrollers, static RAM, and other digital logic circuits. CMOS

technology is also used for a wide variety of analog circuits such as image sensors, data

converters, and highly integrated transceivers for many types of communication.

CMOS was also sometimes referred to as complementary-symmetry metal–oxide–

semiconductor (or COS-MOS). The words "complementary-symmetry" refer to the fact

that the typical digital design style with CMOS uses complementary and symmetrical

pairs of p-type and n-type metal oxide semiconductor field effect transistors (MOSFETs)

for logic functions.

Two important characteristics of CMOS devices are high noise immunity and low static

power consumption. Significant power is only drawn when the transistors in the CMOS

device are switching between on and off states. Consequently, CMOS devices do not

produce as much waste heat as other forms of logic, for example transistor-transistor

logic (TTL) or NMOS logic, which uses all n-channel devices without p-channel devices.

CMOS also allows a high density of logic functions on a chip.

The phrase "metal–oxide–semiconductor" is a reference to the physical structure of

certain field-effect transistors, having a metal gate electrode placed on top of an oxide

insulator, which in turn is on top of a semiconductor material. Aluminum was once used

but now the material is polysilicon.

"CMOS" refers to both a particular style of digital circuitry design, and the family of

processes used to implement that circuitry on integrated circuits (chips). CMOS circuitry

dissipates less power when static, and is denser than other implementations having the

same functionality. As this advantage has grown and become more important, CMOS

processes and variants have come to dominate, so that the vast majority of modern

integrated circuit manufacturing is on CMOS processes.

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CMOS circuits use a combination of p-type and n-type metal–oxide–semiconductor field-

effect transistors (MOSFETs) to implement logic gates and other digital circuits found in

computers, telecommunications equipment, and signal processing equipment. Although

CMOS logic can be implemented with discrete devices (for instance, in an introductory

circuits class), typical commercial CMOS products are integrated circuits composed of

millions (or hundreds of millions) of transistors of both types on a rectangular piece of

silicon of between 0.1 and 4 square centimeters.

CMOS circuits are constructed so that all PMOS transistors must have either an input

from the voltage source or from another PMOS transistor. Similarly, all NMOS

transistors must have either an input from ground or from another NMOS transistor. The

composition of a PMOS transistor creates low resistance when a low voltage is applied toit and high resistance when a high voltage is applied to it. On the other hand, the

composition of an NMOS transistor creates high resistance when a low voltage is applied

to it and low resistance when a high voltage is applied to it.

The image on the right shows what happens when an input is connected to both a PMOS

transistor and an NMOS transistor. When the voltage of input A is low, the NMOS

transistor has high resistance so it stops voltage from leaking into ground, while the

PMOS transistor has low resistance so it allows the voltage source to transfer voltage

through the PMOS transistor to the output. The output would therefore register a high

voltage.

On the other hand, when the voltage of input A is high, the PMOS transistor would have

high resistance so it would block voltage source from the output, while the NMOS

transistor would have low resistance allowing the output to drain to ground. This would

result in the output registering a low voltage. In short, the outputs of the PMOS and

NMOS transistors are complementary such that when the input is low, the output would

be high, and when the input is high, the output would be low. Because of this, the CMOS

circuits' output is by default the inversion of the input.

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Besides digital applications, CMOS technology is also used for analog applications. For

example, there are CMOS operational amplifier ICs available in the market. CMOS

technology is also widely used for RF applications all the way to microwave frequencies.

Indeed, CMOS technology is used for mixed-signal (analog+digital) applications.

ACTUATOR SENSOR INTERFACE

Actuator Sensor Interface(ASI) is the simplest of the industrial networking protocols

used in PLC, DCS and PC-based automation systems. It is designed for connecting

simple field I/O devices (e.g. binary (ON/OFF) devices such as actuators and sensors,

rotary encoders, analog inputs and outputs, push buttons, valve position sensors ...) in

discrete manufacturing and process applications using a single 2-conductor cable.

AS-Interface is an 'open' technology supported by leading automation vendors. AS-

Interface is a highly efficient networking alternative to the hard wiring of field devices. It

is an excellent partner network for higher level fieldbus networks such as *Profibus,

*DeviceNet, Interbus and *Industrial Ethernet, for whom it offers a low-cost remote I/O

solution. It is proven in hundreds of thousands of applications, including conveyors,

process control valves, bottling plants, electrical distribution systems, airport carousels,

elevators, bottling lines and food production lines.

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AS-Interface provides the ideal basis for Functional Safety in machinery

safety/emergency stop applications. Safety devices communicating over AS-Interface

follow all the normal data rules. The required level of data verification is provided by

dynamic changes in the data. This technology is called Safety as Work (or ASi-Safe) and

allows safety devices and standard to be connected to the same network cable. Using

appropriate safe input hardware (i.e. light curtains, e-stop button, door interlock switches

...) AS-Interface can provide safety support up to SIL (Safety Integrity Level) 3 according

to IEC 61508 as well as CAT 4 according to EN954-1.The AS-Interface specification is

managed by AS-International, a member funded organization located in Germany.

Several international daughter organizations exist around the world.

An AS-Interface network requires only a few basic components falling into the followinggeneral categories:

• Scanners and Gateways (also called masters)

• Power supplies and repeaters

• Modules (also called slaves)

• Network cable, installation hardware and useful tool (infrastructure)

1. Scanners and Gateways

The Scanner/Gateway performs two functions. With respect to the AS-Interface network

it is a master, performing the data exchange with the modules and updating its internal

I/O image. The functionality of the master is defined in the Master Profile of the AS-

Interface specification. As part of specification version 3.0 the M4 Master Profile has

been defined. Any given network can only have one Scanner/Gateway. With respect to a

connected PLC/DCS or PC the Scanner/Gateway is a slave. The AS-Interface community

typically uses the word Gateway when the AS-Interface master connects to an upper-

level network like DeviceNet, Profibus or any of the industrial Ethernet flavors. On the

other hand, if it resides on the backplane of a PLC it is usually referred to as a Scanner.

Since AS-Interface communication is based on the Master-Slave communication method,

any network must have only one Master at a time.

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2.Power supply

Any AS-Interface segment must be powered. This is typically accomplished connecting

an AS-Interface power supply. These supplies have certain unique characteristics

regarding internal circuitry and output voltage. Standard 24VDC power supplies can notbe used to directly power a segment. The total length AS-Interface network cable in a

single segment must be no more than 100m. If the total network length must be longer

repeaters can be used. As the repeater galvanically isolates any two segment a new power

supply must be used on the far side of the repeater. A common misconception exists

concerning the number of repeaters in a network. It has been stated that the maximum

length of an AS-Interface network can be 300m, created by using two repeaters. This is

not the case at all! What matters is not how many repeaters are using but rather how

many repeaters any data packet, originating at a Scanner or Gateway, has to cross before

reaching the I/O node. Due to the tight timing constraints defined each packet can at most

travel across two repeaters before reaching an AS-Interface node. This has the following

consequences:

1.

Linear networks with the Scanner/Gateway mounted at one end can be 300m long

2.

Linear network with 600m length can be constructed when the Scanner/Gateway

is mounted in the middle segment3.

Star shaped networks with virtually no length limitation are possible

3. Modules

This is by far the largest group of components and includes binary and analog I/O

modules, stack lights, pushbuttons, sensors with integrated ASIC, valve control boxes, E-

stops, light curtains; in general any device that can exchange data with the PLC. Each

module on the network must have a unique address. For AS-Interface the address space

ranges from 0 to 31, where 0 cannot be used, but is reserved for Automatic Single Node

Replacement. Since adoption of specification 2.11 this address space is further divided

into A and B extended addresses. As a result, using a module designed to support this

addressing mode, it possible to have two modules at each address; one at the A half and

one at the B half .

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The current specification Version 3.0 has adds many the ability to construct many new

types of I/O combinations, including binary modules with 4 inputs and 4 outputs

supporting A/B addressing.

4. Network cable

The vast majority of AS-Interface installations utilize the AS-Interface flat cable, defined

as part of the AS-Interface specifications. While the shape of the cable does not matter

(any other cable can be used) the electrical characteristics of the selected cable matters

greatly. To prevent problems due to improper cable, most professional suggest the AS-

Interface flat cable. This cable is designed to make use of the cable piercing technology.

When an AS-Interface module is installed on the network, piercing needles penetrate the

cable and displace the internal copper strands without cutting them. This allow AS-

Interface modules to be installed anywhere on the network without cutting and preparing

(i.e. removing cable jacket, stripping insulation and possibly applying a ferule) the cable

first. The result is a faster installation without the chance of inadvertent shorts between

the leads.

There are several types of cables available. Yellow cable is usually used to power AS-

Interface modules and enable communication between the field devices and the scanner

or Gateway. Several material are offered to address specific applications needs. The AS-

Interface black cable is typically used to supply modules with 24VDC AUX power. No

communication takes place on this cable. Similar to the yellow cable, the black cable is

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also produced using various jacket material to address the specific needs of the

application. A red jacketed cable has been defined but is virtually unused. Its intended

use was in applications where AC power is supplied to the field nodes. The two leads

inside the AS-Interface cable are brown (+ lead) and blue (- lead) independent of material

makeup and outer jacket color.

5. Other components

Passive taps, flat-to-round cable adapters, handheld addressing tools and many other

accessories are designed to further simplify the installation of AS-Interface networks.

OPTO ISOLATOR

An opto-isolator is a device that uses a short optical transmission path to transfer a

signal between elements of a circuit, typically a transmitter and a receiver, while keeping

them electrically isolated — since the signal goes from an electrical signal to an optical

signal back to an electrical signal, electrical contact along the path is broken.

The opto-isolator is simply a package that contains both an infrared LED and a

photodetector such as silicon diode, transistor Darlington pair, or SCR. The wave-length

response of each device is tailored to be as identical as possible to permit the highest

measure of coupling possible.

A common implementation involves a LED and a phototransistor, separated so that light

may travel through a barrier but electrical current may not. When an electrical signal is

applied to the input of the opto-isolator, its LED lights, its light sensor then activates, and

a corresponding electrical signal is generated at the output. Unlike a transformer, theopto-isolator allows for DC coupling and generally provides significant protection from

serious overvoltage conditions in one circuit affecting the other. If high transmission ratio

is required Darlington photo transistor is used, however higher transmission ratio usually

results in low noise immunity and higher delay.

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With a photodiode as the detector, the output current is proportional to the amount of

incident light supplied by the emitter. The diode can be used in a photovoltaic mode or a

photoconductive mode. In photovoltaic mode, the diode acts like a current source in

parallel with a forward-biased diode. The output current and voltage are dependent on the

load impedance and light intensity. In photoconductive mode, the diode is connected to a

supply voltage, and the magnitude of the current conducted is directly proportional to the

intensity of light. This opto coupler type is significantly faster than one with photo

transistor however transmission ratio is very low. Because of that it is common to

integrate amplifier circuit in same package.

The optical path may be air or a dielectric waveguide. When high noise immunity is

required optical conductive shield may be integrated into optical path. The transmittingand receiving elements of an optical isolator may be contained within a single compact

module, for mounting, for example, on a circuit board; in this case, the module is often

called an optoisolator or opto-isolator. The photosensor may be a photocell,

phototransistor, or an optically triggered SCR or TRIAC. Occasionally, this device will in

turn operate a power relay or contactor.

For analog isolation, special "analog" optoisolators are used. These devices have two

independent, closely matched phototransistors, one of which is typically used to linearize

the response using negative feedback.

Among other applications, opto-isolators can help cut down on ground loops, block

voltage spikes, and provide electrical isolation.

• Most common application is for switched-mode power supplies. They utilise

optocouplers for mains isolation. Because of noisy environment optocouplers with

low transmission ratio are preferred.

• One of the requirements of the MIDI (Musical Instrument Digital Interface)

standard is that input connections be opto-isolated.

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• They are used to isolate low-current control or signal circuitry from transients

generated or transmitted by power supply and high-current control circuits. The

latter are used within motor and machine control function blocks

CIRCUIT BREAKERS

A circuit breaker is an automatically-operated electrical switch designed to protect

an electrical circuit from damage caused by overload or short circuit. Its basic

function is to detect a fault condition and, by interrupting continuity, to immediately

discontinue electrical flow. Unlike a fuse, which operates once and then has to be

replaced, a circuit breaker can be reset (either manually or automatically) to resume

normal operation. Circuit breakers are made in varying sizes, from small devices that

protect an individual household appliance up to large switchgear designed to protect

high voltage circuits feeding an entire city.

All circuit breakers have common features in their operation, although details varysubstantially depending on the voltage class, current rating and type of the circuit

breaker.

The circuit breaker must detect a fault condition; in low-voltage circuit breakers this

is usually done within the breaker enclosure. Circuit breakers for large currents or

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high voltages are usually arranged with pilot devices to sense a fault current and to

operate the trip opening mechanism. The trip solenoid that releases the latch is

usually energized by a separate battery, although some high-voltage circuit breakers

are self-contained with current transformers, protection relays, and an internal control

power source.

Once a fault is detected, contacts within the circuit breaker must open to interrupt the

circuit; some mechanically-stored energy (using something such as springs or

compressed air) contained within the breaker is used to separate the contacts,

although some of the energy required may be obtained from the fault current itself.

Small circuit breakers may be manually operated; larger units have solenoids to trip

the mechanism, and electric motors to restore energy to the springs.

The circuit breaker contacts must carry the load current without excessive heating,

and must also withstand the heat of the arc produced when interrupting the circuit.

Contacts are made of copper or copper alloys, silver alloys, and other materials.

Service life of the contacts is limited by the erosion due to interrupting the arc.

Miniature circuit breakers are usually discarded when the contacts are worn, but

power circuit breakers and high-voltage circuit breakers have replaceable contacts.

When a current is interrupted, an arc is generated - this arc must be contained, cooled,

and extinguished in a controlled way, so that the gap between the contacts can again

withstand the voltage in the circuit. Different circuit breakers use vacuum, air,

insulating gas, or oil as the medium in which the arc forms. Different techniques are

used to extinguish the arc including:

• Lengthening of the arc

•

Intensive cooling (in jet chambers)

• Division into partial arcs

• Zero point quenching

• Connecting capacitors in parallel with contacts in DC circuits

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TYPES OF CIRCUIT BREAKERS

Many different classifications of circuit breakers can be made, based on their features

such as voltage class, construction type, interrupting type, and structural features.

1. Low voltage circuit breakers

Low voltage (less than 1000 VAC) types are common in domestic, commercial and

industrial application, include:

• MCB (Miniature Circuit Breaker)—rated current not more than 100 A. Trip

characteristics normally not adjustable. Thermal or thermal-magnetic operation.

Breakers illustrated above are in this category.

• MCCB (Molded Case Circuit Breaker)—rated current up to 1000 A. Thermal or

thermal-magnetic operation. Trip current may be adjustable in larger ratings.

• Low voltage power circuit breakers can be mounted in multi-tiers in LV

switchboards or switchgear cabinets.

The characteristics of LV circuit breakers are given by international standards such as

IEC 947. These circuit breakers are often installed in draw-out enclosures that allow

removal and interchange without dismantling the switchgear.

Large low-voltage molded case and power circuit breakers may have electrical motor

operators, allowing them to be tripped (opened) and closed under remote control.

These may form part of an automatic transfer switch system for standby power.

Low-voltage circuit breakers are also made for direct-current (DC) applications, for

example DC supplied for subway lines. Special breakers are required for direct

current because the arc does not have a natural tendency to go out on each half cycle

as for alternating current. A direct current circuit breaker will have blow-out coils

which generate a magnetic field that rapidly stretches the arc when interrupting direct

current.

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Small circuit breakers are either installed directly in equipment, or are arranged in a

breaker panel.

The 10 ampere DIN rail-mounted thermal-magnetic miniature circuit breaker is the

most common style in modern domestic consumer units and commercial electrical

distribution boards throughout Europe. The design includes the following

components:

1.

Actuator lever - used to manually trip and reset the circuit breaker. Also indicates

the status of the circuit breaker (On or Off/tripped). Most breakers are designed so

they can still trip even if the lever is held or locked in the "on" position. This is

sometimes referred to as "free trip" or "positive trip" operation.

2.

Actuator mechanism - forces the contacts together or apart.

3.

Contacts - Allow current when touching and break the current when moved apart.

4.

Terminals

5.

Bimetallic strip

6.

Calibration screw - allows the manufacturer to precisely adjust the trip current of

the device after assembly.

7.

Solenoid

8.

Arc divider / extinguisher

2. MAGNETIC CIRCUIT BREAKER

Magnetic circuit breakers use a solenoid (electromagnet) whose pulling force

increases with the current. The circuit breaker contacts are held closed by a latch. As

the current in the solenoid increases beyond the rating of the circuit breaker, the

solenoid's pull releases the latch which then allows the contacts to open by spring

action. Some types of magnetic breakers incorporate a hydraulic time delay feature

using a viscous fluid. The core is restrained by a spring until the current exceeds the

breaker rating. During an overload, the speed of the solenoid motion is restricted by

the fluid. The delay permits brief current surges beyond normal running current for

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motor starting, energizing equipment, etc. Short circuit currents provide sufficient

solenoid force to release the latch regardless of core position thus bypassing the delay

feature. Ambient temperature affects the time delay but does not affect the current

rating of a magnetic breaker.

3. Thermal magnetic circuit breaker

Thermal magnetic circuit breakers, which are the type found in most distribution

boards, incorporate both techniques with the electromagnet responding

instantaneously to large surges in current (short circuits) and the bimetallic strip

responding to less extreme but longer-term over-current conditions.

RESETTABLE FUSES

A polymeric positive temperature coefficient device (PPTC, commonly known as a

resettable fuse) is a passive electronic component used to protect against overcurrent

faults in electronic circuits. They are actually non-linear thermistors, however, and cycle

back to a conductive state after the current is removed, acting more like circuit breakers,

allowing the circuit to function again without opening the chassis or replacing anything.

These devices are often used in computer power supplies, largely due to the PC 97

standard , and in aerospace/nuclear applications where replacement is difficult.

A PPTC device has a current rating. When the current flowing through the device, (which

has a small resistance in the on state) exceeds the current limit, the PPTC device warms

up above a threshold temperature and the electrical resistance of the PPTC device

suddenly increases several orders of magnitude to a "tripped" state where the resistance

will typically be hundreds or thousands of ohms, greatly reducing the current. The rated

trip current can be anywhere from 20 mA to 100 A.

A polymeric PTC device comprises a non-conductive crystalline organic polymer matrix

that is loaded with carbon black particles to make it conductive. While cool, the polymer

is in a crystalline state, with the carbon forced into the regions between crystals, forming

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many conductive chains. Since it is conductive (the "initial resistance"), it will pass a

given amount of current, called the "hold current". If too much current is passed through

the device, the "trip current", the device will begin to heat. As the device heats the

polymer will expand, change from a crystalline state into an amorphous state. The

expansion separates the carbon particles and breaks the conductive pathways, causing the

resistance of the device to increase. This will cause the device to heat faster and expand

more, further raising the resistance. This increase in resistance is sufficient to

substantially reduce the current in the circuit. A small amount of current will still flow

through the device and is sufficient to maintain the temperature of the device and keep it

at the high resistance level ("latching" functionality).

When the power and fault are removed, the PPTC device will cool. As the device cools, itcontracts to its original shape and returns to a low resistance level where it can hold the

current as specified for the device. This cooling usually takes a few seconds, though a

tripped device will retain a slightly higher resistance for hours, slowly approaching the

initial resistance value.

Since a PPTC device has an inherently higher resistance than a metallic fuse or circuit

breaker at ambient temperature, it may be difficult or impossible to use in circuits that

cannot tolerate significant reductions in operating voltage, forcing the engineer to choose

the latter in a design.

Bipolar junction transistor

A bipolar (junction) transistor (BJT) is a type of transistor. It is a three-terminal device

constructed of doped semiconductor material and may be used in amplifying or switching

applications. Bipolar transistors are so named because their operation involves both

electrons and holes, as opposed to unipolar transistors, such as field-effect transistors, in

which only one carrier type is involved in charge flow. Although a small part of the

transistor current is due to the flow of majority carriers, most of the transistor current is

due to the flow of minority carriers and so BJTs are classified as minority-carrier

devices.

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In electronics, a transistor is a semiconductor device commonly used to amplify or

switch electronic signals. A transistor is made of a solid piece of a semiconductor

material, with at least three terminals for connection to an external circuit. A voltage or

current applied to one pair of the transistor's terminals changes the current flowing

through another pair of terminals. Because the controlled (output) power can be much

larger than the controlling (input) power, the transistor provides amplification of a signal.

The transistor is the fundamental building block of modern electronic devices, and is used

in radio, telephone, computer and other electronic systems. Some transistors are packaged

individually but most are found in integrated circuits.

MOSFET

The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or

MOS FET) is a device used to amplify or switch electronic signals. The MOSFET

includes a channel of n-type or p-type semiconductor material (see article on

semiconductor devices), and is accordingly called an NMOSFET or a PMOSFET (also

commonly nMOS, pMOS). It is by far the most common transistor in both digital and

analog circuits, though the bipolar junction transistor was at one time much more

common.

Usually the semiconductor of choice is silicon, but some chip manufacturers, have begun

to use a mixture of silicon and germanium (SiGe) in MOSFET channels. Unfortunately,

many semiconductors with better electrical properties than silicon, such as gallium

arsenide, do not form good semiconductor-to-insulator interfaces and thus are not suitable

for MOSFETs. However there continues to be research on how to create insulators with

acceptable electrical characteristics on other semiconductor material.

To overcome power consumption increase due to gate current leakage, high-κ dielectric

is replacing silicon dioxide as the gate insulator, and metal gates are making a comeback

by replacing polysilicon .

The gate is separated from the channel by a thin insulating layer of what was traditionally

silicon dioxide, but more advanced technologies uses silicon oxynitride. Some companies

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have started to introduce a high-κ dielectric + metal gate combination in the 45

nanometer node.

When a voltage is applied between the gate and source terminals, the electric field

generated penetrates through the oxide and creates a so-called "inversion layer" or

channel at the semiconductor-insulator interface. The inversion channel is of the same

type – P-type or N-type – as the source and drain, so it provides a channel through which

current can pass. Varying the voltage between the gate and body modulates the

conductivity of this layer and makes it possible to control the current flow between drain

and source.

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 it also

suffers from some more technical deficiencies in its role as the standard gate material.

Nevertheless, there are several reasons favoring use of polysilicon as a gate material:

1.

The threshold voltage (and consequently the drain to source on-current) ismodified by the work function difference between the gate material and channel

material. 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 quite

straightforward to tune the work function, so as to achieve low threshold voltages

for both NMOS and PMOS devices. By contrast the work functions of metals are

not easily 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-SiO2 interface has been well studied and is known to have relatively

few defects. By contrast many metal–insulator interfaces contain significant levels

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of 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 performing

transistors. Such high temperature steps would melt some metals, limiting the

types of metals that could be used in a metal-gate based process.

While polysilicon gates have been the de facto standard for the last twenty years, they do

have some disadvantages, which have led to the announcement of their replacement by

metal gates. These disadvantages include:

1.

Polysilicon is not a great conductor (approximately 1000 times more resistive

than metals) which reduces the signal propagation speed through the material. The

resistivity 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

conductivity further, sometimes a high temperature metal such as tungsten,

titanium, cobalt, and more 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 polysilicon alone, because the silicide material is not near the channel.

The process in which silicide is formed on both the gate electrode and the source

and drain regions is sometimes called salicide, self-aligned silicide.

2. When the transistors are extremely scaled down, it is necessary to make the gate

dielectric 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 the

transistor is in the inversion. To avoid this problem, a metal gate is desired. A

variety of metal gates such as tantalum, tungsten, tantalum nitride, and titanium

nitride are used, usually in conjunction with high-k dielectrics. An alternative is to

use fully-silicided polysilicon gates, and the process is referred to as FUSI.

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Insulator

As devices are made smaller, insulating layers are made thinner, and at some point

tunneling of carriers through the insulator from the channel to the gate electrode takes

place. To reduce the resulting leakage current, the insulator can be made thicker bychoosing a material with a higher dielectric constant. To see how thickness and dielectric

constant are related, note that Gauss' law connects field to charge as:

with Q = charge density, κ = dielectric constant, ε0 = permittivity of empty space and E =

electric field. From this law it appears the same charge can be maintained in the channel

at a lower field provided κ is increased. The voltage on the gate is given by:

with V G = gate voltage, V ch = voltage at channel side of insulator, and t ins = insulator

thickness. This equation shows the gate voltage will not increase when the insulator

thickness increases, provided κ increases to keep t ins / κ = constant (see the article on

high-κ dielectrics for more detail, and the section in this article on gate-oxide leakage).

The insulator in a MOSFET is a dielectric which can in any event be silicon oxide, but

many other dielectric materials are employed. The generic term for the dielectric is gate

dielectric since the dielectric lies directly below the gate electrode and above the channel

of the MOSFET.

Junction design

The source-to-body and drain-to-body junctions are the object of much attention because

of three major factors: their design affects the current-voltage ( I-V ) characteristics of the

device, lowering output resistance, and also the speed of the device through the loading

effect of the junction capacitances, and finally, the component of stand-by power

dissipation due to junction leakage.

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The drain induced barrier lowering of the threshold voltage and channel length

modulation effects upon I-V curves are reduced by using shallow junction extensions. In

addition, halo doping can be used, that is, the addition of very thin heavily doped regions

of the same doping type as the body tight against the junction walls to limit the extent of

depletion region.

The capacitive effects are limited by using raised source and drain geometries that make

most of the contact area border thick dielectric instead of silicon.

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me05379notes-4

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