3.0 electronic devices and transducers - 1st half

8/13/2019 3.0 Electronic Devices and Transducers - 1st Half

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Chapter 3:

Electronic Devices and Transducers

• 3.1 Semiconductors

• 3.2 Diodes

• 3.3 Bipolar and Field Effect Transistors

• 3.4 Thyristors and triacs

• 3.5 Transducers



• Semiconductor is a substance, usually asolid chemical element or compound thatcan conduct electricity

• Its conductance varies depending oncurrent or voltage applied

• The conductance of semiconductor isintermediate between conductor andinsulator

• Specific properties of semiconductordepend on the impurities, or dopants addedto it



• An N-type semiconductor carries

current mainly in the form ofnegatively-charged electrons, in a

manner similar to the conduction of

current in a wire



• A P-type semiconductor carries

current predominantly as electron

deficiencies called holes• A hole has a positive electric

charge, equal and opposite to the

charge on an electron

• In a semiconductor material, the

flow of holes occurs in a direction

opposite to the flow of electrons



Material, such as germanium, gallium

arsenide and silicon are some of the good

semiconductor substances



Today, most semiconductor are created with silicon



Semiconductor materials are the

foundation of modern electronics,

including radio, computers, telephonesetc



• Carbon, silicon and

germanium is semiconductor

• They have unique property intheir electron structure-each

has electrons in its outer

orbital

• This allow them to form nice

crystal

• The four electrons form

perfect covalent bonds with

four neighboring atoms

creating a lattice



• In a silicon lattice, all silicon atoms

bond perfectly to four neighbors,

leaving no free electrons to conductelectric current. This makes a

silicon crystal an insulator rather

than a conductor



• A pure silicon crystal is nearly an

insulator - very little electricity will flow

through it

• But all this can be changed through a

process called “doping”



Resistivity Values (Ωm) for

selected materials at 300KConductors

Aluminium 2.73 x 10-8

Carbon (amorphous) 3.5 x 10-5

Copper 1.72 x 10-8

Gold 2.27 x 10-8

Nichrome 1.12 x 10-6

Silver 1.63 x 10-8

Tungsten 5.44 x 10-8

Semiconductors

Silicon (device grade) 10-5 to 1

depends on impurity concentration

Insulators

Fused quartz > 1021

Glass (typical) 1 x 1012

Teflon 1 x 1019



• The behavior of silicon can be turnedinto a conductor by doping it

• In doping, a small amount of an impurity

is mix into the silicon crystal



- In N-type doping, phosphorus orarsenic is added to the silicon in small

quantities.

• Phosphorus and arsenic each have five

outer electrons, so they're out of place when

they get into the silicon lattice.



• The fifth electron has nothing to bond to, so

it's .

• It takes only a very small quantity of the

impurity to create enough free electrons to

allow an electric current to flow through thesilicon.

•

N-type silicon is a good conductor.Electrons have a negative charge, hence the

name N-type.



- In P-type doping, boron or galliumis the dopant.

•Boron and gallium each have only threeouter electrons.

• When mixed into the silicon lattice, theyform "holes" in the lattice

.



• The absence of an electron creates the

effect of a positive charge, hence the name

P-type.

•

Holes can conduct current.

• A hole from a

neighbor, moving the hole over a space.

• P-type silicon is a good conductor



•

A diode is the simplest possible semiconductordevice

• In many application, diode is an electronicdevice that allows current to

• Has two terminals, the anode and the cathode

anode iD cathode

+ -

VD



• N-type and P-type silicon are not that

amazing by themselves; but when two

of them are put together, there will besome very interesting behavior at the

junction. That's what happens in a

diode.



• Even though N-type silicon by itself is a

conductor, and P-type silicon by itself is

also a conductor, the combination shownin the diagram conduct any

electricity

Th ti



• The negative

electrons in the

N-type silicon get

attracted to thepositive terminal

of the battery.

The positiveholes in the P-

type silicon get

attracted to thenegative terminal

of the battery

• No current flows

across the

junction becausethe holes and the

electrons are

each moving in

the wrong

direction



• If the battery is flip around, the diode

conducts electricity just fine. The free

electrons in the N-type silicon are repelled

by the negative terminal of the battery

• The holes in the P-type silicon are repelled

by the positive terminal. At the junction

between the N-type and P-type silicon,

holes and free electrons meet



• The electrons fill the holes. Those holes

and free electrons cease to exist, and newholes and electrons spring up to take their

place.

• The effect is that current flows through the

junction



• Small amount of voltage necessary to get

the diode going.

• In silicon, this voltage is about 0.7 volts.

•

This voltage is needed to start the hole-electron combination process at the

junction



•

Ideal diode would block all current.• A real diode lets perhaps 10 µA through

not a lot, but still not perfect.

•

And if you apply enough reverse voltage(V), the junction breaks down and lets

current through.

•

Usually, the breakdown voltage is a lotmore voltage than the circuit will ever

see, so it is irrelevant



In automobile, diode allow current

from the alternator to charge the

battery when the engine is running.However, when the engine stops,

the diode prevent the battery from

discharging through the alternator.



• A transducer is a device that converts onetype of energy to another

• The conversion can be to/from electrical,electro-mechanical, electromagnetic,photonic, photovoltaic, or any other form ofenergy

• In electric circuit, transducer are used toproduce a voltage ( or sometimes a current)

that is proportional to physical quantity ofinterest, such as distance, pressure ortemperature



• Example: microphones,

loudspeaker

• MicrophonesMicrophones

are transducers which

detect sound signals andproduce an electrical

image of the sound, i.e.,

they produce a voltage or

a current which isproportional to the sound

signal

3.0 electronic devices and transducers - 1st half

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