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EE 201

TOPIC 1:INTRODUCTION

TOSEMICONDUCTOR

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DEFINITION OF SEMICONDUCTOR

A semiconductoris a solidwhoseelectrical conductivityis in between that of a

conductorand that of an insulator, and can be

controlled over a wide range, either permanently or

dynamically.

Example of semiconductor !ilicon " #ermanium

http://en.wikipedia.org/wiki/Solidhttp://en.wikipedia.org/wiki/Electrical_conductivityhttp://en.wikipedia.org/wiki/Conductorhttp://en.wikipedia.org/wiki/Electrical_insulationhttp://images.google.com.my/imgres?imgurl=http://www.woodlands-junior.kent.sch.uk/Homework/victorians/images/inventions/bulb.jpg&imgrefurl=http://www.woodlands-junior.kent.sch.uk/Homework/victorians/inventions.htm&h=101&w=76&sz=2&hl=en&start=14&tbnid=j8E4p-0-iZfSzM:&tbnh=83&tbnw=62&prev=/images%3Fq%3Djoseph%2Bswan%2Bthe%2Binventor%2Bof%2Bbulb%26gbv%3D2%26svnum%3D10%26hl%3Den%26sa%3DGhttp://en.wikipedia.org/wiki/Electrical_insulationhttp://en.wikipedia.org/wiki/Conductorhttp://en.wikipedia.org/wiki/Electrical_conductivityhttp://en.wikipedia.org/wiki/Solid

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Diagrams o !toms

Atom: negative electrons orbit a small

positive nucleus" EE201-semiconductor device
http://images.google.com.my/imgres?imgurl=http://www.woodlands-junior.kent.sch.uk/Homework/victorians/images/inventions/bulb.jpg&imgrefurl=http://www.woodlands-junior.kent.sch.uk/Homework/victorians/inventions.htm&h=101&w=76&sz=2&hl=en&start=14&tbnid=j8E4p-0-iZfSzM:&tbnh=83&tbnw=62&prev=/images%3Fq%3Djoseph%2Bswan%2Bthe%2Binventor%2Bof%2Bbulb%26gbv%3D2%26svnum%3D10%26hl%3Den%26sa%3DG

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Cont##

$ %iny particles of matter calledprotonsand neutronsma&e up

the center of the atom' electronsorbit li&e planets around a star.

$ %he nucleus carries a positive electrical charge, owing to the

presence of protons (the neutrons have no electrical charge

whatsoever), while the atom*s balancing negative charge resides

in the orbiting electrons.

$ %he negative electrons are attracted to the positive protons +ust

as planets are gravitationally attracted by the !un, yet the orbits

are stable because of the electrons* motion.

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Cont##

Electron shells in an atom were formerly designated by letter

rather than by number. %he first shell (n1) was labeled , the

second shell (n2) , the third shell (n/) , the fourth shell(n) , the fifth shell (n3) 4, the sixth shell (n5) 6, and the

seventh shell (n7) 8.

%he maximum number of electrons that any shell may hold is

described by the e9uation 2n2, where :n; is the principle

9uantum number. %hus, the first shell (n1) can hold 2 electrons' the second shell

(n2) < electrons, and the third shell (n/) 1< electrons. (=igure

below)

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& or'it ( s)e**

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8uestion 1

Allumminium has 1/electrons, determine the number of

electron at electron orbit>

8uestion 2

?arbon - 5 electron

#ermanium - /2 electron6hosphorus - 13 electron

E,!MP-E .UESTION

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C/!R!CTERISTICS OF E

NUMER

a*ence:%he electrons in the outer most shell, or valence shell, are &nown as valence

electrons. %hese valence electrons are responsible for the chemical properties of the

chemical elements.

1-/ ev - conductor

- can conduct an electrical current

- low resistance

3-< ev - insulator

- cannot conduct an electrical current

- high resistance ev - semiconductor

- ot easy to accept@throw away valence electron

from another atomic

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How Sharing of Electrons


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Distur'ing actor sta'i*it4 :5

eat

%emperature riseBoping (absorption)

6otential different.


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!d6antages o Si*icon

!i C can accept more overheat before

change to conductor. ess easily damaged by heat when soldering

Dery low lea&age current when reverse

voltage. (#e-high lea&age current)


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P5t47e and N5t47e c)aracteristics

?urrent carrier C free electron and hole

=ree electron C negative current carrierole C positive current carrier

Hole

Free electron

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2 T8PE OF SEMICONDUCTOR:

ntrinsic !emiconductor

- 6ure semiconductors.- Ex #ermanium " !ili&on

Extrinsic !emiconductor- 6ure semiconductor F impurity

- %his process called doping (absorption) process.- 2 categories of impurity C trivalen and pentavalen.

- Ex trivalen-Alluminium, Goron, #allium.

pentavalen-Antimoni, Arseni&, 6hosphorus

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Formation o N5t47e 6ure semiconductor (silicon) F pentavalen atom (arseni&)

%his creates an excess of negative (n-type) electron charge

carriers.

%he electrons are the majority carriers, while holes are the

minority carriersin -type materials

Si

Ar

!iSi Si

Si

Si Si Si

Si

Figure 1.6

Silicon absorb with Arsenik

=ree electron

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http://en.wikipedia.org/wiki/Majority_carrierhttp://en.wikipedia.org/wiki/Minority_carrierhttp://en.wikipedia.org/wiki/Minority_carrierhttp://en.wikipedia.org/wiki/Majority_carrier

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Formation o P5t47e

6ure semiconductor (silicon) F trivalen atom (ndium)

%he purpose of P5t47e do7ingis to create an abundance of

holes a hole behaves as a 9uantity of positive charge

the holes are the majority carriers, while electrons are the

minority carriersin 6-type materials

Si

In

!iSi Si

Si

Si Si Si

Si

Figure 1.7

Silicon absorb with Indium

the vacancy left behind by theelectron is &nown as a hole.

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http://en.wikipedia.org/wiki/Majority_carrierhttp://en.wikipedia.org/wiki/Minority_carrierhttp://en.wikipedia.org/wiki/Electron_holehttp://en.wikipedia.org/wiki/Electron_holehttp://en.wikipedia.org/wiki/Minority_carrierhttp://en.wikipedia.org/wiki/Majority_carrier

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PN 9unctions

%hese are p-type and n-type semiconductors brought together

in close contact, creating what is called the depletion region.

%he *depletion region'is so named because it is formed from

a conducting region by removal of all free charge carriers,

leaving none to carry a current.

%he importance of this contact or +unction is the creating of a

region between the p and n layers where p-type holes can

recombine with n-type free electrons producing light, such as in

light emitting diodes(EBs).

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http://en.wikipedia.org/wiki/Depletion_regionhttp://en.wikipedia.org/wiki/Light_emitting_diodeshttp://en.wikipedia.org/wiki/Light_emitting_diodeshttp://en.wikipedia.org/wiki/Depletion_region

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PN 9unctions

http://en.wikipedia.org/wiki/File:Pn-junction-equilibrium.pnghttp://en.wikipedia.org/wiki/File:PN_Junction_Open_Circuited.svg

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PN 9unctions

n 6 +unction ;it)out an e

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For;ard 'ias

=orward-bias occurs when the P-typebloc& is connected to the

positiveterminal of a battery and the N-typebloc& is connected

to the negativeterminal, as shown below


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For;ard 'ias

%he *holes* in the 6-type region and the electrons in the -type

region are pushed towards the +unction.

%his reduces t)e ;idt) o t)e de7*etion =one# %he positive charge applied to the 6-type bloc& repels the

holes, while the negative charge applied to the -type bloc&

repels the electrons.

As electrons and holes are pushed towards the +unction, the

distance between them decreases. %his *o;ers t)e 'arrier in7otentia*#


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For;ard 'ias

Hith increasing bias voltage, eventually the no conducting

depletion Ione becomes so thin that the charge carriers can

tunnel across the barrier, and t)e e*ectrica* resistance a**s toa *o; 6a*ue#

%he electrons which pass the +unction barrier enter the 6-type

region (moving leftwards from one hole to the next, with

reference to the above diagram).

An electron starts flowing around from the negative terminal tothe positive terminal of the battery.

%he thin depletion Ione produces very little electrical resistance

against the flow of electrons


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Re6erse 'ias

?onnecting the P-typeregion to the negativeterminal of the

battery and the N-typeregion to thepositiveterminal, produces

the reverse-bias effect. Gecause the 6-type region is now connected to the negative

terminal of the power supply, the *holes* in the 6-type region are

pulled away from the +unction, causing t)e ;idt) o t)e no

conducting de7*etion =one to increase#

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http://en.wikipedia.org/wiki/File:PN_Junction_in_Reverse_Bias.png

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Re6erse 'ias

!imilarly, because the -type region is connected to the

positive terminal, the electrons will also be pulled away from

the +unction. %his effectively increases t)e 7otentia* 'arrier and greatly

increases t)e e*ectrica* resistance against the flow of charge

carriers.

=or this reason there will be minima* e*ectric current across

t)e 9unction#

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For;ard and Re6erse 'ias

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RE!>DO?NS -IMIT

At the middle of the +unction of the p-n material, t)e de7*etion

region ;idens ;it) increasing re6erse 'ias#

%he e*ectric ie*d gro;s as t)e re6erse 6o*tage increases# Hhen the electric field increases beyond a critical *e6e* the

9unction 'rea@s do;n and currant begins to flow, usually by

either the Jener or avalanche brea&down processes.

Goth of these brea&down processes are non-destructive and

reversible so long as current density does not exceed levelsthat could cause thermal damage.

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I cur6e

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.UIC> REIE?


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Conc*usion:

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Befine a semiconductor.

Explain the characteristics of -type and 6-type semiconductors.

llustrate the formation of a 6 +unction llustrate the meaning of forward biased voltage and reverse biased

voltage.

dentify the effects when a 6- +unction is supplied with forward biased

and reverse biased voltage on the following items

C

Area of depletion regionC Kunction resistance

C ?urrent flow (including lea&age current)

Explain why brea&down occurs when 6- +unction is reverse biased.

BAB 1 EE201

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