Fabrication and Characterization of Organic Solar Cell

7/26/2019 Fabrication and Characterization of Organic Solar Cell

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Fabrication and Characterization of Organic Solar Cell

CMOS Transistors

It took almost 20 years after the invention of the bipolar transistor

for MOS to make its appearance. Shockley and many others! had tho"ght

of this device first# it $as or sho"ld have been! m"ch more simple% p"t a

plate close to the s"rface of silicon# connect it to a voltage and move thecarriers inside the silicon electro&statically.

'he problem $as the s"rface of silicon. (ere the silicon atoms are

no longer neatly tied "p $ith each other by sharing the o"termost electrons.'hey face an entirely different material# SiO2 or $orse# some covering

$ith "nkno$n imp"rities mi)ed in!. 'his material doesn*t even have a

crystal str"ct"re# it is amorpho"s. In +,- a start"p# /eneral Microelectronics# felt it had lickedthe

problem $ith CMOS and bro"ght o"t the first digital MOS integratedcirc"it. It $as one of the $orst prod"cts ever to hit the market% a large

portion stopped $orking $ithin days. 'he reason% there $ere elements $iththe silicon&dio)ide chiefly sodi"m! that carried an electric charge and co"ld

move. One day the MOS transistor $as perfectly f"nctional# the ne)t day it

$as permanently t"rned on.It took another fe$ years to gain an "nderstanding of MOS s"rface

physics and make stable MOS transistors. 'oday the silicon s"rface is so

$ell "nderstood that $e can deliberately place a charge into the o)ide layerthat stays there for years# probably even cent"ries. It is no$ the dominant

integrated device# being m"ch smaller than the bipolar transistor. 'he

n"mber of MOS transistors prod"ced every year has long s"rpassed then"mber of ants in the $orld. t the time of $riting this book#

semicond"ctor man"fact"rers prod"ced some 100 million transistors for

every person in the $orld per year!.

'he fig"resho$s a cross&section

of the most often

"sed n&$ell! process.'here are many

variations and

refinements this is

only the basic one.In the gate

area the ins"lating

layer SiO2 or nitride#or a combination! is

thinned do$n and silicon is gro$n on top of it. Since the ins"lator is

amorpho"s# the gro$n silicon is not single&crystal# it consists of many smallcrystals $hich do not fit together very $ell th"s it is called poly&crystalline


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silicon or simply poly!.

3e)t the so"rce and drain regions are implanted# "sing a mask. 'he

inside edges are masked by the gate# so they align perfectly to the gate i.e.they are self-aligning!. 'he device is also self-insulating% as long as the

so"rce and drain are at or above the s"bstrate potential "s"ally gro"nd!# the

4"nctions to the s"bstrate are reverse&biased and no b"lky isolation diff"sionis necessary.

For the p&channel transistor the polarities for the so"rce and drain

implants are reversed and these regions are placed inside an n&typediff"sion. In most applications one s"ch n&$ell hosts many p&channel transistors and is simply

connected to the positive s"pply voltage in this

$ay the devices are ins"lated from each other as long as each so"rce and

drain is at or belo$ the positive s"pply

Fig. 1-18: Cross-section of an N-well CMOS process.

.In both the n&channel and p&channel transistors# so"rces and drains

are identical# i.e. yo" can arbitrarily call one the so"rce and the other the

drain. Or one region can do do"ble&d"ty# being the drain for one transistor

and the so"rce for the ne)t one# connected in series.'he p&channel transistor is al$ays at a disadvantage# beca"se holes

are more diffic"lt to move than electrons. 'h"s it $ill have a lo$er gainthan an n&channel device for the same gate o)ide thickness! and be

some$hat slo$er. MOS

transistors# by the $ay#

are called unipolardevices# beca"se they

employ only one type of

carrier# as opposed by thebipolar transistor# in

$hich both electrons andholes are important forthe operation!.

3o$ let*s look at

an n&channel! MOStransistor in more detail.

'he basic idea is to create

a region a channel!


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bet$een so"rce and drain

$hich has the same

polarity n&type!# so thatthere is direct cond"ction bet$een

the t$o. 'his is done $ith a positive

voltage at the gate $hich p"shesholes a$ay from the s"rface and the

device is called an enhancementmodetransistor there are also

depletion-mode devices in $hich a

channel is implanted or diff"sed and

then c"t off $ith a negative gate

voltage!.'his is tr"e only at zero or

very lo$ drain voltage. s the drain

voltage is increased# a depletion region forms aro"nd it. Since there is no$ a voltage drop along

thechannel# $ith the drain side at a higher voltage than the so"rce# the

depletion region along the channel grad"ally increases to$ard the drain#c"tting more and more into the channel. 'h"s the resistance of the channel

increases.

Fig. 1-19: As the drain voltage is increased adepletion region pinches off the channel.


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Fig. 1-!": #rain c$rrent vs. drain voltagewith the gate voltage held constant.

'he initial slope of thedrain voltage 5 drain c"rrent

c"rve is the resistance of the

channel $itho"t any depletionlayers. 'he final slope at the

highest drain voltage represents

its resistance $ith the depletionlayer almost pinching off the

channel. It is an "nfort"nate

fact that this region is calledthe 6sat"ration region6# $hich

clashes badly $ith the earlier

definition for the bipolar

transistor.bove a certain gate

potential# $hich has to be e)ceeded

to attract any carriers to the s"rface the threshold voltage! an MOStransistor is basically a s7"are&la$ device% do"bling the gate voltage res"lts

in fo"r times the drain c"rrent. 'he meas"re of gain is the

transconductance# drain c"rrent divided by gate voltage. So again# like the

bipolar transistor# this is a non&linear device%

Id k ( )W

L

Vgs VT = 2

$here Id 8 drain c"rrent

k 8 transcond"ctance

9 8 channel $idth


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: 8 channel length

;gs 8 gate&to&so"rce voltage

;' 8 threshold voltageor ;gs & ;' 8 gate voltage above the threshold

'he region belo$ the channel also infl"ences the gain. It forms a

back-gate. In an n&$ell n&channel transistor this is the s"bstrate# commonto all devices. 0?!.


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Fabrication and Characterization of Organic Solar Cell