Field-effect transistor utilizing conducting polymers

.~vnthetic Metals. 28 (1989) ('753 C760

FIELD

EFFECT T R A N S I S T O R

I{. Koezuka Materials

C753

UTILIZING

CONDUCTING

POLYMERS

arld A. Tsumura & Electronic

Tsukaguchi-Honmachi

Devices

8-Chome,

Laboratory,

Amagasaki,

Mitsubishi

Electric

Corporation,

1 i,

Hyogo 0{)i, ga[larl

ABSTRACT The

field-effect

polymer, as

polypyrrole

source

and/or

device. I0 ~

applied heating

polymer

(FET) device

and polythiophene, drain

The source

by

after

transistor

current

gate biases. it in air.

FET can be driven

two kinds

has beer) fabricated.

and p o l y t h i o p h e n e

drain

utilizing

can be

works

as a

of the device

It has been also d e m o n s t r a t e d by p r a c t i c a l l y

small

conducting

Polypyrrole

semiconductor

largely m o d u l a t e d

The stability

of

acts

~n

that

by a factor of is

ca.

excellent

that

the

even

conducting

voltages.

INTRODUCTION Since [i],

Carter

organic

future

functional

materials organic

changed

controllability

characteristics their

polymers

p-type There

of

and

of e l e c t r o n i c their

with those conducting

are

conducting

at will

El(!ctronic

active

physical

D(~vice"

components

properties

of

polymers Their

consisting

and e l e c t r o c h e m i c a l

properties

is

with c o n d u c t i n g

of

long

properties doping

importance

polymers

Among

of

electronic

oJ

organi<:

from such a point of view.

noticeable.

by chemical

the

of the d e v i c e s

and

as

can

methods.

since

the

can be e a s i l y o p t ± m i z e d

fabrication.

A number

compared

Chemical

are e s p e c i a l l y

oll "Molecular

much a t t e n t i o n

investigated

so called

backbones

the concept

received

devices.

compounds,

be c o n t r o l l a b l y

after

have

have been a c t i v e l y

conjugated

This

has p r e s e n t e d

materials

some

0379-6779/89/$3.50

devices

have been d e m o n s t r a t e d

characteristics of the devices

polymers

have been

restrictions

have been made clear

with other

placed

organic

synthesized

utilizing to be

materials.

as stable

on the fabrication

of

more

excellent

However,

polymers the

conducting

only

thus

far.

devices

from

© Elsevier Sequoia/Printed in The Netherlands

C754 practical

standpoints.

conducting

polymers

as a b l o c k i n g

electrode.

use of such r e a c t i v e when the device We

metals.

chosen a t r a n s i s t o r

(ii) the device The f a b r i c a t i o n

attempted

[3],

Conducting

but

to

consisting

a

drain

materials

as a semiconductor.

can be m o d u l a t e d

The

of the d e v i c e

(FET) with

five-membered

in

as a s e m i c o n d u c t o r

of

passed between

10 2 with e x t e r n a l l y

air. been

the

FETs

gold layers

an FET

in

have

reported

and stable

consisting

The current

(CH) X has been

heterocycles

thin film is the first actual

by m o r e than

of r e a c t i v e

stability

have a l r e a d y

is

semiconducting

the u t i l i z a t i o n

problem

conducting

transistor

for d e p o s i t e d

transistor

has a

The d e v i c e

for

as

a

electrochemically utilizing

organic

the source

applied

and

the

gate biases

in

polypyrrole,

to

[5,6].

In this study, the

of

device

(i) the t h i n - f i l m

without

The a u t h o r s

[4-6].

polythiophene

both

p-type

to e a s i l y oxidize.

suitable

application

(CH) X itself

polyheteroc~cles

and/or

that device

with

to the d e t e r i o r a t i o n

points:

of a f i e l d - e f f e c t

deposited

drain

leads

diodes

such as indium and a l u m i n u m

have a t e n d e n c y

as the m o s t

promising

be very stable.

stable

source

type

metals

can be f a b r i c a t e d

the

polymers

recognized using

therefore,

in the light of the f o l l o w i n g

[2];

Schottky

function

Such m e t a l s

metals,

one of the most c o m m e r c i a l l y films

example,

low work

is left in air.

have

polymers

For

require

source

semiconductor fabrication

in

we have a p p l i e d drain

another in

conducting

and

the

the

course of the a t t e m p t

addition

to

polymer,

polythiophene

toward all

of a new type FET with c o n d u c t i n g

polymers

plastic and its

used

as

FETs.

a The

characteristics

are reported.

DEVICE

FABRICATION

The Fig.

schematic

i.

An

cross-sectional

n-type

silicon

v i e w of a newly

wafer

(4 - 8 o h m cm,

fabricated -axis,

~-~)x (semiconductor) Au

~(source) H\

\

\

/

\

Si02

/

/

(drain) <\ \

/'

/

/H

/ /

n - 5.

Fig.

i. C r o s s - s e c t i o n a l

gate v i e w of the newly

alloy

fabricated

FET device.

FET is 380

shown Hm

in

thick)

C755

covered

with t h e r m a l l y g r o w n SiO 2 (3000 A thick)

couple

of

layers

gold

layers

(200 A thick),

(2 mm x 4 mm,

separated

300 A thick)

make an ohmic

substrate.

undercoated

with

A

chromium

After p a r t l y r e m o v i n g the other

G a - I n alloy was a t t a c h e d

c o n t a c t with

used as a

from each o t h e r by 6 ~m, were i n s t a l l e d o n t o one

side of the SiO 2 layers by usual ways. the SiO 2 m e c h a n i c a l l y ,

was

it. So that

to the n a k e d s i l i c o n

the s i l i c o n w a f e r

side o[

surface

itself works as a

to gate

electrode. Polypyrrole

(3600 A thick)

was galvarlostatically s y n t h e s i z e d by using

gold (~lectrodes on the SiO 2 as an anode. out

in an a c e t o n i t r i l e

toluenesulfonate.

Using

two

prepared polypyrrole

polythiophene

film

layers as an anode,

rlot

only two p o l y p y r r o l e

used as a s t a r t i n g monomer,

the

FET

function

effect,

that

modulated

by

conditions

polythiophene ions

CiO 4

as was

saturated calomel The

2,2'-bithiophene

The p o l y m e r

completely

them.

If

Otherwise,

the d e v i c e do~s not

current passed between

source

been

adopted

as

a semiconductor electrochemically electrode

previous [5].

display

and

thiophene

15,6].

ones

The h i g h l y

for

The the

doped

u n d o p e d at a potential

not

same

of

0

V

be

undoping

only

hav~ng

polythiophene

with

against

a

(SCE) used as a s t a n d a r d r e f e r e n c e .

c h a r a c t e r i s t i c s of the d e v i c e so f a b r i c a t e d were m e a s u r e d

using a Y o k o g a w a - H e w l e t t

make [ield-

can

FET

thin

to

any

drain

a

coated

the SiO 2 w o u l d not be c o v e r e d with such a

The p o l y t h i o p h e n e so p r e p a r e d had to be u n d o p e d

the

was

solution

The e l e c t r o p o l y m e r i z a t i o n y i e l d e d

any gate biases as f o r m e r l y r e p o r t e d

have

p toluenesulfonate

in an a c e t o n i t r i l e

layers but also the SiO 2 b e t w e e n

well.

is,

conditions

(ca. 600 A thick).

was

film.

p

layers.

including t e t r a e t h y l a m m o n i u m p e r c h l o r a t e .

polythiophene

p y r r o l e an<] t e t r a m e t h y l a m m o n i u m

The d e p o s i t i o n of p o l y p y r r o l e doped with

e l e c t r o p o l y m e r i z e d under g a l v a n o s t a t i c a l

thin

The e ] e c t r o p o l y m e r i z a t i o n was c a r r i e d

solution containing

ions o c c u r r e d o n l y on the gold

two

in v a c u o

by

P a c k a r d 4140B pA/dc v o l t a g e source.

R E S U L T S AND D £ S C U S S I O N The with

fabricated device

the c o n d u c t i o n c h a n n e l

Polypyrrole as

a

the drain.

s i l i c o n wafer, leads

to

field-effect

Two gold The

layers o n t o the SiO 2 are

film works

leads from the

source

i n s u l a t o r and the gate e l e c t r o d e c o n s i s t of SiO 2 and

respectively.

layer

transistor

l e n g t h of ca. 6 ~ m and the c o n d u c t i o n w i d t h of 2 mm.

The u t i l i z a t i o n of the thick s e m i c o n d u c t o r

large leakage c u r r e n t even under no gate bias

semiconductor undoping

i) is an i n s u l a t e d gate

l a y e r s act as a s o u r c e a n d / o r a drain and a p o l y t h i o p h e n e

semiconductor.

and/or

give

(Fig.

(ca.

600 A thick)

was p r e p a r e d

in

[6]. this

Thus, study.

the

a

layer thin The

c o n d i t i o n s as above d e s c r i b e d for p o ] y t h i o p h e n e have been r e p o r t e d to -7 -8 1 (i0 - I0 S cm ) in the s e m i c o n d u c t i n g r e g i m e [5,6].

the c o n d u c t i v i t y

C756 On

the

other hand,

determined undoped

to

the onset p o t e n t i a l

be b e l o w

at the p o t e n t i a l

high c o n d u c t i v i t y Fig.

2 shows

-0.2 V vs.

of p - d o p i n g

SCE by

cyclic

for

of 0 V vs. SCE is still p - d o p e d

to work as a source and a drain the typical

source

current

polypyrrole

voltammetry.

has

been

Polypyrrole

so that

it has

in place of metal

enough

electrodes.

(I S ) - drain voltage(VDs)

curves

at

A

< -I.5 -50

m

{/} l-4

-I

-40 -5

-30 °

00

Fig.

-10

2.

-20 -30 VDS ( V )

I S - VDS c h a r a c t e r i s t i c s

-40

-50 °

of the FET d e v i c e

at

various

gate

voltages

(VG).

various assume

gate the

voltages

(VG)for

equivalence

When the V G

is zero,

the drain

voltages

voltages

are

of the source

applied

indicates

saturation

is clearly

off).

In

this

paper,

we

(I S ) to the drain current (ID). -i0 I0 A even w i t h increasing

to b e l o w

enhanced

as the

negative

The e n h a n c e m e n t

of the I S

under

negative

character

observed

FET.

The I S is @reatl~

to the gate. p-type

fabricated

current

the I S is d e p r e s s e d

(normally

gate biases

the newly

of the p o l y t h i o p h e n e

in high drain

voltages.

film.

This

Good current

I S saturation

in

C757

high

drain voltage region suggests the o c c u r r e n c e of pinch-off near

~lectrode like conventional I

S This

inorganic

FETs.

Under a constant V

can be m o d u l a t e d by more than 103 when the V modulation

ratio

G

the

drain

of -50 V the

DS is varied from 0 V to

is larger compared with that for

the

FET

60

using

V. only

polythiophene as a s e m i c o n d u c t o r layer. In

conventional

saturation

inorganic t h i n - f i l m FETs, the

regiorl is expressed as fol~ows

(I D) is equal to the source current W~Cox I S = ID

drain current

{]D )

[ /], assuming that the drain

in

the

current

(Is).

2 (V G - Vth)

(I)

2L V

repres(,nts the threshold gaLe voltaqe at which the conduction channel begins th to be formed. W and L are the conduction width and the conduction channel

length, respectively.

COX stands for the c a p a c i t a n c e of the gate insulator and

is the carrier m o b i l i t y in the channel. Fig.

3

exhibits the I~_~

VDS

(: V G) c h a r a c t e r i s t i c s

[or our

device

with

£3 %

/ /

Vth

VOS (" VG) (V)

Fig. 3. ~ S vs~ VDS (=V G) plots for the FET device with interconnected gatedrain electrodes. The threshold voltage (Vth) of the device is d e t e r m i n e d to be -16 V by e x t r a p o l a t i n g the straight line toward the VDS axis.

i n t e r c o n n e c t e d g a t e - d r a i n electrodes. voltages mechanism line

is

carrier

as e x p e c t e d from eqn (i).

A straight line is drawn for high This result indicates that

is similar to that of conventional extrapolated

toward the VDS

the

inorganic thin-film

(=V G) axis to yield Vth

FETs.

(-16

drain

operation

V).

m o b i l i t y can be calculated from the slope and eqn (i) to be 6.9 x

The The 10 -5

C758 cm 2 v-lsec -I. the

previous

The ~ is larger than that (ca~ 10 -5 cm2v-lsec -I FET, which leads to larger m o d u l a t i o n ratio

for

in the case the

of

fabricated

device in this study. Important p a r a m e t e r s for FETs,

e_t g. t r a n s c o n d u c t a n c e gm in the saturation

region and the m a x i m u m o p e r a t i n g frequency

f are d e f i n e d by eqn max

(2) and

(3),

respectively.

gm .

8Is I . . . ~V G V D s : C O n s t

W~Cox (V G - Vth)

(2)

L

gm

f

(3)

max 2ZCox LW

The

gm

value has been d e t e r m i n e d to be i0 nS, which has been improved

increasing (3), f

amount of the carrier mobility.

by

the

Using above d e t e r m i n e d gm and

eqn

can be c a l c u l a t e d to be ca. 1 KHz.

max The stability

is an important factor from

practical

device as above fabricated was h e a t - t r e a t e d in air. in

the i m p r o v e m e n t of the c h a r a c t e r i s t i c s rather than the

4).

The I S is d e p r e s s e d b e l o w I0 p A under zero V G.

increases

up

to the level near microampere.

with applied gate voltages,

therefore,

standpoints.

The

The treatment has resulted deterioration

(Fig.

When the V G is -50 V

The m o d u l a t i o n ratio of

reaches about 105 .

the

In other words,

conductivity of the p o l y t h i o p h e n e film can be greatly and r e v e r s i b l y

IS I

S the

controlled

ide

-~ ,6'

Id ,60

-,b

-~o

40

-~

-so

VG ( v )

Fig. 4. I s vs__t.V G plots for the FET device before (O) and after (O) heating device at 120 °C for 1 day in air. The drain voltage (VDs) was -50 V.

the

C759 by e x t e r n a l l y enlargement V - i s e c -I)

applied

voltages

of the m o d u l a t i o n of the carrier

be 28 nS a c c o m p a n y i n g Although been

the

obtained

treatment

treatment chains

the raise

heat-treatment

has b e e n

from

reported

thiophene

gets

length

shorter

~ -con3ugation

gives

wide

before

no

evidence

and after

con iugation

It is, therefore, o]igomers

polymers,

electropolymerized ~

and

length

considered leaves

length.

band width,

to

mobility.

of both p o l y m e r s

to have

[8 ].

large

of the carrier

Polythiophene

The

The gm is also improve<]

often c r y s t a l l i z e

rid of the v o l a t i l e

with relatively

of by c o n v e n t i o n a l

in the channel.

on the c r y s t a l l i n i t y

conjugation mobility

doping methods. -4 2 is due to the increase (2.0 x i0 cm

ratio

mobility

by X-ray diffraction.

bithiophene prepared

with

instead

This

which

that

behind

to

the

heat-

from

2,2'-

than the

that heat

polythiophene

is because

leads

has

the

high

large carrier

[9].

(o]

10-7

o--

/~

[b) .=o

1¢1

i0-I

// ~O't

io-~

IO'

I

I

I

I

i

I

VO (V]

Fig. 5. Effect of the gate i n s u l a t o r t h i c k n e s s on I S - V G characteristics p o l y t h i o p h e n e FETs under c o n s t a n t V D of -i0 V. M e a s u r e m e n t s were carried after h e a t i n g each d e v i c e at 120 C ~or 1 day in air. The channel width the channel length are 1 mm and 3 ~m, respectively. T h i c k n e s s of gate insulator (A): (a) 600, (b) 900, (c) 1300.

The device VDS

and

V G.

polythiophene

mentioned

above

can o p e r a t e

We have also c o n f i r m e d under

small

applying

only by a p p l y i n g

the o p e r a t i o n

voltages.

unpractically

of the EET

A new another

utilizing device

of out and

large on]y

substrate

C760 has

been f a b r i c a t e d

width

of

I

mm

characteristics applied. and

are

saturates

changed

under

from 0 to -20 V.

polymers

of the current reached

operating

5, where

voltages

There

This

between

105 .

current

a problem

FETs with

conduction The

operating

V Of -i0 V DS raises with the having

that the

is VG the

is, the better

is the

FETs with

voltages.

polymers,

polypyrrole

The m o d u l a t i o n

by e x t e r n a l l y

conducting

device

the gate v o l t a g e

layer

of the FET device.

applied

that

ratio

gate biases

practically

low

This

of

polymers.

type

it in air.

that the source

current

with that of c o m m e r c i a l l y

that this p r o b l e m

when

the i n s u l a t i n g

and drain

the

for the device

It has also been d e m o n s t r a t e d

can drive the

in c o m p a r i s o n

constant

I S steeply

by 4 orders

under p r a c t i c a l

even after h e a t i n g

remains

a small

two types of c o n d u c t i n g

source

3 ~m, layer.

study has d e m o n s t r a t e d

for the f a b r i c a t i o n

passed

FET stably works

The thinner

become.

length of

insulating

thin,

The source

can be driven

about

of

layer becomes

we have u t i l i z e d

and p o l y t h i o p h e n e

small

in Fig.

small V G.

characteristics

In summary,

believe

shown

channel

thickness

layer of 600 A can be m o d u l a t e d

conducting

has

various

As the i n s u l a t i n g

insulating

device

w i t h the c o n d u c t i o n

and

will be o v e r c o m e

for our device

available

in the near

a-Si

is

~till

FET devices.

We

future.

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