HOT CARRIERS IN REDUCED GEOMETRY SURFACE-CHANNEL CHARGE-COUPLED DEVICES

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HOT CARRIERS IN REDUCED GEOMETRY SURFACE-CHANNEL CHARGE-COUPLED

DEVICES

André Touboul, J. Lopez, G. Lecoy

To cite this version:

André Touboul, J. Lopez, G. Lecoy. HOT CARRIERS IN REDUCED GEOMETRY SURFACE-

CHANNEL CHARGE-COUPLED DEVICES. Journal de Physique Colloques, 1981, 42 (C7), pp.C7-

183-C7-192. �10.1051/jphyscol:1981721�. �jpa-00221658�

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HOT CARRIERS I N REDUCED GEOMETRY SURFACE-CHANNEL CHARGE-COUPLED DEVICES

A b s t r a c t : I n a surface-channel C.C.D., e l e c t r i c f i e l d s have been computed w i t h the help o f a f i n i t e - d i f f e r e n c e method. The normal f i e l d , higher than the c r i t i c a l v a l u e , can create impact i o n i z a t i o n under t h e gates but not i n the gaps. Nevertheless t h i s f i e l d i s not c r u c i a l f o r the C.C.D. o p e r a t i o n as a s h i f t r e g i s t e r . The l o n g i t u d i n a l f i e l d , defined as the sum o f two components, i s higher than the c r i t i c a l f i e l d but a t the very beginning of t h e t r a n s f e r . A l l those r e s u l t s have been found again f o r a t h e o r e t i c a l device w i t h 1.5 \m gate l e n g t h .

Nevertheless t h i s heating o f t h e c a r r i e r s i s found t o be the major l i m i t a t i o n to the maximum t r a n s f e r frequency.

I . INTRODUCTION

Reducing the dimensions o f surface-channel Charge-Coupled Devices has two main purposes : i n c r e a s i n g t h e d e n s i t y o f i n t e g r a t i o n and o p e r a t i n g the devices a t higher t r a n s f e r r a t e s .

As the charge t r a n s f e r i s c o n t r o l e d by t h e i n t e r f a c e e l e c t r i c f i e l d s , we may expect hot c a r r i e r s which w i l l consequently a f f e c t t h e t r a n s f e r and/or t h e storage behaviour o f the d e v i c e . This work has been focused on the t r a n s f e r o f f r e e c a r r i e r s i n a surface p-channel C.C.D. presenting a very simple e l e c t r o d e s t u c t u r e : one l e - vel o f m e t a l l i c ( A l ) gates w i t h i n t e r e l e c t r o d e gaps. The values o f the e l e c t r i c f i e l d s under t h e gates and i n the gaps could be such t h a t impact i o n i z a t i o n mecha- nisms, hot c a r r i e r s i n j e c t i o n i n t o SiO„ or m o b i l i t y s a t u r a t i o n (and then v e l o c i t y s a t u r a t i o n ) might take place and reduce t h e t r a n s f e r speed. An actual sequence o f t r a n s f e r used f o r C.C.D.'s o p e r a t i o n i s described.The clok pulses V Q I and VQ2 a p p l i e d JOURNAL DE PHYSIQUE

Colloque C7, supplément au n°10, Tome 42, octobre 1981 page C7-183

A. Touboul, J.C. Lopez and G. Lecoy

Centre d'Etudes d'Electronique des Solides, associé au C.N.B.S., Université des Sciences et Techniques du Languedoc, 34060 Montpellier Cedex, France

Résumé : Pour un C C D . à canal en surface, les champs électriques à l'inter- face ont été calculés par une méthode numérique (différences finies). Le champ normal supérieur au champ critique peut donner lieu à de l'ionisation par impact sous le milieu des grilles mais pas dans les espaces interelectrodes.

Néanmoins, le rôle de ce champ a peu de conséquences sur le fonctionnement en registre à décalage de ces dispositifs. Le champ longitudinal, défini comme la somme de deux composantes n'est supérieur au champ critique que pour une très faible partie du stade initial du transfert. Tous ces résultats ont été retrouvés sur une structure théorique ayant une longueur de grille de 1.5 ym.

Néanmoins, cet effet de chauffage des porteurs est la cause principale de la limitation de la fréquence maximum des transferts.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981721

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C7-184 JOURNAL DE PHYSIQUE

t o

two adjacent gates a r e represented i n Fig. 1.

The following sequences will be considered : i ) the c a r r i e r s are stored under

the gate G l ; the VG1 and VG2 s i - I I I I I

gnals a r e respectively equal t o Vst and Vr.

i i ) the VG2 signal swings from Vr t o Vst : the charge t r a n s f e r

t I l l

I 4 I I

s t a r t s . I 1 1 ;

I

&!

ⁱ ¹

i i i ) VGl and VG2 a r e equal du- 1 ^I

;tt : ;

ring the clock pulses overlap. v

;'"

i v ) the t r a n s f e r i s finishing : I

I

^I

f

L e n d of the

VG1 goes to V r . ^II I1

'

' 1

' I

^I transfer

I

For most of clock f r e -

I I ; \ : r

^----!J^st

quencies, these two clock s i - {pulse overlap

gnals present t h i s a n d Fig. 1 : Clock pulses diagram f o r a complete a ~ i s e time

z,

much smaller than sequence of t r a n s f e r (3-phase C.C.D.).

the f a l l time T~ (see f i g . 1 ) .

In order t o evaluate the e l e c t r i c f i e l d s , we have developed a simulation of the elementary t r a n s f e r process between two gates. This 2-D model of the device ope- ration includes the e f f e c t of the interelectrode gap and takes i n t o account the field-dependent mobility.

A 1-D model of the r1.O.S. s t r u c t u r e has given r e s u l t s supporting a "sheet- model" f o r the inversion channel where the f r e e c a r r i e r s a r e located.

2. CHARACTERIZATION AND CALCULATION OF THE ELECTRIC FIELDS Normal components

E N of the f i e l d will be distinguished from longitudinal ones EL ( i n the t r a n s f e r direction :

6

a x i s ) .

To c a l c u l a t e the strength of the f i e l d s , we need a physical model of the s t r u c t u r e defined by the following assumptions :

i ) the oxide layer (thickness : do,) i s ideal i i ) the Si-Si02 i n t e r f a c e i s f r e e of traps

j i i ) we have neglected the quantum e f f e c t s associated with the inversion layer popu- l a t i o n

[

1 1

i v ) f o r the considered i n t e r v a l s of time which control the t r a n s f e r sequences, thermal generation of c a r r i e r s i s not taken i n t o account.

v ) the majority c a r r i e r current i s negligible.

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This s e t of equations i s used f o r a n-type s i l i c o n s u b s t r a t e (p-channel) AV = 0

f o r 0 < y < do,

The boundary c o n d i t i o n s a r e :

" F

~ ( x , y ) + 0 f o r Y ⁺

I

metal (A])

~ ( x , y ) ⁺0 f o r Y ⁺ ^rn

-

p ( x , y ) + 0 f o r Y ^-+ rn S i 02

dox n-type Si

The r e s o l u t i o n of t h e s e equations by means of a f i n i t e d i f f e r e n c e s method under one g a t e leads t o t h e normal f i e l d E N a f t e r t h e o n s e t of t h e deep d e p l e t i o n regime and an i d e a l i n j e c t i o n of minority c a r r i e r s ( t h e s i g n a l ) r e p r e s e n t i n g t h e inversion l a y e r population. The r e s u l t s o f t h i s c a l c u l a t i o n s imply a 1-D m o d ~ l f o r t h e transversal f i e l d : 99 % of t h e s i g n a l c a r r i e r s a r e located i n about 120 A under t h e oxide l a y e r f o r a voltage swing of 10 V on

the

g a t e . For a doping d e n s i t y of N D = 10 15 /cm3, t h e normal f i e l d E N and t h e signal charges

Qs

d i s t r i b u t i o n a r e p l o t t e d i n Fig. 2.

Fig. 2 : Normal f i e l d En and c a r r i e r d e n s i t y QS a t t h e i n t e r f a c e .

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C7- 186 JOURNAL DE PHYSIQUE

Eqs. (1) t o (4) become e a s i e r to solve and the longitudinal f i e l d s can be cal- culated anywhere along the i n t e r f a c e when considering a "sheet-model" f o r the charges and an i n f i n i t e depleted region. The structure under t e s t i s represented in Fig. 3 : i t i s periodic. Different gate lengths ( L : 1 2 + 4

urn)

and gaps ( 2 + 0 . 4

urn)

have been considered. A theoretical s t r u c t u r e ( L = 1.5

urn

and g = 0.1 ym) has a l s o been investigated t o point out s p e c i f i c gate shortening e f f e c t s .

Figure 3 : Longitudinal view of the s t r u c t u r e used f o r the t r a n s f e r model.

gap G1 G 2

The charge t r a n s f e r in the x-direction i s expressed by (3) and ( 4 ) which

l 1 . I t - L 9

give :

J~ ( x , t ) = q

u

{ ~ ~ ( x , t ) p ( x , t )

-

P

'-/2

i

4 A l

The e l e c t r i c f i e l d E L ( x , t ) i s the sum of an external component Ee(x,t) due t o the neighbouring gate voltages C2

]

and of an internal component Ei ( x , t ) induced by the charges

r

³

^1;

^{E i}( x , t ) i s evaluated from the a n a l y t i c expression derived by Carnes

[

3 1 on the basis of the surface potential 1 4

]

:

E i ( x , t ) = - a a p ( x , t )

a x

⁽¹¹⁾

do, =0;12,1~ m

I

n-type S ~ ( N ~ = I O ' ~ C ~ - ~ ) S ~ O ;

The knowledge of VG1 and VG2 corresponding t o the d i f f e r e n t t r a n s f e r sequences applied t o eqs ( 1 ) and ( 2 ) leads t o Ee.

Then ( 8 ) , ( 9 ) and (10) can k solved everywhere t o give the c a r r i e r density p ( x , t )

.

^The

numerical methods used f o r a l l the d i f f e r e n t i a l equations a r e based on a f i n i t e d i f - ference scheme

E,

^{6 1 .}

In the interelectrode gaps, we have derived an analytic expression of the potential which i s a function of the two adjacent gate voltages, the oxide thickness, and the depletion layer depth.

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3. EFFECT OF THE NORMAL FIELD EN

For t h e devices described i n s e c t i o n Z ) , we have c a l c u l a t e d t h i s f i e l d a t t h e i n t e r f a c e . When t h e r e i s no c a r r i e r i n t h e s u r f a c e w e l l , t h e value o f EN under t h e middle o f a gate i s 5.0 x 10 v/cm f o r 4

lvGl

⁼10 V. The presence o f m i n o r i t y c a r r i e r s increases s t r o n g l y t h i s f i e l d and i t reaches 3.5 x 10 v/cm f o r a c a r r i e r d e n s i t y

5

o f 1.5 x 10 /cm2. This s i t u a t i o n i s r e p o r t e d i n f i g u r e 2. 12

I n t h e gaps between two gates, a p o t e n t i a l b a r r i e r takes place. When t h e two gates are biased a t t h e same p o t e n t i a l ⁼ I V G 2 1 = 10 V, t h i s b a r r i e r AVs i s a s t r o n g f u n c t i o n o f t h e gate l e n g t h L and o f t h e gap w i d t h g. The d i f f e r e n t values o f AVs and EN i n t h e gaps a r e given i n t a b l e 1.

Table 1 : Values o f t h e p o t e n t i a l b a r r i e r and EN i n t h e gaps.

The e f f e c t o f t h e gate f i e l d has been e x t e n s i v e l y s t u d i e d i n M.O.S. s t r u c t u r e s

,

8

,

9 7

.

We s h a l l p o i n t o u t i t s s p e c i f i c e f f e c t s on C.C.D.'s operations.

i ) For these t y p i c a l values o f t h e gate voltages (IVst

-

VrI= 10 V), t h e doping 15 3

d e n s i t y (ND = 10 /cm ) and o f t h e o x i d e thickness, t h e normal f i e l d i s always l e s s than t h e o x i d e breakdown f i e l d , nevertheless i t i s s t r o n g e r than the c r i t i c a l f i e l d E which i s 1.8 x 10 v/cm f o r holes 4

[lo]

and can even produce impact i o n i z a t i o n

[: P

11

1

under t h e gates b u t n o t i n t h e gaps. This e f f e c t c o u l d occur b u t f o r more doped s u b s t r a t e s . Nevertheless, t h e r e s u l t i n g change i n c a r r i e r d e n s i t y would n o t be o f a g r e a t importance ( l e s s than on t h e f i r s t - o r d e r o p e r a t i o n of C.C.D.'s.

ii) Another consequence o f t h i s normal f i e l d c o u l d be the i n j e c t i o n o f h o t c a r r i e r s i n t o Si02.As t h e C.C.D.'s under study are implemented on n-type substrate, t h e f r e e c a r r i e r s are holes. The energy b a r r i e r a t the i n t e r f a c e i s o f t h e order o f 3.8 eV

r

8 1 and reduces t h e emission p r o b a b i l i t y . This i n j e c t i o n c o u l d a r i s e from "Channel Hot C a r r i e r s " when t h e c a r r i e r s a r e t r a n s f e r r i n g along t h e device o r from "Substra- t e Hot C a r r i e r s " f o r some t h e r m a l l y generated c a r r i e r s .

Anyway, t h e s h i f t AVT o f t h e t h r e s h o l d v o l t a g e does n o t c o n s t i t u t e a very c r u c i a l l i m i t a t i o n because t h e device working as a delay l i n e i s always biased i n t h e deep d e p l e t i o n regime f o r times s m a l l e r than those r u l i n g t h e thermal genera- t i o n . A s i m p l i f i e d c a l c u l a t i o n o f t h i s s h i f t AVT f o r the above-mentioned data gives

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C7-188 JOURNAL DE PHYSIQUE

values l e s s than 10-2 V.

4. ANALYSIS OF THE LONGITUDINAL FIELD EL DURING THE TRANSFER

The v a r i a t i o n o f t h e m o b i l i t y w i t h t h e e l e c t r i c f i e l d E L ( x ) i s introduced by t h e r e l a t i o n :

1 / B

up

= uo

/{I

+ L E ~ ( X ) / E ~ J ~ }

where po i s the "low f i e l d " i n t e r f a c e m o b i l i t y taken equal t o h a l f t h e b u l k value, Ec i s t h e c r i t i c a l f i e l d and B a parameter equal t o 2 as r e p o r t e d i n

[ l o ]

, p 2 J

.

I n t h i s section, we s h a l l comment t h e r e s u l t s o f t h e f i e l d c a l c u l a t i o n f o r t h e d i f f e r e n t stages o f t h e t r a n s f e r .

i ) storage under t h e g a t e G1 :

The t r a n s f e r s i m u l a t i o n i s i n i t i a t e d w i t h an a r b i t r a r y d i s t r i b u t i o n o f car- r i e r s under the e m i t t i n g gate G 1 under which t h e i r d e n s i t y i s s e t constant.

The values o f t h e e x t e r n a l f i e l d Ee a t t h e middle o f t h e gaps a r e very dependent on t h e gap w i d t h g : Ee = 2.2 ^-t9.43 x 10 V/cm when g i s reduced from 2 t o 4 0.4 um ( g a t e v o l t a g e :

-

10 V). A t t h e edges of t h e gates, t h i s component Ee i n - creases a l s o when g decreases (Ee : 2.62 ^-t7.91 10 V/cm) whereas i t i s n e a r l y un- 4 s e n s i t i v e t o v a r i a t i o n s o f t h e gate l e n g t h L from 12 t o 4 um. For a t h e o r e t i c a l de- v i c e ( L = 1.5 pm and g = 0.1 pm), t h e c a l c u l a t i o n gives f o r a gate v o l t a g e o f

-

5 V:

Ee = 7.61 x 10 V/cm 4 i n t h e mid-gap {Ee = 5.4 x 10 V/cm a t t h e gate edges 4

These values o f Ee a l l o w us t o expect f i e l d s h i g h e r than 10 V/cm f o r gate 5 voltages g r e a t e r than

-

5 V.

I t should be n o t i c e d t h a t a l l t h e values o f Ee a r e h i g h e r than t h e c r i t i c a l f i e l d Ec.

The i n t e r n a l f i e l d Ei has a v e r y s i m i l a r magnitude b u t i t s s i g n i s always op- p o s i t e t o t h e one o f Ee. Under t h e same b i a s c o n d i t i o n s and a t t h e same l o c a t i o n s , t h e computed values a r e :

E. = 5.42 ^+- 9.2 x 10 V/cm i n the mid-gap 4

1 4

{Ei = 1.81 ^f5.4 x 10 V/cm a t the gate edges f o r g v a r y i n g from 2 t o 0.4 urn.

For t h e 1.5 um device, Ei compensates e x a c t l y Ee i n t h e gaps.

The r e s u l t i n g f i e l d EL (eq. 9) i s s m a l l e r than any o f the two components b u t s t i l l h i g h e r than Ec a t the gate edges. T h i s f i e l d EL w i l l p l a y an i m p o r t a n t r o l e d u r i n g the n e x t two stages. I n f i g u r e 4, t h e r e s u l t i n g f i e l d EL and t h e two components Ee, Ei are p l o t t e d f o r a 4 pm gate l e n g t h device.

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F i g . 4 : The l o n g i t u d i n a l f i e l d EL and t h e two components E i

,

Ee a t t h e beginning o f t h e t r a n s f e r .

EL,,,

, = 2 x ~ O ~ V / C M ; Ee,max = 9.43 X 104 V/CM ; E i ,max = 9.23 x 104 V/CM.

I !

i i ) Rise time of V G ~ and c l o c k pulses o v e r l a p

-1 -

T.'

When VG2 i s equal t o Vst

,

EL i s s t i l l h i g h e r than Ec ; then, the f i r s t p a r t o f t h e t r a n s f e r r e d c a r r i e r s w i l l reach t h e s a t u r a t i o n v e l o c i t y o n l y i n t h e gaps.

This w i l l l a s t from 1 t o

lo-'

nsec according t o t h e e l e c t r o d e dimensions which impo- se t h e c l o c k frequency. A t t h e beginning o f t h e pulses overlap, t h e values o f EL are equal t o 1.8 10 V/cm a t t h e gate edges f o r t h e 4 pm device. 4

For the 1.5 vm device, t h i s f i e l d i s more important (even f o r VG =

-

5 V) : EL = 2.75 x 10 V/cm i n t h e gap 4

1

EL = 3.12 x 10 V/cm a t t h e G2 gate edge 4

The e x t e r n a l f i e l d Ee decreases t o lower values i n t h e gaps and i t s d i r e c t i o n changes a t t h e exact mid-gap. The t r a n s f e r i s c o n t r o l e d e x c l u s i v e l y by Ei.

The r e s u l t i n g f i e l d EL a t t h e end o f t h e o v e r l a p has t y p i c a l values o f 3 t o 6 x 10 V/cm f o r any device. An e q u i l i b r i u m i s reached f o r t h e c a r r i e r s d i s t r i b u t i o n 3 under t h e two gates (see F i g . 5 ) . An extremum i s obtained i n t h e midgap :

Fig. 5 : D i s t r i b u t i o n o f t h e c a r r i e r s a t t h e end o f t h e pulse o v e r l a p stage

( t h e i n d i c a t e d time gives t h e dura- ~ ( 0 . 6 n r e c )

t i o n o f t h e overlap)

A : L = 1 2 ~ m ; g = 2 urn -.

B : L = 4 v m ; g = 0 . 4 v m I

- - -

C : L = 1 . 5 ~ ~ ; g = 0.1 pm ^{e ) :} ^,t' ---'.

1 '

I

, '

O b l l l i

'

^I ¹ ⁸

I

C ~ I I

-WB

t GI L P G2

I

A

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C7- 190 JOURNAL DE PHYSIQUE

consequently E i will be null and change i t s direction f o r gaps larger than 0.1pm.

This i l l u s t r a t e s the e f f e c t of the potential b a r r i e r AVS in the gaps reported in t a b l e 1, of section 3.

i i i ) Fall time of VG1 : f i n a l stage of the t r a n s f e r

During t h i s stage, i n s p i t e of the importance of the external f i e l d Ee, EL i s l e s s than Ec f o r any device because of the values of E i . These 3 f i e l d s are reported i n Fig. 6 f o r VG1 = VG2/2

The calculation gives very high values f o r EL a t the r i g h t edge of G2 but as no t r a n s f e r i s possible, these f i e l d s have no e f f e c t on the c a r r i e r s .

5. CONCLUDING REMARKS

We have shown t h a t the e f f e c t s of EL are more important than the ones of the normal f i e l d E N when considering the c a r r i e r s t r a n s f e r .

i ) The calculation of the two components Ee and Ei of EL have given in most of the cases values higher than Ec (except a t the end of the overlap). Meanwhile, an effec- t i v e heating of the c a r r i e r s was found possible only during a very l i t t l e f r a c t i o n of the t r a n s f e r sequence because of the simultaneous action of these two opposite f i e l d s .

This i l l u s t r a t e s the obligation of considering both influences of the charge d i s t r i b u t i o n and of the external f i e l d a s was suggested i n [ 1 3 ] .

i i ) To point out the e f f e c t of the field-dependent mobility (eq. 12), we have compa- red the computed c a r r i e r density when assuming a constant mobility with the previous- l y described r e s u l t s . We have selected the r i s e time of VG2 and the pulse overlap stages during which we could show t h a t t h e heating of the c a r r i e r s happened. A t the

'Lax

E

¹

end of the VG2 r i s e time, the c a r r i e r density under t h e center of G2 i s only Fig. 6 : Variation of

the resulting f i e l d E L ,

Ee and Ei f o r

0.5

V G ~ = V~2/2 =

-

5 V ( a t ha1 f t h e fa1 1 time of VG1).

0

- E.

11

n

I

1;

- I I

-

I

!

-

_' 'L

t

$

'

gap

11

\ E ~ g a P

Ee

i

!

.

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1.2 x 10 8 whereas t h e c a l c u l a t i o n , on t h e basis o f t h e constant m o b i l i t y , g i - 9 -2

ves 7.3 x 10 cm

.

A t t h e r i g h t edge o f G2 where no c a r r i e r i s found, t h e same c a l c u l a t i o n gives 8 . 8 x 10 4

A t t h e middle o f t h e o v e r l a p stage, t h e c a r r i e r d i s t r i b u t i o n s are v e r y s i m i - l a r a t t h e l e f t edge and under t h e c e n t e r o f G 2 whereas t h e d e n s i t y i s t h r e e times h i g h e r a t i t s r i g h t edge. T h i s a c c e l e r a t i o n o f t h e t r a n s f e r i s c l e a r l y r e l a t e d t o the use o f a constant m o b i l i t y .

Furthermore, t h e e q u i l i b r i u m i s reached i n 60 psec i n s t e a d of 80 psec under t h e two gates G1 and G2 e q u a l l y biased a t Vst.

These c a r r i e r d i s t r i b u t i o n s have been p l o t t e d i n F i g . 7 f o r t h e two values o f t h e m o b i l i t y .

Fig. 7 : Comparison o f t h e c a r r i e r d i s t r i b u t i o n under GI and G2 a f t e r the r i s e time o f VG2 and a t t h e o v e r l a p f o r d i f f e r e n t mo- b i l i t y laws :

A : constant m o b i l i t y vp=vo B : field-dependent mobi-

l i t y y p = p p ( E ) X.-

tma*

%

^max

According t o an a c t u a l 3-phase c l o c k p u l s e diagram (see Fig. I ) , t h e 4 vm device m i g h t be operated a t c l o c k frequencies o f t h e order o f 35 MHz. I n t h a t case, the time i n t e r v a l d u r i n g which c a r r i e r s a r e heated represents o n l y 1.1 % o f t h e c l o c k period. For t h e 1 . 5 um device, t h e t h e o r e t i c a l c l o c k frequency i s 258 MHz and the time when c a r r i e r h e a t i n g should occur i s 0.8 % o f t h e c l o c k p e r i o d which i s o f t h e same order o f t h e preceeding one. T h i s r e s u l t i s r a t h e r s u r p r i s i n g because as- s o c i a t e d w i t h t h e r e d u c t i o n o f t h e dimensions were expected more e f f e c t s o f t h e mo- b i l i t y s a t u r a t i o n . The r e d u c t i o n o f t h e c l o c k frequency due t o t h a t e f f e c t i s o n l y o f 1 . 5 %.

i i i ) For t h i s non-overlapping gate s t r u c t u r e under t e s t , t h e geometrical parameter which c o n t r o l s t h e e l e c t r i c f i e l d s i s t h e gap width. I t s i n f l u e n c e on EL has been found t o be more preponderant than t h e r e d u c t i o n o f t h e gate l e n g t h (even f o r L as small as 1 . 5 pm).

Furthermore, t h i s " t h e o r e t i c a l " 1 . 5 lim device has presented a behaviourwhich i s mainly s i m i l a r t o t h e one o f more conventional devices.

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C7-192 JOURNAL DE PHYSIQUE

REFERENCES

[ l

1

F. STERN, C.R.C. C r i t i c a l R e v i e w

i n

S o l . S t a t . S c i e n c e s , p. 499, (1974)

[

2

1

J . E . CARNES, I4.F. KOSONOCKY e t a l . , I E E E J. S o l . S t . C i r c .

E,

p.322 (1971)

[

3 1 J.E. CARNES, W.F. KOSONOCKY e t a 1

. ,

I E E E T r a n s . E l . D e v . w , p . 7 9 8 , (1972) [ 4 ] C.K. KIM, F1. LENZLINGER, J.A.P.

42,

^p. 3586, (1971)

[

5 1 R.S. VARGA, " F l a t r i c e i t e r a t i v e a n a l y s i s " , P r e n t i c e - H a l l

,

E n g l e w o o d C l i f f s N.J. (1962)

[ 6

]

^C.H. CHAN, S.G.C. CHAMBERLAIN, S.S.E. g , p . 491 (1974) [ 7

]

^S.A. ABBAS, R.C. DOCKERTY, A.P.L.

7,

p. 147 (1975)

[ 8 ] T.H. NING, P.W. COOK e t a l . , I E E E T r a n s . E l . Dev.

w,

p. 346 (1979) 1 9

]

P.E. COTTRELL, R.R. TROUTMAN, T.H. NING, I E E E T r a n s . E l . D e v .

ED-26,

p. 520 (1979)

[ l o

]

C. JACOBONI, C. CANAL1 e t a l . , S.S.E.

20,

p. 77 (1977)

[

111 K. HESS, C.T. SAH, S.S.E.

2,

p. 1025 (1979)

[ 1 2 ] K. HESS, C.T. SAH, I E E E T r a n s . E l . Dev. E , p . 1399 (1978)

[

1 3 1 ^M.H. ELSAID, S.G. CHAFlBERLAIN, L.A.K. WATT, S.S.E.

3,

p. 6 1 (1977)