CURRENT PATH IN AMORPHOUS-SILICON FIELD EFFECT TRANSISTORS

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CURRENT PATH IN AMORPHOUS-SILICON FIELD EFFECT TRANSISTORS

M. Matsumura, S. Kuno, Y. Uchida

To cite this version:

M. Matsumura, S. Kuno, Y. Uchida. CURRENT PATH IN AMORPHOUS-SILICON FIELD EFFECT TRANSISTORS. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-519-C4-522.

�10.1051/jphyscol:19814111�. �jpa-00220728�

CURRENT PATH I N AMORPHOUS-SILI'CON F I E L D EFFECT TRANSISTORS

M. Matsumura, S.I. Kuno and Y. Uchida

Department of Physical EZectronics, Tokyo I n s t i t u t e o f TechnoZogy, Oh-okayama, Meguro-ku, Tokyo 152, Japan

Abstract.- On-resistance of amorphous-silicon field effect transistors with staggered electrodes was investigated. It was found that dependences of the on-resistance on geometrical parameters were classified into two groups. The origin was attributed to the residual resistance between the nt el-ectrode and the channel which was formed at the silicon-silicon dioxide interface. The resistance was analyzed by taking space charge effect into account, and we found that it changes in accordance with sample preparation conditions. It is pointed out that caution should be taken not only in transistor design but also in mobility evaluation and gap-state-density evaluation.

Introduction.- One of the important parameters of amorphous-silicon field effect transistors(a-Si FETs) (1) is the on-off current ratio. Recently, we obtained a high on-off current ratio (more than 10 8 ) by using transistors with a staggered electrode structure (2). However, from a simple geometrical consideration, the maximum obtain- able value was estimated to be 104. In this paper, we would like to report that 1) this discrepancy arised from a drastic decrease of the residual resistance of the active n- a-Si caused by the space charge effect, 2) this value changes by sample preparation conditions because it varies with localized state density, thickness and Fermi potential, and 3) the current path in the a-Si FETs changes from sample to sample according to the residual resistance.

Experimental Results.- Figure 1 shows a cross-sectional view of our FETs. The FET. had

-

^t

nf-n -n structure with staggered electrodes and was prepared on a crystal silicon substrate, with various n electrode spacing t L and n- island length L'. The gate oxide thickness and channel width W

were fixed at 0.5pm and 200um.

respectively. The films were

4

L'

>

deposited by the hot cathode arc

L

discharge decomposition method (3). ^5---)

I

Uniformity of the FET characteristics

was excellent (on-current and off-

n+ n+

current variations were less than 50%) but reproducibility was not good.

i : ^:--I-:L

^I

i

^:

^path+,!

.-L-->--:

n- ---l $R! ,

Figure 2 shows typical drain current ID vs. gate voltage VG

characteristics. The maximum current

Tox Si02

was 130uA and the minimum current was

about IPA. Thus the on-off current

G

ratio y is more than 10 8

.

Fig.1. Cross-sectional view of FET with Under low drain voltage VD and staggered electrode.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814111

JOURNAL DE PHYSIQUE

high

V

_G conditions, a group of I'ETs on a substrate showed ID-VD characteristics as shown in Fig.3. The on-resistance ^{--L, Q}

V

R

on =dVD/dID

E -5

was not given by L and was proportional m to Lt. Thus the current is considered to 0 flow along the Si-Sio2 interface from

-J-6

one edge of the n- island to the other

- 7

edge of the n- island as shown by a solid line in Fig.1. However, a group of FETs on another substrate showed I -V

D D

- 8

characteristics as shown in Fig.4. The

on-resistance was proportional to L and

-9

did not depend on L'. Thus it can be

considered that the current first

crosses the n- layer downwards, then flows

-10

along the Si-Si02 interface and again

crosses the n- layer upwards, as shown by

-11

dotted lines in Fig.1.

The different path that the current took was caused by the resistance R between the n electrode and the channel t formed at the Si-Si02 interface. When R

is high, the FETs show the former

0

characteristics because the voltage drop

VG ( V )

f Fig.2. Typical semilogarithmic

between the upper n electrode and the dependence of ID on V lower channel is high. And when R is V was fixed at 40V. G'

low, the FET show the latter D

Fig.3. I -V characteristics of the FETs with the current path shown by s8li8 line in Fig.1.

T h e o r e t i c a l Results,- I n case where t h e c u r r e n t flows a l o n g t h e d o t t e d l i n e s i n Fig.1 i f we n e g l e c t channel r e s i s t a n c e , Ron equals t o 2R and i s given by

where p i s t h e s p e c i f i c r e s i s t a n c e of t h e n- l a y e r . d t h e t h i c k n e s s of t h e a c t i v e n- l a y e r . Since t h e o f f - r e s i s t a n c e Roff i s expressed by

y becomes a s

R (L-L1 )L

y= off/Ron=

b2

I n s e r t i n g t y p i c a l v a l u e s i n t o L,LV and d . y was c a l c u l a t e d t o be 104, which i s much l e s s than t h e e x p e r i m e n t a l l y o b t a i n e d value. This discrepancy can be explained by t h e f a c t t h a t R i s a space charge l i m i t e d r e s i s t a n c e s i n c e a high r e s i s t i v e n- l a y e r was sandwitched between t h e nt e l e c t r o d e and t h e channel.

F i g u r e 5 shows t h e c a l c u l a t e d p o t e n t i a l and c a r r i e l : d e n s i t y d i s t r i b u t i o n s i n

t t

t h e n -n--n a-Si diodes. The l o c a l i z e d s t a t e d e n s i t y d i s b r i b u t i o n assumed i s shown i n Fig.6. When t h e c u r r e n t f l o w s , a l a r g e number 0:: excess e l e c t r o n s appear i n t h e n- l a y e r , r e s u I t i n g i n a d r a s t i c d e c r e a s e of i t s r e s i s t a n c e . ~ o w e v e r , d i s t a n t from t h e nt cathode, excess e l e c t r o n d e n s i t y d e c r e a s e s and r e s i s t i v i t y approaches i t s

e q u i l i b l i u m value. Thus t h e space charge l i m i t e d r e s i s t a n c e can be observed o n l y i n t h e case of extremely narrow n- l a y e r and can n o t be observed i n conventional FE measurements. When t h e n- l a y e r i s 0.5urn and t h e c u r r e n t d e n s i t y i s l ~ / c m ~ , which a r e

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V I L J

Fig.5. C a r r i e r and p o t e n t i a l d i s t r i b u t i o n s i n

Fif.6. Localized s t a t e d e n s i t y d i s t r i b u t i o n assumed i n a n a l y s i s .

nf -n--nt a-Si diodes.

t y p i c a l v a l u e s i n a - S i FETs, t h e v o l t a g e drop a c r o s s t h e n- l a y e r was c a l c u l a t e d t o be about &V, which i s

4

o r d e r s i n magnxtude l e s s t h a n t h e v a l u e c a l c u l a t e d from s p e c i f i c r e s i s t a n c e . Thus t h e high c u r r e n t r a t i o o b t a i n e d e x p e r i m e n t a l l y can be explained by t h e d e c r e a s e of R

.

S i n c e R changes w i t h l o c a l i z e d s t a t e d e n s i t y d i s t r i b u t i o n and n- t h i c k n e s s and s i n c e i t i s c o m p a r a b l e t o t h e d r a i n v o l t a g e of t h e a-Si FETs, t h e c u r r e n t p a t h changes w i t h sample p r e p a r a t i o n c o n d i t i o n s .

It i s important t o express t h i s space charge e f f e c t a n a l y t i c a l l y . When we assume t h e l o c a l i z e d s t a t e d e n s i t y n e a r Fermi l e v e l t o t a k e t h e form nToexp(E;/a) and when t h e t h i c k n e s s d of t h e n- l a y e r i s much l e s s than t h e c h a r a c t e r i s t i c t h i c k n e s s do d e f i n e d by

ao=cap0J/ (qnTo(a+kT) )

.

Ron becomes

Ron=Rono ( ~ f k T ) (d/do) a / ( a t k T ) / ( 2 a + k ~ )

,

where J i s t h e c u r r e n t d e n s i t y and Rono t h e r e s i s t a n c e when t h e s p e c i f i c r e s i s t a n c e i s i t s e q u i l i b l i u m value.

Conclusion.- The importance of r e s i d u a l r e s i s t a n c e i n a-Si FETs was p o i n t e d o u t . Lack of r e p r o d u c i b i l i t y i s p a r t l y caused by t h i s r e s i s t a n c e . ~ n d s i n c e t h e e f f e c t i v e channel l e n g t h changes with t h e r e s i s t a n c e , c a r e f u l examination of t h i s r e s i s t a n c e i s necessary f o r evaluation of m o b i l i t y and l o c a l i z e d s t a t e d e n s i t y by FE method.

To e l i m i n a t e t h i s r e s i s t a n c e , coplanar e l e c t r o d e s t r u c t u r e must be a p p l i e d . References.

~)P.G. LeComber, W.E. Spear and A . Ghaith, Elect. ~ e t t . , s , 1 7 9 ( 1 9 7 9 ) . 2)H. Nayama and M. Matsumura, ~ ~ ~ , % , 7 5 4 ( 1 9 8 0 )

3)M. Matsumura and Y. Uchida, t o be presented a t t h i s conference.