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Radiation effects in n-channel MOSFETS
Solid-State Electronics, 1974, Vol. 17, pp. 663-666. Pergamon Press.
Printed in Great Britain
RADIATION EFFECTS IN n-CHANNEL MOSFETS DIETRICH ONNASCH and HANS PETER REIMERDES Institut for Strahlenschutz, 2301 Stohl, institut for Reine und Angewandte Kernphysik, Universit~it Kiel, 2300 Kiel, West Germany
(Received 9 March 1973) Abstract--The radiation damage of commercially available n-channel depletion MOSFET's (3NI28) is investigated up to a dose of 6 Mrad (SiO2) Co-60 y-rays. The upper bound of admissible radiation dose for these devices is about 100 krad (SiO~) owing to the decrease of forward transconductance. The change of the turn-on-voltage is less than 1 V C-V-characteristics obtained by measuring the MOS charging current in response to a linear voltage ramp point to the growth of interface states with a monoenergietic level below the conduction band of Si.
C C,,, D [ Io L n v(t) Vo V~, VR V~ tR x, Z /x /xeff Q
2. EXPERIMENTAL METHODS
NOTATION MOS capacitance oxide capacitance dose frequency drain current channel length electron concentration charging voltage drain-source voltage gate-source voltage maximum ramp voltage turn-on voltage ramp time inversion layer width channel width electron mobility effective mobility of channel electrons charge stored in the MOS capacitor
E x p e r i m e n t s were p e r f o r m e d o n n - c h a n n e l d e p l e t i o n M O S F E T ' s R C A - t y p e 3N128 h a v i n g a gate t u r n - o n - v o l t a g e of - 3 V , a f o r w a r d t r a n s c o n d u c t a n c e of 7.3 m S (at VD = 15V, Io = 5mA), a n d an i n p u t c a p a c i t a n c e of 6 pF. Nine d e v i c e s h a v i n g similar electrical c h a r a c t e r i s t i c s were irradiated in steps to d o s e s of 9, 26, 90, 260, 870 up to 5200 krad ( S i O 0 w i t h a Co-60 y - s o u r c e . T h e d o s e rate generally u s e d was 2-1 k r a d ( S i O 0 / h r . D u r i n g irradiation t h r e e t r a n s i s t o r s e a c h w e r e b i a s e d w i t h 0 V (T~ - T3), with - 2 . 1 V ( T 4 - T6), a n d w i t h - 4 . 2 V ( T 7 - Tg) applied b e t w e e n gate a n d bulk. T h e t r a n s f e r c h a r a c t e r i s t i c s , the f r e q u e n c y dep e n d e n c e of the noise figure of the d r a i n c u r r e n t , a n d the voltage d e p e n d e n c e of the differential M O S - c a p a c i t a n c e were m e a s u r e d for e a c h transistor a f t e r e v e r y irradiation step. N o i s e figure measu r e m e n t s w e r e p e r f o r m e d with the T r a n s i s t o r Noise A n a l y s e r H P 4470. T h e differential capacit a n c e was o b t a i n e d b y m e a s u r i n g the M O S c h a r g i n g c u r r e n t in r e s p o n s e to a positive or n e g a t i v e linear voltage r a m p [4-6]. T h e c u r r e n t t h r o u g h the MOSdiode,
1. INTRODUCTION To i n v e s t i g a t e r a d i a t i o n effects o n the electrical p r o p e r t i e s of M O S - d e v i c e s , simple s t r u c t u r e s are r e q u i r e d w h i c h are m a n u f a c t u r e d using a p r o d u c tion p r o c e s s similar to t h a t used to f a b r i c a t e integr a t e d circuits. W e h a v e c h o s e n c o m m e r c i a l l y available n - c h a n n e l - d e p l e t i o n M O S F E T ' s w i t h o u t prot e c t i o n diodes. T h e effect of y - r a y s on MOSd e v i c e s c a n be divided into two c o m p o n e n t s , (1) the build up of a positive c h a r g e d e n s i t y in the oxide a n d (2) the c r e a t i o n of t r a p p i n g or e l e c t r o n a c c e p t o r states at the Si-SiO2 i n t e r f a c e [ I - 3 ] . T h e first process is e m p h a s i z e d in c u r r e n t literature. Our measu r e m e n t s p r o v e the s e c o n d p r o c e s s to b e m o r e effective in c h a n g i n g electrical p r o p e r t i e s for the r a d i a t i o n d a m a g e studies of the here i n v e s t i g a t e d commercial n-MOSFET.
dQ_ ~, I -
dt
dv x tit- = C ( V )
VR t~-
is directly p r o p o r t i o n a l to the c a p a c i t a n c e C ( V ) a n d c a n be r e c o r d e d o n an oscilloscope, or for sufficiently long r a m p t i m e s tR, can be amplified and plotted. T h e m a x i m u m r a m p voltage VR was 10 V, the r a m p slope varied b e t w e e n l0 s a n d 0.07 V/sec. D r a i n a n d s o u r c e w e r e s h o r t e n e d to bulk. 663
664
DIE IRI('H ONNASCH a n d HANS PETER RF;IMERI]ES 3. E X P E R I M E N T A l . R E S U L T S
In Fig. 1, I,> V, c h a r a c t e r i s t i c s for V<; = 0 are p r e s e n t e d with the dose as p a r a m e t e r for the t r a n s istor T~. W e note a d e c r e a s e of the d r a i n - c u r r e n t by a f a c t o r of 10 at a b o u t 5 Mrad. In Fig. 2, the corresp o n d i n g 1, - V<, c h a r a c t e r i s t i c s are p r e s e n t e d . Evid e n t l y e v e n b y an i n c r e a s e of gate voltage the preirradiation s a t u r a t i o n c u r r e n t s c a n n o t be r e a c h e d a n y m o r e . T h e t r a n s c o n d u c t a n c e is strongly red u c e d : the t u r n - o n voltage, h o w e v e r , varies by less t h a n 1 V. T h e r e d u c t i o n of the drain c u r r e n t for V:, = 1 V a n d Va = O V is plotted v s dose in Fig. 3 for nine t r a n s i s t o r s . N o t e t h a t in spite of equal initial electrical t a b l e p r o p e r t i e s t h e s a m p l e s c a t t e r i n g is considerable. T h e influence of biasing c o n d i t i o n s during irradiation o n t h e p o s t - i r r a d i a t i o n b e h a v i o r of the transistors cannot be established. T h e m e a s u r e m e n t s of the noise figure yield the e x p e c t e d 1If c u r v e in the interval f r o m 10 Hz to 10 k H z (Fig. 4). T r a n s i s t o r T, s h o w s a typical inc r e a s e in noise figure w h i c h is r e p r e s e n t e d in Fig. 5. T h e only e x c e p t i o n was f o u n d in t r a n s i s t o r 7"7. Its initially high noise figure d e c r e a s e d in the b e g i n n i n g of irradiation a n d i n c r e a s e d only at high d o s e s app r o a c h i n g the level of the noise figure of the o t h e r irradiated t r a n s i s t o r s . T h e c a p a c i t a n c e c u r v e s for p o s i t i v e and n e g a t i v e voltage r a m p slopes are p r e s e n t e d in Fig. 6 for irradiation d o s e s of 2 6 , 2 6 0 and 5200 krad (SlOe). T h e f o r m a t i o n of a dip within e a c h c u r v e with increas-
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Fig. I. Drain current vs drain voltage before and after five irradiation steps for transistor T, with all electrodes shortened during irradiation.
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Fig. 3. Dose-dependence of the redtiction of drain currenl for nine transistors T, T.,.
665
Radiation effects in n-channel MOSFET's 4oi 36
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Fig. 4. Frequency dependence of noise figure for transistor T, at I,, = 10 txA, V~ = 5V, source resistance 100 kfL D in Krad (SiO2): 26, --(D---O--, 90, - A - - A - - , 260, + - - + - , 870, - - - x - - - x , 5200--O- -O-.
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Fig. 6. Differential gate to source-drain-substrate capacitance normalized to oxide capacity vs gate voltage for transistor T, for three iradiation doses. The solid curve is measured with a positive ramp slope, the dashed curve with a negative slope.
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Fig. 5. Noise figure vs dose at a frequency of 100 Hz for transistor T, (typical) and T~ (highest noise). ins dose is evident. Its position, however, is dependent on the sign of the ramp slope. The position of the minimum of the C - V - c u r v e s depends on the position and magnitude of the dent. The distribution of minima is within 0'5 V. These results are nearly the same for all transistors. A d e p e n d e n c e of the C - V - c u r v e s on the amount of ramp slope cannot be established.
Unfortunately information concerning the structure of the transistor has not b e c o m e available. E v e n with knowledge of the oxide thickness and doping density, h o w e v e r , as named in ref. [7], quantitative estimates can hardly be p e r f o r m e d since very extensive data about doping profile, lateral nonuniformities of doping, oxide thickness etc. over the area of the M O S structure are necessary to give detailed explanations of the radiation effects. The large scatter in the radiation response of the individual devices (Fig. 3) results from the scatter of these data. According to the equation Z I,, = L t~eff C,,~ ((V~ - V ~ ) V , ~ - V,;72) the reduction of drain current (Fig. 3) can in principle be explained by a shift of VT or a decrease of /xe~. In Figs. 1 and 2 there is, h o w e v e r , only a shift of V~ within 1 V. A reduction of the effective mobility, h o w e v e r , could be understood assuming that the mobility is limited by scattering at ionized impurity centers [8]. The density of these ionized centers is probably increased by the radiation. The
666
DIETRICH ONNASCH and HANS PETER REIMERI)ES
small positive shift of Vr, already r e p o r t e d by H o l m e s - S i e d l e a n d Z a i n i n g e r [ 2 ] for s o m e t y p e s of n - c h a n n e l M O S F E T ' s , c a n also be a s c r i b e d to the g e n e r a t i o n of i n t e r f a c e states. F r o m the i n c r e a s e in the 1If noise for all t r a n s i s t o r s but o n e (Fig. 51, an i n c r e a s e of the i n t e r f a c e state d e n s i t y is c o n c l u d e d [9]. T h e relative high p r e - i r r a d i a t i o n noise figure of t r a n s i s t o r T: m a y a c c o u n t for its u n u s u a l b e h a v i o r . It c a n be a s s u m e d that high irr a d i a t i o n d o s e s c a u s e a new stable state at the Si-SiO2 i n t e r f a c e w h i c h is not affected f r o m the p r e - i r r a d i a t i o n state. During m e a s u r e m e n t s the f e r m i level was n e a r the v a l e n c e b a n d . T h u s acc o r d i n g to the e x p l a n a t i o n of the 1/f noise an ext e n d e d i n t e r f a c e state d i s t r i b u t i o n a b o v e the vale n c e b a n d c a n be a s s u m e d . In m e a s u r i n g C - V c u r v e s in M O S F E T ' s b y the quasi static t e c h n i q u e , the s w e e p rate is not limited b y t h e t h e r m a l g e n e r a t i o n as in M O S - d i o d e s [5], bec a u s e t h e carriers are i n j e c t e d into t h e c h a n n e l f r o m drain a n d s o u r c e s h o r t e d to s u b s t r a t e . T h u s t h e r e is n o d e p e n d e n c e on the a m o u n t of s w e e p rate e v e n at r a m p slopes of 10 ~ V/sec. T h e d e f o r m a t i o n of t h e C - V c u r v e s m a y b e a t t r i b u t e d to t h e g r o w t h of lateral i n h o m o g e n e t i e s as r e c e n t l y d e m o n s t r a t e d by m e a s u r e m e n t s of Williams a n d W o o d s [ 1 0 ] . A n o t h e r a n d p r o b a b l y b e t t e r e x p l a n a t i o n for the f o r m a t i o n of the d e n t is the g r o w t h of a m o n e n e r g e tic e l e c t r o n a c c e p t o r level b e l o w the c o n d u c t i o n b a n d of S i l l 1]. T h e r a d i a t i o n d e p e n d e n t i n c r e a s e in d e n s i t y of s u c h a state has already b e e n d e m o n strated b y W a g e m a n n and Br/iunig[ 12l. Also S i x o n et al.[13] e s t a b l i s h e d a m a x i m u m within the static c a p a c i t a n c e of irradiated M O S s t r u c t u r e s a n d assigned it to a v a r i a t i o n of i n t e r f a c e state d e n s i t y in t h e gap. T h e s t r u c t u r e of t h e dips is p r o b a b l y d e p e n d e n t on the e m p t y i n g (filling) of deep traps as the F e r m i level is s w e p t up (down) t h e f o r b i d d e n gap in the d e p l e t i o n region of t h e d i o d e - o x i d e layer. T h e diff e r e n t position of the dent with i n c r e a s i n g or dec r e a s i n g r a m p is thus c a u s e d b y t h e different c h a r g e state of the m o n o e n e r g e t i c e l e c t r o n a c c e p t o r cen-
ters f r o m w h i c h a different surface potential follows [ 1 I1. T h e r e d u c t i o n of drain c u r r e n t , the i n c r e a s e of noise figure, and the f o r m a t i o n of the d e n t within the C - V c h a r a c t e r i s t i c imply the c r e a t i o n of n e w i n t e r f a c e states by the radiation. T o sum up, the c o n c l u s i o n s of the p r e s e n t w o r k a b o u t radiation effects in c o m m e r c i a l n - c h a n n e l d e p l e t i o n MOSF E T ' s ( R C A 3N128) are; (I) T h e c h a n g e of oxide c h a r g e d e n s i t y is relatively insignificant for the electrical b e h a v i o r of this device. (2) T h e u p p e r b o u n d of r a d i a t i o n dose for t h e s e d e v i c e s is a b o u t I()~rad (SiO~) owing to the d e c r e a s e of f o r w a r d t r a n s c o n d u c t a n c e . (3) T h e d e g r a d a t i o n of t r a n s c o n d u c t a n c e is c a u s e d by e n h a n c e d ionized i m p u r i t y and d e f e c t c e n t e r s c a t t e r i n g of the e l e c t r o n s in the c h a n n e l . (4) C - V c h a r a c t e r i s t i c s point to the g r o w t h of oxide i n t e r f a c e c e n t e r s with a m o n o e n e r g e t i c level below the c o n d u c t i o n b a n d of St.
REFERENCES
I. K. H. Zaininger and A. G. Holmcs-Siedle, RCA Rer. 28, 208 {1967). 2. A. H. Holmes-Siedle and K. H. Zaininger. Solid-St. Technol. 12, 40 (1969). 3. E. H. Snow, A. S. Grove and D. J. Fitzgerald, Proc. IEEE 55, 1168 (19671. 4. N. G. Dillman, Rev. Sci. Instr. 39, 619 (1968). 5. W. K. Kappallo and J. P. Walsh, Appl. Phys. Lefts 17, 384 (1970). 6. M. Kuhn, Solid-St. Electron. 13, 873 (19701. 7. E. W. Kreutz, H. Pagnia and W. Waidlich, Phys. Status Solidi (a) 5, 691 (1971). 8. N. S. Murphy, F. Berz and I. Finn, Solid-St. Electron. 12, 775 (1969). 9. C. T. Sah and F. H. Hielscher, Phys. Rer. Letts 17, 956 {19661. 10. R. Williams and W. H. Woods, J. appl. Phys. 43, 4142 (19721. 11. F. P. Heimann and G. Warfield, IEEE Trans. Electron Devices 12, 167 (1965). 12. H. O. Wagemann and B. Brfiunig, Phys. Letts 29A, 456 ( 19691. 13. P. Sixon, P. l)ansas, and J. P. Courat, Solid-St. Electron. 15. 133 (19721.
Zusammenfassung--Die Strahlenschiidigung von kommerziellen n-Kanal-Verarnmng-MOSFET's (3NI28) wird bis zu Dosen yon 6 Mrad (SiO~) Co-60--/-Strahlung untersucht. Ein einwandfreier elektrischer Betrieb der Bauelemente h6rt im Bereich yon etwa 100 krad (SiO~) durch den Abfall der Steilheit auf. Die A,nderung der Schwellspannung betrfigt weniger als 1 V. C-V-Kennlinien, die mit einer linear mit der Zeit ansteigenden oder abfallenden Ladespannung aufgenommen werden, deuten auf die Entstehung yon neuen Grenzschichtzustfinden rnit einem monoenergetischen Niveau unter dem Si-Leitungsband hin.
Printed in Great Britain
RADIATION EFFECTS IN n-CHANNEL MOSFETS DIETRICH ONNASCH and HANS PETER REIMERDES Institut for Strahlenschutz, 2301 Stohl, institut for Reine und Angewandte Kernphysik, Universit~it Kiel, 2300 Kiel, West Germany
(Received 9 March 1973) Abstract--The radiation damage of commercially available n-channel depletion MOSFET's (3NI28) is investigated up to a dose of 6 Mrad (SiO2) Co-60 y-rays. The upper bound of admissible radiation dose for these devices is about 100 krad (SiO~) owing to the decrease of forward transconductance. The change of the turn-on-voltage is less than 1 V C-V-characteristics obtained by measuring the MOS charging current in response to a linear voltage ramp point to the growth of interface states with a monoenergietic level below the conduction band of Si.
C C,,, D [ Io L n v(t) Vo V~, VR V~ tR x, Z /x /xeff Q
2. EXPERIMENTAL METHODS
NOTATION MOS capacitance oxide capacitance dose frequency drain current channel length electron concentration charging voltage drain-source voltage gate-source voltage maximum ramp voltage turn-on voltage ramp time inversion layer width channel width electron mobility effective mobility of channel electrons charge stored in the MOS capacitor
E x p e r i m e n t s were p e r f o r m e d o n n - c h a n n e l d e p l e t i o n M O S F E T ' s R C A - t y p e 3N128 h a v i n g a gate t u r n - o n - v o l t a g e of - 3 V , a f o r w a r d t r a n s c o n d u c t a n c e of 7.3 m S (at VD = 15V, Io = 5mA), a n d an i n p u t c a p a c i t a n c e of 6 pF. Nine d e v i c e s h a v i n g similar electrical c h a r a c t e r i s t i c s were irradiated in steps to d o s e s of 9, 26, 90, 260, 870 up to 5200 krad ( S i O 0 w i t h a Co-60 y - s o u r c e . T h e d o s e rate generally u s e d was 2-1 k r a d ( S i O 0 / h r . D u r i n g irradiation t h r e e t r a n s i s t o r s e a c h w e r e b i a s e d w i t h 0 V (T~ - T3), with - 2 . 1 V ( T 4 - T6), a n d w i t h - 4 . 2 V ( T 7 - Tg) applied b e t w e e n gate a n d bulk. T h e t r a n s f e r c h a r a c t e r i s t i c s , the f r e q u e n c y dep e n d e n c e of the noise figure of the d r a i n c u r r e n t , a n d the voltage d e p e n d e n c e of the differential M O S - c a p a c i t a n c e were m e a s u r e d for e a c h transistor a f t e r e v e r y irradiation step. N o i s e figure measu r e m e n t s w e r e p e r f o r m e d with the T r a n s i s t o r Noise A n a l y s e r H P 4470. T h e differential capacit a n c e was o b t a i n e d b y m e a s u r i n g the M O S c h a r g i n g c u r r e n t in r e s p o n s e to a positive or n e g a t i v e linear voltage r a m p [4-6]. T h e c u r r e n t t h r o u g h the MOSdiode,
1. INTRODUCTION To i n v e s t i g a t e r a d i a t i o n effects o n the electrical p r o p e r t i e s of M O S - d e v i c e s , simple s t r u c t u r e s are r e q u i r e d w h i c h are m a n u f a c t u r e d using a p r o d u c tion p r o c e s s similar to t h a t used to f a b r i c a t e integr a t e d circuits. W e h a v e c h o s e n c o m m e r c i a l l y available n - c h a n n e l - d e p l e t i o n M O S F E T ' s w i t h o u t prot e c t i o n diodes. T h e effect of y - r a y s on MOSd e v i c e s c a n be divided into two c o m p o n e n t s , (1) the build up of a positive c h a r g e d e n s i t y in the oxide a n d (2) the c r e a t i o n of t r a p p i n g or e l e c t r o n a c c e p t o r states at the Si-SiO2 i n t e r f a c e [ I - 3 ] . T h e first process is e m p h a s i z e d in c u r r e n t literature. Our measu r e m e n t s p r o v e the s e c o n d p r o c e s s to b e m o r e effective in c h a n g i n g electrical p r o p e r t i e s for the r a d i a t i o n d a m a g e studies of the here i n v e s t i g a t e d commercial n-MOSFET.
dQ_ ~, I -
dt
dv x tit- = C ( V )
VR t~-
is directly p r o p o r t i o n a l to the c a p a c i t a n c e C ( V ) a n d c a n be r e c o r d e d o n an oscilloscope, or for sufficiently long r a m p t i m e s tR, can be amplified and plotted. T h e m a x i m u m r a m p voltage VR was 10 V, the r a m p slope varied b e t w e e n l0 s a n d 0.07 V/sec. D r a i n a n d s o u r c e w e r e s h o r t e n e d to bulk. 663
664
DIE IRI('H ONNASCH a n d HANS PETER RF;IMERI]ES 3. E X P E R I M E N T A l . R E S U L T S
In Fig. 1, I,> V, c h a r a c t e r i s t i c s for V<; = 0 are p r e s e n t e d with the dose as p a r a m e t e r for the t r a n s istor T~. W e note a d e c r e a s e of the d r a i n - c u r r e n t by a f a c t o r of 10 at a b o u t 5 Mrad. In Fig. 2, the corresp o n d i n g 1, - V<, c h a r a c t e r i s t i c s are p r e s e n t e d . Evid e n t l y e v e n b y an i n c r e a s e of gate voltage the preirradiation s a t u r a t i o n c u r r e n t s c a n n o t be r e a c h e d a n y m o r e . T h e t r a n s c o n d u c t a n c e is strongly red u c e d : the t u r n - o n voltage, h o w e v e r , varies by less t h a n 1 V. T h e r e d u c t i o n of the drain c u r r e n t for V:, = 1 V a n d Va = O V is plotted v s dose in Fig. 3 for nine t r a n s i s t o r s . N o t e t h a t in spite of equal initial electrical t a b l e p r o p e r t i e s t h e s a m p l e s c a t t e r i n g is considerable. T h e influence of biasing c o n d i t i o n s during irradiation o n t h e p o s t - i r r a d i a t i o n b e h a v i o r of the transistors cannot be established. T h e m e a s u r e m e n t s of the noise figure yield the e x p e c t e d 1If c u r v e in the interval f r o m 10 Hz to 10 k H z (Fig. 4). T r a n s i s t o r T, s h o w s a typical inc r e a s e in noise figure w h i c h is r e p r e s e n t e d in Fig. 5. T h e only e x c e p t i o n was f o u n d in t r a n s i s t o r 7"7. Its initially high noise figure d e c r e a s e d in the b e g i n n i n g of irradiation a n d i n c r e a s e d only at high d o s e s app r o a c h i n g the level of the noise figure of the o t h e r irradiated t r a n s i s t o r s . T h e c a p a c i t a n c e c u r v e s for p o s i t i v e and n e g a t i v e voltage r a m p slopes are p r e s e n t e d in Fig. 6 for irradiation d o s e s of 2 6 , 2 6 0 and 5200 krad (SlOe). T h e f o r m a t i o n of a dip within e a c h c u r v e with increas-
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665
Radiation effects in n-channel MOSFET's 4oi 36
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Fig. 6. Differential gate to source-drain-substrate capacitance normalized to oxide capacity vs gate voltage for transistor T, for three iradiation doses. The solid curve is measured with a positive ramp slope, the dashed curve with a negative slope.
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Fig. 5. Noise figure vs dose at a frequency of 100 Hz for transistor T, (typical) and T~ (highest noise). ins dose is evident. Its position, however, is dependent on the sign of the ramp slope. The position of the minimum of the C - V - c u r v e s depends on the position and magnitude of the dent. The distribution of minima is within 0'5 V. These results are nearly the same for all transistors. A d e p e n d e n c e of the C - V - c u r v e s on the amount of ramp slope cannot be established.
Unfortunately information concerning the structure of the transistor has not b e c o m e available. E v e n with knowledge of the oxide thickness and doping density, h o w e v e r , as named in ref. [7], quantitative estimates can hardly be p e r f o r m e d since very extensive data about doping profile, lateral nonuniformities of doping, oxide thickness etc. over the area of the M O S structure are necessary to give detailed explanations of the radiation effects. The large scatter in the radiation response of the individual devices (Fig. 3) results from the scatter of these data. According to the equation Z I,, = L t~eff C,,~ ((V~ - V ~ ) V , ~ - V,;72) the reduction of drain current (Fig. 3) can in principle be explained by a shift of VT or a decrease of /xe~. In Figs. 1 and 2 there is, h o w e v e r , only a shift of V~ within 1 V. A reduction of the effective mobility, h o w e v e r , could be understood assuming that the mobility is limited by scattering at ionized impurity centers [8]. The density of these ionized centers is probably increased by the radiation. The
666
DIETRICH ONNASCH and HANS PETER REIMERI)ES
small positive shift of Vr, already r e p o r t e d by H o l m e s - S i e d l e a n d Z a i n i n g e r [ 2 ] for s o m e t y p e s of n - c h a n n e l M O S F E T ' s , c a n also be a s c r i b e d to the g e n e r a t i o n of i n t e r f a c e states. F r o m the i n c r e a s e in the 1If noise for all t r a n s i s t o r s but o n e (Fig. 51, an i n c r e a s e of the i n t e r f a c e state d e n s i t y is c o n c l u d e d [9]. T h e relative high p r e - i r r a d i a t i o n noise figure of t r a n s i s t o r T: m a y a c c o u n t for its u n u s u a l b e h a v i o r . It c a n be a s s u m e d that high irr a d i a t i o n d o s e s c a u s e a new stable state at the Si-SiO2 i n t e r f a c e w h i c h is not affected f r o m the p r e - i r r a d i a t i o n state. During m e a s u r e m e n t s the f e r m i level was n e a r the v a l e n c e b a n d . T h u s acc o r d i n g to the e x p l a n a t i o n of the 1/f noise an ext e n d e d i n t e r f a c e state d i s t r i b u t i o n a b o v e the vale n c e b a n d c a n be a s s u m e d . In m e a s u r i n g C - V c u r v e s in M O S F E T ' s b y the quasi static t e c h n i q u e , the s w e e p rate is not limited b y t h e t h e r m a l g e n e r a t i o n as in M O S - d i o d e s [5], bec a u s e t h e carriers are i n j e c t e d into t h e c h a n n e l f r o m drain a n d s o u r c e s h o r t e d to s u b s t r a t e . T h u s t h e r e is n o d e p e n d e n c e on the a m o u n t of s w e e p rate e v e n at r a m p slopes of 10 ~ V/sec. T h e d e f o r m a t i o n of t h e C - V c u r v e s m a y b e a t t r i b u t e d to t h e g r o w t h of lateral i n h o m o g e n e t i e s as r e c e n t l y d e m o n s t r a t e d by m e a s u r e m e n t s of Williams a n d W o o d s [ 1 0 ] . A n o t h e r a n d p r o b a b l y b e t t e r e x p l a n a t i o n for the f o r m a t i o n of the d e n t is the g r o w t h of a m o n e n e r g e tic e l e c t r o n a c c e p t o r level b e l o w the c o n d u c t i o n b a n d of S i l l 1]. T h e r a d i a t i o n d e p e n d e n t i n c r e a s e in d e n s i t y of s u c h a state has already b e e n d e m o n strated b y W a g e m a n n and Br/iunig[ 12l. Also S i x o n et al.[13] e s t a b l i s h e d a m a x i m u m within the static c a p a c i t a n c e of irradiated M O S s t r u c t u r e s a n d assigned it to a v a r i a t i o n of i n t e r f a c e state d e n s i t y in t h e gap. T h e s t r u c t u r e of t h e dips is p r o b a b l y d e p e n d e n t on the e m p t y i n g (filling) of deep traps as the F e r m i level is s w e p t up (down) t h e f o r b i d d e n gap in the d e p l e t i o n region of t h e d i o d e - o x i d e layer. T h e diff e r e n t position of the dent with i n c r e a s i n g or dec r e a s i n g r a m p is thus c a u s e d b y t h e different c h a r g e state of the m o n o e n e r g e t i c e l e c t r o n a c c e p t o r cen-
ters f r o m w h i c h a different surface potential follows [ 1 I1. T h e r e d u c t i o n of drain c u r r e n t , the i n c r e a s e of noise figure, and the f o r m a t i o n of the d e n t within the C - V c h a r a c t e r i s t i c imply the c r e a t i o n of n e w i n t e r f a c e states by the radiation. T o sum up, the c o n c l u s i o n s of the p r e s e n t w o r k a b o u t radiation effects in c o m m e r c i a l n - c h a n n e l d e p l e t i o n MOSF E T ' s ( R C A 3N128) are; (I) T h e c h a n g e of oxide c h a r g e d e n s i t y is relatively insignificant for the electrical b e h a v i o r of this device. (2) T h e u p p e r b o u n d of r a d i a t i o n dose for t h e s e d e v i c e s is a b o u t I()~rad (SiO~) owing to the d e c r e a s e of f o r w a r d t r a n s c o n d u c t a n c e . (3) T h e d e g r a d a t i o n of t r a n s c o n d u c t a n c e is c a u s e d by e n h a n c e d ionized i m p u r i t y and d e f e c t c e n t e r s c a t t e r i n g of the e l e c t r o n s in the c h a n n e l . (4) C - V c h a r a c t e r i s t i c s point to the g r o w t h of oxide i n t e r f a c e c e n t e r s with a m o n o e n e r g e t i c level below the c o n d u c t i o n b a n d of St.
REFERENCES
I. K. H. Zaininger and A. G. Holmcs-Siedle, RCA Rer. 28, 208 {1967). 2. A. H. Holmes-Siedle and K. H. Zaininger. Solid-St. Technol. 12, 40 (1969). 3. E. H. Snow, A. S. Grove and D. J. Fitzgerald, Proc. IEEE 55, 1168 (19671. 4. N. G. Dillman, Rev. Sci. Instr. 39, 619 (1968). 5. W. K. Kappallo and J. P. Walsh, Appl. Phys. Lefts 17, 384 (1970). 6. M. Kuhn, Solid-St. Electron. 13, 873 (19701. 7. E. W. Kreutz, H. Pagnia and W. Waidlich, Phys. Status Solidi (a) 5, 691 (1971). 8. N. S. Murphy, F. Berz and I. Finn, Solid-St. Electron. 12, 775 (1969). 9. C. T. Sah and F. H. Hielscher, Phys. Rer. Letts 17, 956 {19661. 10. R. Williams and W. H. Woods, J. appl. Phys. 43, 4142 (19721. 11. F. P. Heimann and G. Warfield, IEEE Trans. Electron Devices 12, 167 (1965). 12. H. O. Wagemann and B. Brfiunig, Phys. Letts 29A, 456 ( 19691. 13. P. Sixon, P. l)ansas, and J. P. Courat, Solid-St. Electron. 15. 133 (19721.
Zusammenfassung--Die Strahlenschiidigung von kommerziellen n-Kanal-Verarnmng-MOSFET's (3NI28) wird bis zu Dosen yon 6 Mrad (SiO~) Co-60--/-Strahlung untersucht. Ein einwandfreier elektrischer Betrieb der Bauelemente h6rt im Bereich yon etwa 100 krad (SiO~) durch den Abfall der Steilheit auf. Die A,nderung der Schwellspannung betrfigt weniger als 1 V. C-V-Kennlinien, die mit einer linear mit der Zeit ansteigenden oder abfallenden Ladespannung aufgenommen werden, deuten auf die Entstehung yon neuen Grenzschichtzustfinden rnit einem monoenergetischen Niveau unter dem Si-Leitungsband hin.