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Effects of field-dependent mobility on transfer

efficiency in m.o.s. b.b.d. analogue delay linesJ. W. Haslett, M.Sc, Ph.D., and M. L. Kejariwal, M.Sc.

Indexing terms: Charge-coupled devices, Delay lines, Monolithic integrated circuits

Abstract

The effects of field-dependent mob ility on transfer inefficiency due to intrinsic transfer limita tions, dynamic-dramconductance and threshold-voltage modulation are calculated for an m.o.s. bucket-brigade delay line. The resultsshow significant differences over previous theories at higher clock frequencies, where line performance is criticalif quantisation errors are to be minimised in analogue applications.

1 Introduction

In a conventional m.o.s. b.b.d ., charge transfer no rmally takesplace through the channel of a simple m.o.s triode, whose gate-draincapacitance has been artificially increased to form one of the storagebuckets in the delay line.

1'2

The drain diffusion also acts as a sourcefor the next triode in the line, and so on. At low and intermediateclock frequencies, finite dynamic-drain condu ctance and threshold-voltage mo dulation, due to source and drain-voltage variations duringcharge transfer, constitute the most significant effects contributing tocharge-transfer inefficiency. At higher clock frequencies, the transferinefficiency increases due to the finit e time required for charge toflow from one capacitor to the next in the line. This mechanismrepresents an inherent limitation to the high-frequency performanceof the line , and is determined by the gain characteristics of thetransistors used. Unfortunately, in many applications the length ofthe time delay is fixed and it is desirable to minimise q uantisationerrors by taking many samples per cycle of the maximum inputfrequency of interest. This implies a high clock rate and a long line,which is a cond ition where intrinsic transfer inefficiency can becomeimp ortant. It is therefore of interest to examine the factors affectingthe intrinsic transfer process.

The most simple model of charge transfer assumes a transistor witha square-law characteristic of the form

3

In = -(VBR ~ 0)

where |3 is a factor that includes carrier mobility and device geo metry.If the source capacitance at the beginning of transfer is charged toVs, then the charge to be transferred is simply described by

-Q = c(vg8-vT) (2 )

where Cis the oxide overlap capacitance. Since the drain current mustbe equal to dQ/dt if no charge is lost to the gate circuit, then combin-ing eqns. 1 and 2 gives the charge remaining in the source after time tas

(3 )

(4 )

Once this quan tity is known, the intrinsic transfer-inefficiencyparameter e,- is readily calculated from4

= dQ(jl€i

dQ(o)

where r represents the time available for transfer and is normally equalto one clock period. Applying eqn. 4 to eqn. 3 leads to the simpleresult

€,- = 1 +2C

2 (5 )

Eqn. 5 indicates that at low clock frequencies e,- is small and dominatedby other effects such as nonzero drain conductance, while at higherfrequencies it degrades rapidly toward unity, so tha t the intrinsic

transfer process dominates in this range.

Paper 7802E, first received 12th July and in revised form 24th Septem ber 1976

Dr. Ha slett and Mr. Kejariwal are with the Department of Electrical Engineering,Faculty of Engineering, The University of Calgary, 2920 24th Avenue NW ,Calgary, Alberta, Canada T2N 1N4

Some attempts have been made by Sangsters

to reduce the effectsof dynamic drain conductance by employing an m.o.s. tetrode con-figured switch to effect the charge transfer. In these devices, theremaining transfer inefficiency can be attri but ed largely to intrinsiceffects similar to eqn . 5 even at intermediate frequencies.

The intrinsic transfer inefficiency represen ts a high-frequencylimitation in both types of devices, and we have found that field-dependent mobility plays an important role in the overall high-frequency delay-line performance. It is the purpose of this paper to

examine these effects.

2 M.O.S. triode structures

1.2 Intrinsic transfer inefficiency

The intrinsic-transfer-inefficiency-parameter calculation iseasily modified to include field-depen dent mobility usingTrofimenkoff s

6emperical relationship, given by

Mm = (6)

where jLt0 is the low field mob ility, e is the electric field, and ec is anempirical con stant. This expression is most accurate for bulk mobilitycalculations, but can be made to give a reasonable fit t o p-channelsurface-mobility curves if ju0 and Ec are properly chosen.10

The m.o.s transistors operate only in the saturated region of theirID-VDS characteristics during transfer and eqn. 6 leads directly to

h{sat) =

(Vgs-VT)VDSfsat)

-V l PS (.sat)

where VDS ( M f ) is given by

(sat) = -LE r+LEA 1 +Was ~

LE,

(7)

(8)

The charge to be transferred is still given by eqn. 2 and a combination

of eqns. 2, 7 and 8 leads to an implicity solution for Q(T) of the form

1 +2g(r)7

CV T

- 1 - 1 +2(2(o)7

CV r

2yC+ ln

1 +2 Q ( T ) 7

CV T

- 1

1 +2Q (o)7

CVr,

(9)

where y — Vrl{Le c).Applying eqn. 4 to eqn. 9 leads directly to

€im ~

2Q(r)y

CV T

- 1

2Q(o)y

CVr, - 1

00)

Eq n. 10 is evaluated for a given initial charge to be transferred, Q(o),by interatively solving eqn. 9 for Q(r) and sub stituting both values ineqn. 10. Comparisons of e,- and e^ are shown in Figs. 1 and 2 asfunctions of clock frequency and signal-bias voltage for two sets of

PROC. IEE, Vol. 124, No. 2, FEBRUARY 1977 109



devices with identical characteristics except for channel length. As canbe seen, the inefficiency increases markedly at shorter channel lengthsowing to the effect of field-dependent mobility.

There has also been some discussion in the literature on the effectsof varying clock waveform.

3When a trapezoidal waveform is used,

the effects of field-depen dent mobility are readily included and itcan be shown that this leads to another implicit equation for Q(T)of the form

In

- I - * ?

In

where

and

u(p) K t

7

- - + K7

In

K t

y

2m C 1

my' "(r) =

;- ' • ^ C l t '

01 )

1 +2Q(r)7

CV T

andrwz is the slope of the clock waveform.Numerical solution of eqn. 11 leads to the value of eitm) the

transfer inefficiency parameter for a trapezoidal waveform includingeffects of field-depend ent mobility. This result is compared directlywith that of Berglund,

3in Figs. 3a and 3b , for the cases where

mobility is assumed con stant. As can be seen, the difference in thevalue of e obtained when accounting for field-dependent mobility isgreater than an order of magnitude in these cases.

If one is to be convinced that these differences are important, thenit is necessary to examine the effects of m obility on dynamic-drainconductance and on threshold-voltage modulation due to source anddrain voltages as well.

2.2 Dynamic-drain conductance

The expression for the saturated drain current in eqn. 1assumes that the current depends on gate-source voltage only. Inpractice, the drain potential also has a small effect on drain current,

producing a finite nonzero dynamic output conductance in the sat-uration region of operation. The presence of this conductance hasbeen shown to lead to incomplete charge transfer, and, in general, itcan be shown

3'4

that

Many authors have described models which account for the effectsof gate and drain-voltage variations, substrate impurity concentrationand oxide thickness on channel shortening in a saturated device.Frohman-Bentchkowsky and Grove

7have derived a relatively simple

model that accounts for many of these factors, and which can bewritten as

Ld

2e s

(YDS ~( 1 3 )

10

10

-3

£UU

a

uu

10- 5

10

10

- 6

10

- 8

10 10J 10

frequency. Hz10

ed = ^ (12)Sm

where the output conductance g^ and the gate transconductancegm are both evaluated in the saturation region at the end of thetransfer period.

Fig. 2

Comparison of intrinsic-inefficiency variations with clock frequencyincluding and excluding field-dependent mobility effects

eim

10- 5

-6£10

•oco

10

- 810

uJ1

?ec

0 2 4 6 8 10 12signal bias voltage (Vs-V,), V

Fig. 1Comparison of intrinsic-transfer-inefficiency variations with biasvoltage including and excluding field-dependent m obility effects.

e .

10"' 1 03

io2

10

.2J J

a;

J J

10

E

uJ -5

uj " fi10

6

\

\

/

/

/

/

/

/ / .

/ • • -

/

/

.-10'

3 5 7 9 11 13signal bias voltage(V S-V T ), V

a

105

106

107

frequency, Hz

Fig. 3

Intrinsic-transfer-inefficiency variations

L - 12 M«H

R(m)V= 1 S V , CQ X = 0 - 0 3 3 6 M F / c m

2,C = OS pF , Mo = 200 cm

2V ' S " ' , / = SO kH z

'eism

• ' €ismleisa With bias voltage using trapezoidal clock wav ef or m s/= 1 MHzb With clock frequency using trapezoidal clock waveforms Vc = 15V,

110 PROC. IEE, Vol. 124, No. 2, FEBRUARY 1977



parameter spresdvowing to the averaging nature of the line. In

addition, the threshold-voltage modulation due to source voltage has

been accurately modelled for these devices. Butler et a/.8 have taken

this factor into account using

Consider eqn . 5; then

VT = VTO+aVs (18)

where VTO is the threshold voltage at zero source voltage and a is a

constant. A more accurate result is obtained using

VT =2qesNa

(19)

where Vx = QmS + 20F — Vss, and 6mS is the metal-semiconductor

work function, QF the Fermi potential and Vss the voltage equivalentof the effective interface charge. Theoretical and measured curves for

two typical unmatched transistors are shown in Fig. 7, and are foundto be in good agreement if it is assumed that the initial differences are

due to different Vss levels, a reasonable assumption based on physicalconsiderations. It should be noted that mismatch in initial values of

VT will not affect the small-signal line performance, but only reducesthe dynamic range of the line.

- 4 0

-3-5

-3 0

- 2 5

-1 -2 -3 -4 -5

source/substrate voltage, V

Fig. 7

Comparison of measured and theoretical variations of threshold voltagewith source-to substrate potential for two unmatched transistors

Tr 1 theoretical Tr 1 observedTr 2 theoretical — • Tr 2 observed

Using eqn. 19 it can be shown that the contribution to transferinefficiency due to threshold voltage is given by

(20)

where Vx has the same meaning as that described in eqn. 19.

The overall line performance can then be calculated to includethe effects of intrinsic transfer, dynamic drain conductance and

theshold-voltage modulation, both with and without field-dependentmobility. These calculated curves can be compared directly withmeasured results to indicate whether or not intrinsic transferefficiency is the limiting factor, and whether or not field-dependentmobility is important.

The measurements were performed using the impulse inputtechnique for c.c.d.s.

9which eliminates errors due to sampler and

source follower attenuation. Fig. 8 shows a comparison of measuredand theoretical results for a large value of capacitance, both includingand excluding field-dependent mobility in the calculations. The

observed results are considerably different from the theoretical curveif mobility is assumed constant, but quite satisfactory agreement is

obtained if mobility variations are included.Since nominal mobility and channel length can differ significantly

from device to device, it is important to consider the sensitivity of theexpressions for transfer inefficiency to these parameters, if com-parison with the experiment is to be meaningful.

2C2

BQOT\Since et < 1 always, — T

y I L J

5% h i L

> 1 and

/• *

2C2

1 +PQoA

2C2

(21)

2C= - 2 at worst, and

y J \ Jso a 5% change in Mo orL would result in a 10% change in et.

Similarily, expressions can be derived for the sensitivities of e^

to 7 and Q(T). The analytic expressions are somew hat complicated

and it is easier to substitute the appropriate values and vary them asfollows:Consider

7 = 0-4

26(0)7 = 2y(Vgs-V T) _

CVT VT

2£Cr)7 .CVT

Then eim = 0 1 1 2 3 .

If 7 is changed by 10% then e^ = 0-115, a change of about 2-5%.The sensitivity of e^ to changes in 7 is approximately 0-25.Similarily, errors in Q(T) as determined from eqn. 9 can easily be

estimated. In the example above, if Q(T) is changed by 10%; i.e.

7 = 04

26(0)7 = 4

CVT

an d

CVT

then eim = 0-132, and the sensitivity of e in

slightly less than 2.

to errors in Q(T) is

10" 10-frequency, Hz

Fig. 8

Comparison of measured and theoretical variations in transferinefficiency for a discrete 28-stage m.o.s. b.b.d., as a function of

clock frequency

>— observed e— calculated e m

calculated e

,i« = 2VVc - 15V

C = 1 nF

The sensitivities of individual-stage transfer inefficiency are

therefore low valued in all cases. Because of the averaging nature of

the line, and because the parameters of each stage vary in bothdirections from the average, the overall accuracy of the measurementswill be better than ± 10%. This type of error is not in any sense largeenough to explain the observed differences shown in Fig. 8, particularlysince relatively long channels are involved. Similar agreement has beenobtained using smaller and larger line capacitances. U nfortunately, it

was not possible for us to vary the channel length in these deviceswhile keeping all other device parameters c onstan t. The observed

differences areexpected to increase substantially when shorterchannels are used.

4 Conclusions

The effects of field-dependent mobility on transferinefficiency due to intrinsic transfer, dynamic drain conductance and

112 PROC. IEE, Vol. 124, No.,2\ FEBRUARY, 1977



threshold-voltage modulation have been calculated for an m.o.s. triode

bucket-brigade delay line. It has been found that the intrinsic transfer

rate is affected most significantly by the field dependence, and that

some compromise in delay-line performance is inevitable if the

channel is made too small. The results are readily applicable to any

triode delay line, and a good estimate of line performance can be

obtained by ignoring the effects of the field on dynamic drain con-

ductance and threshold-voltage modulation in most situations.

5 Acknowledgments

This work was partially supported by grant A7776 from the

National Research Council of Canada.

6 References

1 SANG STER, F.L.J. , and TE ER, K.: 'Bucket-brigade electron ics-n ew

possibilities for delay, time axis conversion and scanning', IEEE J. of Solid-

State Grcuits, 1969, SC-4, pp. 1 31- 13 6

2 SANG STER, F.L.J. : 'T he bucket-brigade delay l ine, a shift register for

analogue signals ',Philips Tech. Rev., 1970, 31 , pp. 97—110

3 BERG LUN D, C.N., and BOLL, J.H.: 'Performance l imitations of the IGFET

bucket-brigade shift register' , IEEE Trans., 1 9 7 2 , ED-19, p p . 8 5 2 - 8 6 0

4 BERG LUN D, C.N., and THO RNB ER, K.K.: 'A fundamental comp arison of

incomp lete charge transfer in charge transfer devices', BellSyst. Tech. J.,1 9 7 3 , 5 2 , p p . 1 4 7 - 1 8 2

5 SANG STER, F.L.J. : 'Integrated bucket-brigade delay l ine using MOS

tetrodes', Philips Tech. Rev., 1970, 31 , pp. 266

6 TRO FIM EN KO FF, F.N.: 'Field-dependent mobili ty analysis of the field-

effect transistor' ,Proc. IEEE, 1 9 6 5 , 5 3 , p p . 1 7 6 5 - 6 6

7 FROHMAN-BENTCHKOWSKY, D. , and GRO VE, A.S .: 'Conduc tance of

MOS transistors in saturation',IEEE Trans. , 1969, ED-16, p p . 1 0 8 - 1 1 3

8 BUTLER, W.J. , BARRO N, MB . , and PUCKETTE, CM . : 'Bucke t -br igade

bandwidth characterist ics ', Electron. Lett., 1 9 7 2 , 8 , p p . 4 4 9 -4 5 0

9 VANSTO NE, G.F . , ROBE RTS, J .B.G., and LONG, A.E. : Th e measure -

men t of charge residual for CCD transfer using impulse and freq uency

response', Solid-State Electron., 1 9 7 4 , 1 7 , p p . 8 8 9 -8 9 5

10 MANSOUR, I.R.M., TALKHAN, E.A., and BARBOOR, A.I. : 'Investigation

on the effect of drift-field dependent mobili ty on MOST characterist ics —

Pts. I and IV, IEEE Trans., 1 9 7 2 , ED-19, p p . 8 9 9 - 9 1 6

Conference Publication 146Millimetric waveguide systems(9th—12th November, Savoy Place, Londo n)

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ACX, M., AILLE T, C , and DU PUIS, P.H.: ' l .F. and baseband circuit

design and repeater performance'

ADAM, J.D., and COLLINS, J.H.: 'Magnetic delay lines for group

delay equalisation in millimetric waveguide communication

systems '

ALL ANIC , J.: 'Helical waveguide-manufacturing continu ous process'

ALSBERG, D.A., ANDERSON, J .C., CARLIN, J .W., FOX, P.E.,

GERDINE, M.A., HARRIS, S. , THOMSON, D.J . , VIGNALI, E.,

WEST, T.J., WILLIAMS, S.D., and YOUNG, D.T.: 'Mechanical

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RUTLEDGE, D.R.: 'Design considerations for installed WT4

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BAPTISTE, C , and H ERLE NT, Y.: 'Circular waveguide in France '

BECKLEY, D.J., and PIGOTT, A.C.: 'Possible application of wave-guides in the main network of the United Kingdom Post Office'

BERN ARDI, P. , BERT OLA NI, F. , and FALCIASECC A, G.:

'Characterisation of circular waveguides of different

manufacture '

BERNAR DI , P . , FALCIASECCA, G. , FERREN TINO, A. , GRASSO,

G., and OCCHINI, E.: 'Development of a continuous circular

waveguide'

BOMER, R.P.: 'Automated a t tenuat ion measurements of the UK

Post Office TEoi mode circular waveguide field-trial route'

BORLEY, D.R., and LAWRENCE, P.J . : 'Synchronisat ion and

multiplexing of digital tributaries for a waveguide system'

BORLEY, D.R., and WOOLGAR, B.K.: 'Digital equipment for a

waveguide system'

BOUVET, J .V., DUCHEMIN, J .P. , FUNCK, R., OBREGON, J . , and

GIBEAU, P.: 'Double-drift silicon avalanche diodes for milli-metr ic appl icat ions (26-42 GHz) '

BO YD, R.J . Jun., and THOMSON, D.J .: 'Geom etr ic requirements for

and fabrication of WT4 waveguide quality tubing '

BROWN, R.J . , BROSTRUP-JENSEN, P., SCHOTTLE, J .J . , and TU,

P.J.: 'WT4 line equalisation'

CARLIN, J.W., and MOORTHY, S.C.: 'Waveguide curvature loss: a

new mechanism'

CARRATT, M.: 'Manufacture and installation of circular waveguide'

FRANKLIN, A., BOMER, R.P., WATSON, J .R., and EVANS, T.A.:

'Pressurisation fault location and repair techniques for the UK

Post Office waveguide system'

FRANKLIN, A., and MURPHY, T.G.: 'Installation of the UK Post

Office field-trial waveguide'

GARLICHS, G.: 'Route planning for dielectric-lined waveguides with

opt imised bends '

62 papers, 250pp., 297 X 210mm, photolitho, soft covers,1976. Price £10:35in the UK and £12-10 overseas (£6-95 to members of the IEE, the IEEE, theloP or the I ERE). Orders, with remittances, should be sent to: PublicationSales Department, IEE, Station House, Nightingale Road, Hitchin, H erts. SG5JRJ, England

GOKG OR, H.S., HOWARD, A.M., and PUR CELL , J .J . : 'Mil l imetre-

wave silicon impatt and p-i-n diodes '

GREED, R.B., POWELL, I.L., and WATSON, B.K.: 'Millimetre-wave

commutating hybrid filter with increased selectivity'

GROVES, I .S. , BORLEY, D.R., and CLARK, B.L.: 'High-speed

millimetre-wave phase modulators using p-i-n diodes '

GUENTHER, R.P., ALSBERG, D.A., BROSTRUP-JENSEN, P., and

HAUSER, W.M.: 'Maintenance and reliability of the WT4

system'

HAR KLESS , E.T., ABELE, T.A., and WANG, H.C.: 'WT4 frequency

mult ip lexing system'

HEUN, H.J . , KUHN, E., and RAU BAL, H.: 'An experimental 528

Mb/s 4-phase d.p.s.k. regenerative repeater'

HUISH, P.W., GROVES, I.S., and LEWIS, D.E.: 'Millimetre-wave

oscillators for waveguide communication systems'

HUTCHISON, P.T., BARNES, C.E., BROSTRUP-JENSEN, P.,

HARKLESS, E.T., MUISE, R.W., and NARD1, A.J.: 'Repeater

design and performance in the WT4 system'

ISHIO, H., MOCHIDA, Y., and SAKA TA, T.: 'Comp uter s imulat ion

of a modem circuit for millimetric waveguide transmissionsystem'

KAN MU RI, N., and FU JIWA RA , Y.: 'Millimetre-wave synthesised

sweeper'

KENYON , N.D ., HOLMES, W.H., and CLAPHAM, W.J .: 'Digi tal

transmission experim ents on the UK Post Office field-trial

waveguide'

KOCH, R.: 'Italian research on millimetric waveguide systems'

KRAHN, F.: 'Aluminium waveguide with aluminium-oxide dielectric

lining'

KUHN, E., and SPORLEDER, F.: 'Performance of an experimental

264 Mbit/s 2-D.P.S.K. guided millimetre-wave transmission

system at 30 GHz'

LO VECCHIO, L , and M AGNI, G.: 'Laying-technology tes ts in the

Italian field trial'

MAHIEU, J .R.: 'Frequ ency m ult ip lexer network using broadbandcirculators for millimetric waveguide systems'

MAY, C.A.: 'Millimetric waveguide-system research and development

in the UK'

MILLS, B.A., BODONY1, J ., and WATSON, B .K.: 'Construct ional

technology of waveguide components for millimetre-wave

appl icat ions '

MOHAMM ED, S.A., and KENY ON, N .D.: ' In terference effects in

commutating-type channelling filters for the millimetric-

waveguide system'

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plant for lightweight millimetric waveguide'

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PROC. IEE, Vol. 124, No. 2, FEBRUARY 1977 113

1977 - iee - effects of field-dependent mobility on transfer efficiency in m.o.s. b.b.d. analogue...

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