current under pulsed operation. Typical threshold currents were 20mA with a differential efficiency of 23% per facet. Some devices showed laser operation at threshold currents as low as 15mA. Fig. 6 shows the typical output spectrum at 1.2 x I,*; the measured full-width half maximum was 0.3 nm, which was limited by the resolution of the spectrometer. Although the wavelength is slightly shorter than previous results these devices show significant improvements in terms

1 ‘-1

1174/61 wavelength, nm

Fig. 6 Inuerse eficiency as Junction of cavity length for broad-area lasers

Cavity losses, ai = 19cn-’; internal efficiency q, = 30% per facet

of threshold current and efficiency for chemical sensing, eye- safe IR sources and LIDAR applications.

Acknowledgments: We should like to thank P. Rees of EPI for material growth, T. A. DeTemple for a preprint (see footnote), and A. Delage, D. P. Halliday and K. M. McGreer of NRC for optical mode profile calculations.

26th August 1992 M. Davies, M. Dion, D. C. Houghton, I. Z. Sedivy and C. M. Vigneron (Solid State Optoelectronics Consortium, Institute for Micro- structural Sciences, National Research Council, Montreal Road, Ottawa, Ontario, K I A OR6, Canada)

Referenees

FOROUIUR, S., LARSSON, A., KSENDZOV, A., LANG, R. I., “I’HILL, N., and scorr, M. D.: ‘Room-temperature operation of MOCVD- grown GaInAs/lnP strained-layer multiquantum-well lasers in 1 . 8 ~ range’, Electron. Lett., 1992,28, (lo), pp. 945-947 noun, D. P., w n m L u , R. U,, ENSTROM, R. E., STEWART, T. R., DIGN- SEPPE, N. G., mwwm, F. z., and COOPER, D. E.: ‘1.5 < L < 1 . 7 ~ strained multiquantum-well InGaAspnGaAsP diode lasers’, Elec- tron. Lett., 1992,28, (l), pp. 37-39 FOROUHAR, s., KSENDOV, A., LA”, A., and TEMKIN, H.: ‘InGaAs/ InGaAsP/lnP strained-layer quantum-well lasers at 2 pm’, Elec- tron. Lett., 1992,28, (15), pp. 1431-1432

and BOULEY, I. c.: ‘Influence of ptype doping of the cladding layer on the threshold current density in 1 3 p m lasers’. SPIE, 1362, Physical Concepts of Materials for Novel Optoelectronic Device Applications 11: Device Physics and Applications, 1990, pp. 617- 622 ADAM, A. R.: ‘Band structure engineering for low-threshold high- efficiency semiconductor lasers’, Electron. Lett., 1986, 22, (3, pp. 249-250 CASEY, H. c . , and CARTER, P. L.: ‘Variation of intervalence band absorption with hole concentration in p-type InF”, Appl. Phys. Letts., 1984.44, (l), pp. 82-83

SERMAGE, B., BLEZ, M., KAZIMIERSKI, C., OUGAZZADEN, A., MIRCFA, A.,

NEW AND FAST MOSFET PARAMETER EXTRACTION METHOD

C. Scharff, J. C. Carter and A. G. R. Evans

Indexing terms: Field-effect transistors, MOS deuices and structures

A new technique for the extraction of submicrometre MOSFET parameters (effective threshold voltage, low field mobility, effective channel length, parasitic series resistance and mobility degradation parameter) is described. The method has been tested on transistors down to 0.3 pm effec- tive gate length. The technique is compared with others and is found to be fast, Bccurate and simpler to implement and can be carried out with standard measurement equipment.

Introduction: Parameter extraction on submicrometre MOS transistors requires that short channel effects are taken into account. Many extraction methods proposed in the literature are either fast and simple to implement but do not provide enough accuracy [l-31 or costly and time-consuming [4, 51. We propose a fast, efficient, simple and accurate technique to characterise MOS transistors. This technique is compared to a technique developed by S. Jain [SI (although slightly modified by us in order to make it more accurate) and another tech- nique called Dual Linear Fitting [l].

Polysilicon gate CMOS transistors with defined gate lengths of O.qO.7, 1, 2 and 4pm and oxide thickness of 40nm were fabricated in a standard process. The l,/VGs character- istics of n-channel devices were measured with an HP4145 analyser at a drain bias VDs of 30mV and no substrate bias (&, = 0). The channel lengths of the transistors were mea- sured with a scanning electron microscope (SEM) by looking at the written transistor gate from above (plan view). The lengths found by the SEM include twice the thickness of the conformal LTO layer (low-temperature oxide) which is located above the polysilicon gate; this does not affect the

2006

accuracy of the methods since the LTO thickness is constant for all transistors.

T h e new method, theory: The starting point of the analysis is eqn. 5.5.2 from Reference 6 that describes the drain current of a device operating in nonsaturation:

where Vr, the effective threshold voltage, differs from the threshold voltage by a barrier lowering term

V; = V, - AVAL, VDs) (2)

Because our extraction method is based on measurements made in the nonsaturation region of operation there is no effective drain-source potential reduction due to pinch-off; however, potential will be dropped across source and drain resistances, so that the effective s o u r d r a i n potential is V, - IDRs, where Rs is the sum of the source and drain resist-

ances. We will assume that 1/21DR, is much smaller than V, - V; so that we do not need to consider the effective reduction in gate source potential. For small channel devices the asdefined channel length L will be significantly larger than the effective channel length L,,, mainly because of sourcdrain dopant diffusion. Hence eqn. 1 becomes

(3)

ELECTRONICS LE77ERS 8th October 1992 Vol. 28 No. 21

This equation can be reduced for VDs > 0 and Vs, = 0 to

Solving (4) for IDS and using V G T = Vas - V;, B POCOX W / L e f f , 0 = 0, + BR, the equation becomes

B VDS Vcr I , = - 1 + ev,, (5)

It is apparent that, if equation (5) is valid, then a plot of IDJ1 + W,,) against V,, will result in a straight line if the correct 0 and V, parameters are chosen. Initially two 0 values and one V' value are guessed and I D J l +el',,) plotted against VGs. The 0 values are chosen so that one is larger and the other smaller than the final true value. The initial V, is chosen by extrapolating the lDs/VGs curve at maximum slope back to zero ordinate; however, the value of this guess is not important. Linear regression is performed on the two curves containing the initial guesses and a new 0 is guessed in a way to reduce the standard deviation. By repeating this process the two guesses converge to a single unique value of 6 and V, with a minimum standard deviation. It should be noted that at each new guess V, is calculated from the intercept of the curve with the VGs axis.

Only a certain portion of the IDs/VGs characteristic is plotted-the lower interval limit is the gate voltage at which the maximum transconductance (9, = SIDs/SVDs) occurs, which is equal to the point with maximum slope on the curve, and the upper interval limit is V,Ag,) + 1 V-in this way we can be certain that the strong inversion model is valid and, by imposing the upper limit, reduce the inaccuracy due to the bias-dependent series resistance RAV,,), i.e. we assume that series resistance R , is constant within the extraction interval but not outside. The threshold voltage can be obtained by extrapolating the straight line to ordinate 0, and B can be obtained from the slope. Fig. 1 illustrates the extraction pro- cedure.

AL, po , R, and 0, are obtained by comparing the transistor to a long channel reference with equal width (we recommend L , = 30pm):

(7)

Eqns. 6 and 7 are adapted from [SI. Eqn. 8 can be found in [7] in a slightly different notation.

Mod$cation of the Jain technique: We have compared our results with a method by Jain; however, we do not use the equation proposed in [5] for the calculation of the series resistance but eqns. 8 and 9 instead. Eqn. (8) provides higher accuracy since the effect of mobility degradation is removed from the series resistance. We also do not measure g , by phase-sensitive detection. We obtain g , by smoothing the ID&! curves and calculating the fist derivative using numencal methods. This technique is preferred over phase- sensitive detection since it is fast and convenient.

extraction -interval -

0 2 5 , I

-me

-- exirapoiaieo I 0 05 ~ I,(

" 0 " T O 5 1 0 1 5 2 0 2 5

vGs,v llooilj

Fig. 1 Extraction procedure

Results and discussion: Table 1 shows the results of our tech- nique in comparison with the two other methods. The results for the Jain and our new method are obtained with 4pm transistors as reference transistors, and the results for the Dual Linear Fitting technique are found by using 0.4,0.7, 1, 2 and 4pm transistors. The accuracy of the Jain and our method could be improved by using large reference tran- sistors, but the results obtained with the 4pm transistors are exact enough to prove that our new method is working well. We have also used the methods with channel lengths deter- mined from cross-section SEM instead of plan-view SEM and found that the results are similar, however, it is much quicker to obtain the plan-view length.

The threshold voltage found by our method is on average 11 mV smaller than the values obtained with the Jain tech- nique. The other values bo, AL, Bo, Rs) are very close to the Jain results. Slight differences between both techniques can be observed because our method does assume that the series resistance is constant within the extraction interval, whereas the Jain technique does not make this assumption.

One disadvantage of the Jain method is that g , must be measured, which means that either the electrical (phase-sensi- tive detection) or the numerical (curve smoothing and first derivative calculation) expenditure is higher than with our method. Additional inaccuracy is incorporated in the Jain technique if g, is calculated numerically, which is due to the necessary curve smoothing algorithm. We want to point out here that the difference between our method and the Jain method is the way V ; and B are being extracted and that all further steps are identical.

The Dual Linear Fitting algorithm is simple to implement but not very accurate because firstly it assumes that R , does not vary with VG and secondly the method extracts the param- eters for five transistors with different channel lengths, assuming that all parameters are constant for this set of tran-

Table 1 PARAMETERS OBTAINED FROM n-CHANNEL DEVICES

AL e, R, fin Method L L,," v: l u n e v m z v - ' s - '

Jain 0.4 1.33 0.115 626 0.7 1.80 0.388 62 1 1.0 2.17 0406 646

63 1 Mean

New Method 0.4 1.33 0.093 620 0.7 1.80 0.378 615 1.0 2.17 0.405 635

623

610

~ - -

~ - - Mean Dual Fitting - - -

ELECTRONICS LETTERS 8th October 1992 Vol. 28

lun

1.056 1.101 0.914 1.024

1.044 1.088 0.931 1.021

1.012

No. 21

Ohm

0.113 11.67 0,111 13.03 0.116 10.48 0.113 11.73

0,109 10.85 0.107 12.09 0,108 11.50 0.108 11.48

0.108 12.23

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sistors. Thus, small variations in AL and R, with channel length (which is very likely) are being neglected, which is why this method gives the lowest values for the mobility and the channel length reduction in Table 1.

Conclusion: We have described a new, rapid and simple parameter extraction method for CMOS transistors. Our method requires only an I,&& curve and an additional ref- erence curve to extract the effective threshold voltage, low field mobility, effective channel length, parasitic series resist- ance and mobility degradation parameter. Comparison with other techniques has shown that our method is fast and accu- rate.

17th September I992

C. Scharff, I. C. Carter and A. G. R. Evans (Microelectronics Group, Department of Electronics & Computer Science, Mountbatten Building, Uniuersity of Southampton, Southampton SO9 S N H , United Kingdom)

References 1 zuo, z. P., DEEN, M. I., and WANG, J.: ‘A new method for extracting

short-channel length or narrow-channel width MOSFET linear parameters’. Canadian Conference of Electrical and Computer Engineering, Montreal, Canada, 17-20 Sept 1989, pp. 1038-1041

2 CHANG, L., and BERG, I.: ‘A derivative method to determine a MOSFETs effective channel length and width electrically’, IEEE Electron. Dev. Left., 1986, EDG7(4), pp. 229-231 PENG, K. L., and mo~owrrz , AI. A.: ‘An improved method to determine MOSFET channel length’, IEEE Electron Deu. Left., 1982, EDL-3(12), pp. 360-362

3

4 m u , B. I., and KO, P. K.: ‘A capacitance method to determine channel lengths for conventional and LDD MOSFETs’, IEEE Electron. Dev. Lett., 1984, EDGY1 I), pp. 491-493

5 JAIN, s.: ‘Measurement of threshold voltage and channel length of submicron MOSFETs’, IEE Proc. I, 1988,13Y6), pp. 162-164

6 rsrvror$ Y. P.: ‘Operation and modeling of the MOS transistor’ (McGraw Hill, New York, 1987)

7 JAIN, s.: ‘A new method for measurement of MOSFET channel length’, Jpn. J. Appl. Phys., 1988,27(8), pp. L1559-Ll561

Appendix : po = constant low field mobility Cox = gate oxide capacitance per unit area W = channel width L = defmed channel length Le,, = LSE, - AL, effective channel length LSE, = L measured by SEM L,,, = length of a reference transistor AL = channel length reduction V, = gate-to-source voltage V, V, = effective threshold voltage V,, = drain-to-source voltage 8, V,, = source-to-substrate voltage E, = critical electric field V,, = gate drive

= transistor gain ! = fitting parameter I,, = drain-to-source current R, = series resistance g = conductance g, = transconductance

= long channel threshold voltage

= channel mobility degradation constant

BANDPASS LINE CODES

L. FouchB, L. Botha and H. C. Ferreira

Indexing terms: Codes and coding, Line coding, ADSL, Spec- tral shaping

Table 1 NUMBER O F Kth-ORDER ZERO-DISPARITY TERNARY CODE WORDS FOR VARIOUS VALUES O F CODE-WORD LENGTH n

n K = l K = 2 K = 3 K = 4

In the letter we present block codes and finite-state-machine codes with higher-order spectral zeros at DC for applications where a speech channel and a baseband digital signal are simultaneously transmitted on the same twisted pair, as in the asymmetrical digital subscriber line (ADSL).

Asymmetric digital subscriber lines (ADSL) [l] have been proposed to transmit DS1 rate in one direction and a low-bit- rate (16 Kbit/s) control channel in the opposite direction, while still providing the conventional analogue telephone service, all on the single twisted pair of copper wires of any nonloaded local telephone loop. The frequency band between DC and lOkHz should thus be left open for insertion of the speech channel and for immunity against impulse noise [ 13.

Block codes with higher-order spectral zeros at DC were considered for use on the above-mentioned control channel, or for providing a low-cost data channel in conjunction with analogue telephone communications in systems where the ADSL is not yet considered. The first K + 1 moments of a codeword are defined as [2]:

u k = z I * x , k ~ { O , l , ..., K} (1) , = 1

where uk is the kth code word moment and i the position of the code symbol x, in the code word (x,, x2, . . . , x.) of length n symbols. A code word is of Kth-order zero disparity if uk = 0. The set of all Kth-order zero-disparity code words of length n is called the Kth-order disparity code of length n. Schouhamer-Immink [2] presented a Table which contains the number of zero-disparity binary code words for different values of K and n.

Similar Tables for q-nary codes, where q = 3, 4 and 5, were determined through computer searches and are presented in Tables 1-3. Schouhamer-Immink [2] showed, for the binary case, that there are no valid code words for values of n that

4 3 1 1 1 6 21 1 1 1 8 109 9 1 1

10 623 43 1 1 12 3949 137 3 1 14 26191 483 17 1 16 181061 2301 69 1 18 1290835 12331 247 1 20 9429433 68617 779 33

are not multiples of 4. This property also holds for q = 4 and short code-word lengths, as can be seen from Table 2. Note in Tables 1 and 3 that there are some entries indicating one code word for K = 1, 2 and 3. These entries represent the all-zero code word. In each case, integer code symbols were chosen such that the same Euclidean distance between adjacent signal levels and symmetry about the zero signal level are main- tained.

If q is even, the set of code symbols A = {uj}, can be calcu- lated by [3]

U J J = 2 . - q - 1 (2)

Table 2 NUMBER OF Kth-ORDER ZERO-DISPARITY 4-LEVEL CODE WORDS FOR VARIOUS VALUES O F CODE-WORD LENGTH n

n K = l K = 2 K = 3 K = 4

4 6 2 0 0 6 0 0 0 0 8 410 22 4 0

10 0 0 0 0 12 47990 842 0 0 14 0 0 0 0 16 7030132 61226 506 0

2008 ELECTRONICS LETERS 8th October 1992 Vol. 28 MO. 21

__ ~ -