Influence of pn Junctions on the Mobile Charge Density in MOS TransistorsEarl S. Schlegel Citation: Journal of Applied Physics 42, 425 (1971); doi: 10.1063/1.1659615 View online: http://dx.doi.org/10.1063/1.1659615 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/42/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Silicon fiber with p-n junction Appl. Phys. Lett. 105, 122110 (2014); 10.1063/1.4895661 Heavy ion microbeam studies of diffusion time resolved charge collection from p-n junctions AIP Conf. Proc. 576, 531 (2001); 10.1063/1.1395365 Estimation of doping density in HgCdTe p-n junctions using scanning laser microscopy Appl. Phys. Lett. 72, 52 (1998); 10.1063/1.120642 SURFACESTATE RELATED l/f NOISE IN pn JUNCTIONS AND MOS TRANSISTORS Appl. Phys. Lett. 12, 287 (1968); 10.1063/1.1651995 Room Temperature Operated pn Junctions as Charged Particle Detectors Rev. Sci. Instrum. 31, 74 (1960); 10.1063/1.1716807

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JOURNAL OF APPLIED PHYSICS VOLUME 42. NUMBER 1 JANUARY 1971

Influence of p-n Junctions on the Mobile Charge Density in MOS Transistors*

EARL S. SCHLEGEL

Pllilco-Ford Corporation, Microelectronics Division, 1400 Union Meeting Road, Blue Bell, Pennsylvania 19422

(Received 6 March 1970; in final form 1 June 1970)

Empirical data shows that the mobile charge density in the oxides of MOS and bipolar microcircuits depends on the proximity of a p-n junction. This data is consistent with a model in which positive mobile ions migrate from oxides over n-regions to those over p-regions. Several practical consequences of this phenomenon are discussed.

The yield, performance, and reliability of bipolar and MOS integrated circuits are strongly dependent on the availability of technology for forming (on silicon) Si02 layers having specific well-controlled levels of charge and state densities. A measure of the importance of such technology is given by the fact that well over a thousand papers have been publishedl

•2 over the last six years on MIS theory and technology. A large part of the work in that literature was performed on simple MOS capacitors because of the simplicity of their fabrication and because of the ease of interpretation of the data obtained from them. In a relatively small num­ber of papers, mobile ion density measurements have been reported for MOS transistors. There has been very little, if any, published information on studies of the correlation of the amount of mobile charge in MOS capacitors and MOS transistors.

From a consideration of the physics, one should ex­pect that the mobile ion density in the oxide would be affected by the fringing field in the oxide where a p-n junction intercepts the silicon surface. That is, the

silicon. The thickness of the oxide was 1550 A. The n-on-p devices were made with a phosphorus diffusion into 5-0 cm boron-doped (111 )-oriented silicon, fol­lowed by etching away the phosphosilicate layer so that any mobile ions would not be immobilized by the phosphorus. The final oxide thickness was 2200 A.

The mobile charge densities in these devices were calculated from the shift along the voltage axis of the C-V or I-V characteristic curve during a bake at 300°C under applied voltages of ±12 V, for 12 min plus the cooling time. A summary of these measured charge densities is given in Table II. In each case the data shown is an average of measurements taken on at least five devices.

TABLE I. Transistor channel dimensions.

n-on-p p-on-n

--------- ---------Length Width Length Width

built-in field that drifts holes toward the p-region and Short channel 0.6 5.0 0.4 4.0 electrons toward the n-region should, where it fringes (mil) into the oxide, drift positive mobile ions toward the Long channel 1.0 1.0 1.0 0.5

region of the oxide over the p-region and negative =(=m=il=)=================== mobile ions toward that over the n-region. A number of investigators have reported this type of behavior for ions on the outer surface of the oxide.a- s In this letter, experimental data is presented that demonstrates simi­lar behavior for mobile charge in the interior of the oxide.

Mobile ions in the oxide have been shown to be positiveS-9 by numerous investigators and therefore should be expected to be drifted in the vicinity of the

These data show that, consistent with the hypothesis, the density of mobile ions in the oxide over the n-region in p-on-n structures decreases in the vicinity of the p-region; conversely, the density of mobile ions in the oxide over the p-region in n-on-p structures increases in the vicinity of the n-region.

These data show that

p-n junction from the region over the n-region to that (1) The field at the edge of a p-n junction influences over the p-region. Experimental data supporting this the mobile charge density in the oxide layer over the hypothesis is given below. silicon in microcircuit structures.

We have made a comparison of the measured charge (2) This influence must be taken into account when densities in MOS capacitors and MOS transistors of measured mobile charge densities are compared. each of two channel lengths, fabricated on the same (3) p-on-n MOS devices should be expected to be chips. This was done for both n-on-p and for p-on-n more stable than n-on-p or complementary MOS de­structures. All of the capacitors were 30X30 mi12. The vices for a given level of contamination (or easier to transistor channel regions (under the aluminum gate make to a given level of stability). metal) had the dimensions given in Table 1. (4) Double-diffused npn bipolar devices should be

The p-on-n devices were fabricated by a diffusion of expected to be more stable than double-diffused pnp boron into 5-0 cm phosphorus-doped (111 )-oriented bipolar devices. (npn bipolar devices can also be ex-

425

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426 EARL S. SCHLEGEL

TABLE II. Mobile charge density in different structures.

p-on-n n-on-p

MOS capacitor, 7 X 1011 cm-2

30X30 mil2

MOS transistor, 1. 8 0.7 I-mil channel length

MOS transistor, 0.4 2.1 0.4- or O. 6-mil channel length

pected to be more stable than pnp bipolar devices be­cause of the gettering by the phosphosilicate formed during the emitter diffusion. Yamin10 has shown that the boron emitter diffusion of a pnp device would weaken any gettering effect of the phosphosilicate formed by the base diffusion. Also, the positive QS8

adds to the stability of an npn and degrades that of a pnp device because it decreases the likelihood that the surface of the region of highest resistivity (the collector region) becomes intrinsic or inverted.)

* The research in this paper was sponsored in part by the Electronics Research Center under NASA Contract NAS12-544.

1 E. S. Schlegel, IEEE Trans. Electron Devices 15, 951 (1968). 2 E. S. Schlegel, IEEE Trans. Electron Devices 14, 728 (1967). 3 W. Shockley, H. J. Queisser, and W. W. Hooper, Phys. Rev.

Lett. 11, 489 (1963). 4 W. Shockley, W. W. Hooper, H. J. Queisser, and W. Schroen,

Surface Sci. 2, 277 (1964). • E. S. Schlegel, G. L. Schnable, R. F. Schwarz, and J. P.

Spratt, IEEE Trans. Electron Devices 15, 973 (1968). 6 E. H. Snow, A. S. Grove, B. E. Deal, and C. T. Sah, J. Appl.

Phys. 36,1664 (1965). 7 E. Yon, W. H. Ko, and A. B. Kuper, IEEE Trans. Electron

Devices 13,276 (1966). 8 T. M. Buck, F. G. Allen, J. V. Dalton, and J. D. Struthers,

J. Electrochem. Soc. 114,862 (1967). 9 S. R. Hofstein, IEEE Trans. Electron Devices 13, 227 (1966). 10 M. Yamin, IEEE Trans. Electron Devices 13, 256 (1966).

JOURNAL OF APPLIED PHYSICS VOLUME 42, NUMBER I' JANUARY 1971

Distribution Coefficient of Germanium in Gallium Arsenide Crystals Grown from Gallium Solutions

F. E. ROSZTOCZY* AND K. B. WOLFSTIRN

Bell Telephone Laboratories, Incorporated, Murray Hill, New Jersey 07974

(Received 30 April 1970; in final form 15 June 1970)

Gallium arsenide crystals doped with germanium were grown from gallium solutions at 900°-875°C. The Ge concentration in the liquid was varied from 0.004 to 56 at.%, and the Ge concentration in the GaAs crystals determined using radiotracer and other techniques. The Ge concentration in the solid varied linearly with increasing Ge concentration in the liquid up to 5 at. % and kG. = (Ge,) / (Gel) = 0.0083± 0.001. Above 5-at.% Ge in the growth solution, kG. increased. At low doping levels Ge acts predominantly as a simple acceptor substituting on arsenic sites. At high doping levels, in the extrinsic range, the Ge concentration in the solid is considerably greater than the free carrier concentration. The form in which the excess Ge exists is not known.

INTRODUCTION

This study of the behavior of Ge in GaAs is a con­tinuation of previous work dealing with the amphoteric character of Ge and Si in GaAs,l-3 Earlier investiga­tions4-8 showed that Ge could act as a donor or an acceptor in GaAs. The variation of the optical and elec­trical properties of the crystals with increasing Ge con­centration in the growth solution has been studied in detail.1·9-11 The distribution coefficient of Ge [kGe= (Ge.) / (Gel) ] for melt-grown GaAs has been reported as 0.018 by Whelan et at.,4 0.03 by Weisberg et at.,12 and more recently as 0.01 by Willardson and AllredY The distribution coefficient of Ge for GaAs grown from Ga solution around 900°C has been variously estimated by Kressel et af.1° as 0.08, by Constantinescu and Petrescu-Prahova9 as 0.012, and by Rosztoczy et at.1 as 0.008.

The present investigation was undertaken to deter­mine the relationship between the Ge concentration in the Ga-GaAs-Ge solution and in the GaAs crystals grown from that solution at 900°-875°C. The Ge con­centration in the solid (Ge8 ) was determined by radio­tracer techniques supplemented by mass spectrographic and electron probe analyses. The total impurity con­centration (NI~Ge.) were also deduced from room­temperature Hall mobility.

EXPERIMENTAL

A. Crystal Growth and Sample Preparation

The epitaxial layers were grown from Ga solutions by cooling from 900° to 875°C. The tipping method and the Hall-effect measurements were described previously 1

and will not be repeated here. The epitaxial layers grown

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