IEEE TRANSACTIONS ON ELECTRON DEVICES. VOL. 36. NO. I I . NOVEMBER I Y X Y 263 1

three-dimensional effects in the comers of the emitter and that two- dimensional considerations fail to explain many results for a large variety of experimental conditions.

The experimental variations involve active base implants before and after the spacer formation, various base link implants between the extrinsic and the intrinsic base, and implanted and polysilicon emitters. In all cases the forward characteristics were ideal down to picoampere collector currents.

Without any exception it is found that the emitter-base reverse characteristics of narrow emitters (14 * 0.8 pm2) show higher re- verse leakage currents than large emitters (14 * 10 pm’), even when compared on an absolute scale. In addition it appears that a large walled emitter (i.e., with two sides overlapping the field ox- ide) tends to show far better reverse characteristics than a large non-walled emitter with a common rectangular geometry. The dif- ference in leakage current at a given reverse bias can be as high as two orders of magnitude!

Similar unexpected trends are observed in precision measure- ments on current gain degradation after constant current stress: 8 pC of reverse charge is sufficient to cause a 40-percent reduction in the current gain (at 1 nA) of a 14 * 10 pm2 walled emitter, whereas it causes less than a 10-percent reduction in the current gain of a 14 * 0.8 pm2 non-walled emitter.

These results can be explained qualitatively in terms of the 3-D concentration profiles of the boron under the oxide sidewall spacer in the comer of an emitter. The lateral supply of boron from two sides of p+ polysilicon causes a local increase of the boron con- centration in the comer of the emitter, which exceeds the boron concentration along a straight emitter edge. If the emitter width becomes less than about twice the radius of curvature of the comer, boron is supplied from three lateral directions, and the boron con- centration in the comer is further increased. Since tunneling cur- rents are strongly dependent on dopant concentrations, this causes a localization of reverse currents in the comers of the emitter, and thereby a strong localization of the stress-generated traps.

In summary, the experimental evidence presented above shows that emitter-base reverse tunneling and current gain degradation in advanced bipolar devices cannot be explained from emitter edge effects but are inherently limited by 3-D effects in the comers of the emitter. [l] G . P. Li er al. , “Identification and implication of a perimeter tunneling

current component in advanced self-aligned bipolar transistors,” IEEE Trans. Electron Devices, vol. 35, no. 1, p. 89, 1988.

[2] G . P. Li et a l . , “On the narrow-emitter effect of advanced shallow- profile bipolar transistors,” IEEE Trans. Electron Devices, vol. 35, no. 11, p. 1942, 1988.

VIB-8 Heteroepitaxial YBa2Cu307 - .-PrBa2Cu30, - x-

YBa2Cu307-* Weak Links Grown by Laser Deposition-C. T. Rogers,‘ A. M. S . Hegde,’ T . Venka te~an ,~ X. D. Wu,’ B. D ~ t t a . ~

Many of the applications envisioned for the high-temperature superconductors (HTSC) lie in the area of Josephson electronics, but realization of this technological promise requires a controllable HTSC weak-link fabrication process. We have developed such a process for producing weak links from the high-temperature super- conductor YBa2Cu307 - -* (YBCO), using PrBa2Cu307 ~x (PBCO) as a lattice-matched semiconducting barrier layer: The first step is the growth of a heteroepitaxial four-layer structure of YBCO- PBCO-YBCO-Au, using our standard laser deposition process [l]. All layers are grown during a single cycle of the vacuum system. We use Rutherford backscattering, both in the random and chan- neling modes, and X-ray scattering to verify layer stoichometry and heteroepitaxial growth of the orthorhombic perovskite struc- ture, c-axis normal to the substrate. Both YBCO layers have sharp superconducting transitions above 80 K upon removal from the vacuum system. Single devices with areas A from 2.5 X lo-‘ cm2 to 2 x cm2 are isolated with a four-step fabrication procedure involving standard photolithography and Ar ion milling [2]. These devices show current-voltage characteristics similar to those ob- served for low-temperature superconductorhormal metalisuper- conductor (SNS) devices. We find good scaling of the critical cur- rents I , with A and scaling of the resistances, R, with 1/A; the typical values of the I$J product of 3.5 mV are consistent with traditional SNS behavior. Further, we observe Shapiro steps in re- sponse to 100-GHz millimeter-wave radiation and oscillation of the dc supercurrent in a transverse magnetic field thus demonstrating that both the ac and dc Josephson effect occurs in these devices. These preliminary results suggest that our multilayer process can be optimized for a variety of Josephson devices essential in pro- ducing electronic circuitry from the HTSC systems.

‘Bellcore, Red Bank, NJ 07701. ’Physics Department, Rutgers University, Piscataway. NJ 08854. ’Indian Institute of Science, Bangalore, India 560 012. 4Physics Dept., Middlebury College. Middlebury, VT 05753.

[l] A. Inam, M. S. Hegde, X. D. Wu, T . Venkatesan. P. England, P. F. Miceli, E. W. Chase, C. C. Chang, J.-M. Tarascon, and J . B. Wacht- man, Appl. Phys. Lerr., vol. 53, p . 908, 1988.

[2] C. T. Rogers, T. L. Cheeks, P. England, S. W. Chan, H. G. Craig- head, T . Venkatesan, IEEE Trans. M a g . , vol. 21, p. 1309, 1989.