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Leakage current mechanisms in SiGe HBTs fabricated using selective and nonselective epitaxy

Schiz, J. F. W., Lamb, Andrew C., Cristiano, Fuccio, Bonar, J. M., Ashburn, Peter, Hall, Stephen and Hemment, P. L. F. (2001) Leakage current mechanisms in SiGe HBTs fabricated using selective and nonselective epitaxy IEEE Transactions on Electron Devices. pp. 2492-2499.


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SiGe heterojunction bipolar transistors (HBTs) have been fabricated using selective epitaxy for the Si collector, followed in the same growth step by nonselective epitaxy for the p+ SiGe base and n-Si emitter cap. DC electrical characteristics are compared with cross-section TEM images to identify the mechanisms and origins of leakage currents associated with the epitaxy in two different types of transistor. In the first type, the polysilicon emitter is smaller than the collector active area, so that the extrinsic base implant penetrates into the single-crystal Si and SiGe around the perimeter of the emitter and the polycrystalline Si and SiGe extrinsic base. In these transistors, the Gummel plots are near-ideal and there is no evidence of emitter/collector leakage. In the second type, the collector active area is smaller than the polysilicon emitter, so the extrinsic base implant only penetrates into the polysilicon extrinsic base. In these transistors, the leakage currents observed depend on the base doping level. In transistors with a low doped base, emitter/collector and emitter/base leakage is observed, whereas in transistors with a high doped base only emitter/base leakage is observed. The emitter/collector leakage is explained by punch through of the base caused by thinning of the SiGe base at the emitter perimeter. The emitter/base leakage is shown to be due to a Poole-Frenkel mechanism and is explained by penetration of the emitter/base depletion region into the p+ polysilicon extrinsic base at the emitter periphery. Variable collector/base reverse leakage currents are observed and a variety of mechanisms are observed, including Shockley-Read-Hall recombination, trap assisted tunneling, Poole-Frenkel and band to band tunneling. These results are explained by the presence of polysilicon grains on the sidewalls of the field oxide at the collector perimeter

Item Type: Article
Divisions : Faculty of Engineering and Physical Sciences > Electronic Engineering > Advanced Technology Institute > Ion Beam Centre
Authors :
Schiz, J. F. W.
Lamb, Andrew C.
Cristiano, Fuccio
Bonar, J. M.
Ashburn, Peter
Hall, Stephen
Hemment, P. L. F.
Date : 1 November 2001
Additional Information : Schiz, J.F.W., Lamb, A.C., Cristiano, F., Bonar, J, Ashburn, P., Hall, S., & Hemment, P.L.F. (2001). Leakage current mechanisms in SiGe HBTs fabricated using selective and nonselective epitaxy. IEEE Transactions on Electron Devices, 48, 2492 - 2499. © 2001 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE.
Depositing User : Mr Adam Field
Date Deposited : 27 May 2010 14:39
Last Modified : 31 Oct 2017 14:01

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