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

Abstract

Abstract-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.

Silicon-germanium for ULSI

“…4. Schematic diagram of SiGe HBT produced using selective epitaxy [17]. lems can exist with excess leakage currents at the collector periphery [17].…”

“…Schematic diagram of SiGe HBT produced using selective epitaxy [17]. lems can exist with excess leakage currents at the collector periphery [17]. An advantage of HBT's is the reduced charge storage and this has initiated a re-appraisal of saturated logic families which can operate at very low voltages.…”

0.18 Micron BiCMOS Process with Novel Structure SiGeC HBT

“…The major application area of SiGe HBTs is for amplifiers including power amplifiers (PA) and low noise amplifiers (LNA). High voltage SiGe HBTs and high speed SiGe HBTs are convenient for fabrication in one process flow and easy to form PA and LNA circuits on one chip, and the low noise figure of SiGe HBT meets the requirements of LNA (2) (3). SiGe HBTs have some key advantages over RFCMOS: 1.…”

SiGe HBTs on bonded wafer substrates

“…The transistor layers are grown using selective epitaxial growth (SEG) for the Si collector, followed in the same growth step by non-selective epitaxial growth (NSEG) for the p 1 SiGe base (nominally 12% Ge) and the n-Si emitter cap [2]. The selective Si collector was grown at 9008C and the non-selective SiGe base and Si emitter cap at 7508C.…”