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In this paper a novel electrical method is described which allows the extraction of bandgap narrowing within the base of SiGe heterojunction bipolar transistors due to heavy doping effects and the presence of germanium. In addition it is shown that the methods sensitivity to doping tails makes it ideal for determining the presence of parasitic energy barriers due to B outdiffusion from the base, a cause of major concern for HBT technology. The analysis is applied to SiGe and SiGeC HBTs showing that background carbon incorporation (R 1020m2-3) completely suppresses TED.

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A comparison is made of the electrical effects of carbon in n-and p-type in situ doped polycrystalline Si 1Ϫy C y and Si 0.82Ϫy Ge 0.18 C y layers. Values of resistivity as a function of temperature, effective carrier concentration and Hall mobility are reported. The n-type polycrystalline Si 1Ϫy C y and Si 0.82Ϫy Ge 0.18 C y films show dramatic increases in resistivity with carbon content, rising from 0.044 ⍀ cm to 450 ⍀ cm ͑0 and 0.8% C͒ and 0.01 ⍀ cm to 2.4 ⍀ cm ͑0 and 0.6% C͒, respectively. In contrast, the increase in B-doped films is much less severe, rising from 0.001 ⍀ cm to 0.939 ⍀ cm ͑0 and 7.9% C͒ and 0.003 ⍀ cm to 0.015 ⍀ cm ͑0 and 4% C͒ for the Si 1Ϫy C y and Si 0.82Ϫy Ge 0.18 C y layers, respectively. The grain boundary energy barrier, determined from the temperature dependence of the resistivity, is found to vary as the square of the C content in the n-type polycrystalline Si 1Ϫy C y and Si 0.82Ϫy Ge 0.18 C y layers, but linearly in the p-type Si 1Ϫy C y layers. The square law dependence seen in the n-type layers for C contents up to 0.9% is explained by an increase in the grain boundary trap density due to the presence of carbon, whereas the linear relationship seen in the p-type layers for C contents between 2% and 8% is explained by a shift in the grain boundary trap energy toward the valence band. Finally, lower values of grain boundary energy barrier are obtained in p-type Si 0.82Ϫy Ge 0.18 C y layers with a C content of 4% than in equivalent Si 1Ϫy C y layers, which could be explained by a larger shift in trap energy toward the valence band.

The sheet resistance, effective carrier concentration, and Hall mobility of in situ boron-and phosphorus-doped polycrystalline Si 0.82Ϫy Ge 0.18 C y films are presented for carbon contents between 0% and 4%. Phosphorus and boron doping levels of 4ϫ10 19 and 2ϫ10 20 cm Ϫ3 were achieved for the n-and p-type layers, respectively, and remained largely unaffected by carbon content. The phosphorus-doped films showed a dramatic increase in sheet resistivity and a corresponding drop in effective carrier concentration and Hall mobility. In contrast, the boron-doped films showed only a minor increase in resistivity. This is attributed to interstitial carbon increasing the defect density and also shifting the defect energy levels at the grain boundaries towards the valence band. This causes an increase in the grain-boundary energy barrier in n-type layers, but leaves the p-type layers largely unaffected.