ELECTRICAL DETERMINATION OF BANDGAP NARROWING IN BIPOLAR-TRANSISTORS WITH EPITAXIAL SI, EPITAXIAL SI1-XGEX, AND ION-IMPLANTED BASES

The apparent bandgap narrowing in bipolar transistors with epitaxial S i, epitaxial SiGe and ion implanted bases is measured from the tempera ture dependence of the collector current density J(c)(T). A graph of l nJ(c)(T)/J(o)(T) as a function of reciprocal temperature is plotted, a nd the apparent bandgap narrowing obtained from the slope. For epitaxi al bare transistors, in which the boron base profiles are abrupt, a li near J(c)(T)/J(o)(T) characteristic is obtained, which allows the unam biguous determination of the apparent bandgap narrowing, The measured values for epitaxial Si bases are in good agreement with the theoretic al model of Klaassen over a range of base doping concentrations. For S i0.88Ge0.12 and Si0.87Ge0.13 epitaxial base heterojunction bipolar tra nsistors (HBT's), values of bandgap narrowing of 119 and 121 meV are o btained due to the presence of the Ge, which can be compared with theo retical values of 111 and 118 meV. For the implanted base transistor, the J(c)(T)/J(o)(T) characteristic is not linear, and its slope is lar ger at high temperatures than at low, This behavior is explained by th e presence of a tail on the ion implanted profile, which dominates the Gummel number of the transistor at low temperatures.