Impurity-density distribution in the base region of drift transistors

The nature of the impurity grading in the base of drift transistors is studied by measuring the dependence of the base transit time on collector voltage. Provided that modulation of collector-depletion layer width caused by change of collector voltage occurs only in the base material, as in the case of an alloyed collector, it is possible to deduce the base-region field parameterm=\Delta V/(kT/q)from such transit-time measurements. By this means, the validity of assumed distributions of impurity density may be verified; in particular, it may be established whether the impurity grading approximates an exponential or a complementary error function (erfc) form. Results are given for a number of drift-transistor samples, most of which are believed to have undergone impurity diffusion into the base material from a constant surface concentration during fabrication. In all cases, however, interpretation of measured data indicates a base impurity-density distribution approximating exponential rather than erfc form to be present.