High-frequency transistors by the diffused-meltback process employing three impurities

The diffused-meltback process for making transistor structures involves growing a crystal containing a donor and an acceptor impurity, cutting the crystal into pellets, melting and refreezing part of a pellet, and then diffusing. The meltback process produces through segregation a spatial discontinuity in concentrations of both impurities, with the same impurity predominating throughout the entire pellet. The diffusion process then results in the appearance of a (base) region of opposite conductivity type very close to the meltback interface. If a single donor and acceptor are used, independent control of excess impurity concentrations in emitter, base, and collector regions is not normally practicable. However, if three impurities are used (i.e., two acceptors and one donor for a germanium p-n-p transistor), they may be so chosen that a different one is predominant in each region. Some limitations on the impurity concentrations that may be obtained are inherent in the crystal growing and segregation processes, but these are not important in the fabrication of a high-frequency signal transistor.