Base profile design for high-performance operation of bipolar transistors at liquid-nitrogen temperature

Measurements of thin epitaxial-base polysilicon-emitter n-p-n transistors with increasing base doping show the effects of bandgap narrowing, mobility changes, and carrier freezeout. At room temperature the collector current at low injection is proportional to the integrated base charge, independent of the impurity distribution. At temperatures below 150 K, however, minority injection is dominated by the peak base doping because of the greater effectiveness of bandgap narrowing. When the peak doping in the base approaches 10¹⁹ cm^-3, the bandgap difference between emitter and base is sufficiently small that the current gain no longer monotonically decreases with lower temperature but instead shows a maximum as low as 180 K. The device design window appears limited at the low-current end by increased base-emitter leakage due to tunneling and by resistance control at the high-current end. Using the measured DC characteristics, circuit delay calculations are made to estimate the performance of an emitter-coupled logic ring oscillator at room and liquid-nitrogen temperatures. It is shown that if the base doping can be raised to 10¹⁹ cm^-3 while keeping the base thickness constant, the minimum delay at liquid-nitrogen temperature can approach the delay of optimized devices at room temperature