High performance operation of silicon bipolar transistors at liquid nitrogen temperature

The feasibility of liquid nitrogen temperature operation of homojunction Si bipolar transistors is examined. Measurements of thin base NPN transistors with increasing base doping show the competing mechanisms of bandgap narrowing with mobility and freeze-out. When the bandgap difference between emitter and base becomes small, a maximum in current gain versus temperature is observed because the increase in mobility dominates at temperatures as low as 180 Kelvin. Delay calculations are made to compare the performance of an ECL ring-oscillator at room and liquid nitrogen temperature. It is shown that little performance degradation occurs if the base doping can be raised to 1 - 2 × 10¹⁹cm^-3to prevent freeze-out of the base doping, while keeping the base thickness within 1000 Å. The upper limit on base doping appears to be set by increased base-emitter leakage due to tunneling.