Monte Carlo analysis of the space-charge effect in AlGaAs/GaAs Ballistic Collection Transistors (BCT's) under high current injection

AlGaAs/GaAs Ballistic Collection Transistors (BCTs) are investigated by self-consistent Monte Carlo simulation, focusing on the space-charge effect in the collector region. In addition to the conventional BCT collector structure (i-p(+)-n(+)), modified collector structures which have n(-)-p(+)-n(+) and p(-)-p(+)-n(+) doping profiles are examined. By taking account of the fact that the collector delay time is composed of transit time and capacitance charging time, it is shown that the n(-)-p(+)-n(+) collector structure is effective for the suppression of the base-widening effect (Kirk effect) compared to the i-p(+)-n(+) or p(+)-p(+)-n(+) structure. Donors in the n(-) layer compensate for the negative space charges produced by near-ballistic electrons. For a simulated BCT with an n(-)-p(+)-n(+) collector, the smaller collector capacitance charging time leads to improvement in current-gain cutoff frequency under high current injection.