Monte Carlo analysis of the space-charge effect in AlGaAs/GaAs ballistic collection transistors (BCTs) 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/sup +/-n/sup +/), modified collector structures which have n/sup -/-p/sup +/-n/sup +/ and p/sup -/-p/sup +/-n/sup +/ 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/sup -/-p/sup +/-n/sup +/ collector structure is effective for the suppression of the base-widening effect (Kirk effect) compared to the i-p/sup +/-n/sup +/ or p/sup -/-p/sup +/-n/sup + /structure. Donors in the n/sup -/ layer compensate for the negative space charges produced by near-ballistic electrons. For a simulated BCT with an n/sup -/-p/sup +/-n/sup +/ collector, the smaller collector capacitance charging time leads to improvement in current-gain cutoff frequency under high current injection.<
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