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⁺-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