MONTE-CARLO ANALYSIS OF THE SPACE-CHARGE EFFECT IN ALGAAS/GAAS BALLISTIC COLLECTION TRANSISTORS (BCTS) UNDER HIGH-CURRENT INJECTION

AlGaAs/GaAs Ballistic Collection Transistors (BCT's) 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.