The new approach of generating THz signals, not by harmonic extraction, but by ballistic reflection is studied with optimized material concepts. We use an ensemble self-consistent Monte Carlo model to evaluate the generation of THz signals from ultrafast current pulses produced by quasi-ballistic electron transport and reflection in AlSb/InAs/AlSb heterostructures with a well of 200 nm length. The small electron effective mass in the -valley of InAs and the large energy separation between and satellite valleys allow electrons in InAs to propagate at a high speed at 300 K for a distance longer than 450 nm at an applied field of 20 kV/cm. The superior electron speed and the large conduction band offset of InAs/AlSb heterointerface are found to be effective to generate strong current oscillations at sub-picosecond time scales under a sinusoidal excitation.