We investigated the characteristics of sub-picosecond radiation pulses from mid-size-gap multi-energy arsenic-ion-implanted GaAs photoconductive antennas in terahertz time-domain spectroscopy experiments. At fixed fluence excitation, both the bandwidth and the negative peak of terahertz waveform minimum reveal increasing trends with increases in the antenna's gap. We reproduce both the measured terahertz waveforms and increasing trends numerically, by calculating a model that incorporates the

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... ap-enhanced bias field with wave equation. We deduce from our simulation that the gap-dependent behavior stems from the fact that an antenna with a larger gap has a stronger trap-enhanced bias field near the anode edge and thereby induces a larger space-charge screening effect and bandwidth.