We present a numerical study of a high speed GaAs lateral p-i-n (LPIN) photodiode. The LPIN is a planar structure composed of interdigitated p(+) and n(+) wells. A metal-semiconductor-metal (MSM) photodiode with identical finger spacing and geometry is simulated for comparison. When pulsed with an 827 nm optical source at 0.68 mW/cm(2), the lateral p-i-n exhibited improved frequency performance and responsivity compared to the MSM. The dark current and capacitance are similar in magnitude between the two devices. Based on these results, it is concluded that a lateral p-i-n can be an attractive alternative to standard MSM photodetectors. (C) 1998 American Institute of Physics.