Most contemporary architectures of photodetectors based on two-dimensional materials include global gates for carrier density control and local p–n junctions in the channel. We study the dependence of photocurrent in such detectors on the light modulation frequency, fully taking into account the effects of distributed resistance and gate-channel capacitance. The decay of photocurrent with modulation frequency governs the response time. We find that the maximum modulation frequency is largely determined by the position of light-sensitive junction with respect to the middle of the channel. Largest modulation frequency is achieved for junctions in immediate vicinity of either source or drain contacts, while fast roll-off of the modulation characteristic is observed for junction in the middle of the channel.