Zn-diffusion experiments were performed in an open furnace with InP and In0.53Ga0.47As using Zn3P2 and Zn3As2 as source materials, respectively. A reproducible diffusion, as expected from the literature, was observed with InP in the temperature range between 400 and 600 degrees C and with In0.53Ga0.47As at 550 degrees C. Area-selective diffusion was performed through evaporated Al2O3 masks. Anomalously enhanced lateral diffusion under the mask observed with In0.53Ga0.47As was suppressed by a 200 nm thick InP cap layer between the ternary layer and the mask. A dark current density of as low as 40 pA/mu m(2) at a reverse bias of 5 V, measured with lateral diodes, shows that a nonleaky lateral pn junction was formed, which is effective in separation of optically generated free carriers. The response time at 15 V reverse bias was 35 ps for a photodetector with an electrode separation of 6 mu m. Simultaneously, low RC charging times in a 50 Omega load of 10 ps were found for the same bias conditions indicating that the transient behavior of the lateral pn photodiode is dominated by electron drift.