Negative differential resistance (NDR) was observed in the dc photocurrent-reverse bias characteristics at 1.55 μm wavelength of a planar InP/InGaAs/InP double-heterojunction p-i-n photodiode, which is designed for high-speed operation. In the bias range of measurements, two NDR regions can be observed, which are suggested to be mainly caused by the drop of average electron velocities in both the InGaAs absorption and InP buffer layers, i.e., the transferred electron effect. For providing evidence, power dependence of these NDR regions was investigated. Besides, another high-responsivity p-i-n photodiode was fabricated and characterized for comparison. © 1999 American Institute of Physics.