We have fabricated and tested metal–semiconductor–metal (MSM) photodetectors based on nitrogen-ion-implanted GaAs. Nitrogen ions with energy of 700 and 880 keV, respectively, were implanted into epitaxial GaAs films at an ion concentration of 3×1012 cm−2. Ti/Au MSM photodetectors with 1-μm-wide fingers were fabricated on top of the implanted GaAs. In comparison to low-temperature-grown GaAs photodetectors, produced in parallel in identical MSM geometry, the 880 keV N+-implanted photodetectors exhibited almost two orders of magnitude lower dark current (10 nA at 1 V bias) and the responsivity more than doubled (>20 mA/W at 20 V bias). Illumination with 100-fs-wide, 810 nm wavelength laser pulses, generated ∼2.5-ps-wide photoresponse signals with amplitudes as high as 2 V. The 2.5 ps relaxation time was the same for both the ion-implanted and low-temperature-grown devices and was limited by the MSM capacitance time constant. © 2003 American Institute of Physics.