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We have developed and characterized ultrafast and highly sensitive photodetectors based on GaAs nanowhiskers fabricated with an improved top-down etching method. We evaluated the material properties of the etched nanowhiskers by micro photoluminescence measurements and studied the effect of post annealing on nanowhiskers' luminescence. The nanowhiskers were integrated into the coplanar striplines for device testing using DC and time-resolved electro-optic characterization techniques. Our photodetectors exhibit a very low dark current below 500 pA at 10 V bias as well as a high responsivity of 0.19 A/W at 30 V, and a cut-off frequency of 1.3 THz. These characteristics make the GaAs nanowhisker photodetectors very promising candidates for high-speed optoelectronics and efficient THz emitters.