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Summary form only given. We present the study of an ultrawide-band long-wavelength p-i-n vertical illuminated photodetector under high illumination using an electro-optic (EO) sampling technique. Our structure design utilizes an InGaAs/InP double heterostructure with a 180-nm-thick absorption layer and 3-/spl mu/m/sup 2/ active area. Graded bandgap layers were inserted at the hetero-interfaces to minimize carrier trapping and to reduce series resistance. The diode RC time constant and parasitic capacitance of the detector were minimized by undercut etching and by using an air-bridged metal. A record 120-GHz bandwidth for long-wavelength detectors was obtained under low illumination.