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We report very high-speed operation of InP/InGaAsP/InGaAs avalanche photodiodes with separate absorption, grading and multiplication regions (SAGM-APDs). For low multiplication values (M0Â¿7) the bandwidth of these APDs is relatively insensitive to the gain and is determined by the hole transit time and the RC time constant. In this gain region bandwidths as high as 5.5 GHz have been achieved. For higher multiplication values the frequency response exhibits a constant gain-bandwidth product. We have observed gain-bandwidth products as high as 40 GHz, the highest value reported to date for a device of this type.