Frequency response of InP/InGaAsP/InGaAs avalanche photodiodes with separate absorption "grading" and multiplication regions

We have measured the frequency response of InP/ InGaAsP/InGaAs photodiodes with separate absorption, "grading," and multiplication regions (SAGM-APD's) for a wide range (
2 \leq M_{0} \leq 35
) of dc gains. The results are explained in terms of a theoretical model which incorporates the transit time of carriers through the depletion region, the RC time constant, the accumulation of holes at the valence band discontinuity of the heterojunction interfaces, and the gain-bandwidth limit.