Previously we reported on the pulse1 and frequency response2 of InP/InGaAsP/InGaAs avalanche photodiodes with separate absorption, grading, and multiplication regions (SAGM-APD). In this paper we describe several modifications we have made to the device structure, shown schematically in Fig. 1, to achieve significant improvement in both the frequency and pulse response. The changes include (1) reduction of the total depletion width to decrease the transit time of the free carriers, (2) use of low-resistance substrates to reduce the RC time constant, (3) introduction of an additional transition layer to eliminate effectively hole trapping at the heterojunction interfaces, and (4) increasing the carrier concentration in the multiplication region to reduce avalanche buildup time.
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