Space charge effects on the bandwidth of Ge/Si avalanche photodetectors

In this work we investigated the effects of bias voltage and incident power on the bandwidth of SACM Ge/Si APD using numerical analyses and compared them with experimental results. APD temporal response near breakdown voltage is dominated by electrical-field-initiated impact ionization process, thus the strength and distribution of electrical field offer clear explanation of the variation of bandwidth. Beyond the widely-discussed space-charge effect in the multiplication layer, which account for the reduction of electrical field, we here argue that, as the incident power increases the excess holes in the absorption layer also results in an overall and significant reduction of electric field in the multiplication layer. With these excess holes in consideration, an effective equivalent voltage is used to describe quantitatively this effect of incident power.