High-speed and low dark current InGaAs/InAlAs avalanche photodiodes with P-type absorption layers

In this paper, an undepleted p-type absorption layer is introduced into the InGaAs/InAlAs separate absorption, grading, charge, and multiplication avalanche photodiode to reduce the dark current and improve the bandwidth. With a multiplication factor 3, the proposed APD has a 28.5 GHz bandwidth and a dark current in the order of picoamperes. In contrast, the bandwidth of an APD with an intrinsic absorption layer is only 23 GHz at the same gain, and the dark current is two orders of magnitude greater. P-type graded doping absorption layers are introduced to form a quasi-field to accelerate electrons transport in the absorption layer. The effects of the concentration profile on the bandwidth were examined, and we found that optimizing the gradient of the concentration profile greatly increased the bandwidth. The effects of the doping concentration and the thickness of the charge layer on the electric field distribution and the operating range of the device were also investigated. The parameters selected for the charge layer allow a wide operating range for the device.