InP/InGaAs based Hi-Lo avalanche photodetectors for high speed optical communications

We present an overview on the progress of InP/InGaAs based Hi-Lo APD's, which are important for long-haul optical fiber communications. Much of recent research efforts have been focused on improving the operation reliability, the gain-bandwidth (GB) product, and reducing the excess noise factor. To achieve a high GB product and a reliable operation, the reduction of the thickness of the multiplication layer and an optimum design of the internal electric field distribution are essential. The concept of the planar InP/InGaAs APD is very important from this perspective and the Hi-Lo APD's are expected to play an important role. Hi-Lo APD employs an n-InP buffer layer, an InGaAs absorption layer, InGaAsP grading layers, an n-type field control layer, an i-InP multiplication layer, and a diffused p-InP layer. One- and two-dimensional analysis shows that the multiplication layer width (MLW) should be 0.2 ∼ 0.3 μm for a hundred GHz of GB product, and the charge density of field control layer should be 3.5 × 10¹²/cm². The suppression of edge breakdown is essential for the curved junction followed by diffusion process. We carried out the analysis of the avalanche gain factor as functions of epitaxial layer parameters. Our calculation shows that the avalanche gain factor depends critically on the MLW and the charge densities of the field control layer. Another important result is that the InGaAs thickness may be allowed within 1.0 μm in order to obtain 0.3 μm of MLW for suppression of avalanche gain factor at device edge.