Optical EngineeringGain and bandwidth analysis and comparison for gallium arsenide and silicon avalanche photodiodes with very thin multiplication layer
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We describe the calculation of gain and bandwidth of an n + -i-p + avalanche photodiode (APD) for a multiplication layer down to tens of nanometers. The computed results are used to make a comparative study of gallium arsenide (GaAs) and silicon (Si) APDs. In the analysis, the depletion region is discretized into equal energy segments to take into account the discontinuous nature of impact ionization in the multiplication layer due to dead-space effect. Also, the carrier diffusion from undepleted regions is considered to study the effect of low bias on the frequency response. Carrier distribution within the structure is obtained by a numerical solution of coupled equations and recurrence relations. The model is verified with some experimental data taken from literature. Results show that gain increases with bias more rapidly for thinner multiplication layer. The Si APD is thinner than GaAs APD for the same gain at a given bias. Diffusion causes significant reduction of bandwidth at a low gain, with the change being sharper for GaAs APD than for Si APD.