Coupled Equivalent Circuit for High-Speed Photodiodes

A coupled equivalent circuit for high-speed Si photodiodes is proposed, which is composed of a traditional resistance-capacitance-limited equivalent circuit and a carrier transit-limited equivalent circuit connected by a voltage controlled current source. All the resistances and capacitances in the carrier transit-limited equivalent circuit have physical meaning according to Laplace Transform. The proposed coupled equivalent circuit was applied to the Si p-i-n photodiodes and agreed well with the measured reflection coefficients and frequency response curves. Again, the minority carrier lifetime, diffusivity and interface state is verified to be the main limiting factors to the frequency response. This improved modeling can provide an effective way to design high speed photodiodes.