# Photodetector Bandwidth Simulation with RC Parasitic Effects

In this post we discuss bandwidth calculation with external RC components included, performed using the transient solver mode in CHARGE.

Background

The Application Gallery example Vertical Photodetector, demonstrates how to obtain the bandwidth of the photodetector using either transient or small signal AC simulation. These calculations are based on the current directly obtained from the contact, labeled as I which is available in the results once the simulation is run.

Simulation Setup

When setting up the simulation for a model with external RC, the time integration option in the transient tab needs to be set to TR-BDF2.

Also, since the simulation can become more demanding in terms of convergence, modifying convergence settings might be required (e.g. increasing global iteration limit in the Advanced settings of the solver). Detailed information on how to approach convergence troubleshooting is presented in this article.

RC Bandwidth

Once the RC components are added, in order to calculate the total bandwidth, we need to study the current flowing to the source (indicated below as Is).

Here in this configuration, the calculation can be performed by utilizing the voltage response of the contact (Vc), as Is is given by (Vs – Vc)/R. This information is available in the results for the contact once the simulation is run. For example, for the vertical photodetector and given Rse=100 Ohm and Csh = 0.1nF the time dependency of Vc is obtained as below:

The bandwidth calculation can be performed similarly to what is presented in the VPD example and by using the provided script (vertical_photodetector_3dB_bandwidth.lsf). The only difference would be to fetch Vc instead of I:

Examples

Performing this calculation for the mentioned setup (VPD with Rse = 100 Ohm and Csh = 0.1nF) leads to a total bandwidth of ~15.5 MHz:

This value approximates the bandwidth obtained for the RC component, derived by f = 1/(2$\pi$ RC), demonstrating the total bandwidth being primarily restricted by the external RC (note that the standalone VPD demonstrates a bandwidth of ~4 GHz).

In another example, the bandwidth derived for a VPD with Rse = 40 Ohm and Csh = 1pF is ~2.3 GHz:

Reflecting the more balanced contribution of the photodetector’s speed and the external RC to the total bandwidth.

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