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Sentaurus TCAD issue in transient simulation

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Newbie level 4
May 7, 2011
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Hi there,

I am trying to perform a mixed-signal simulation in Sentaurus TCAD with the following circuit:
- a photodiode device (3D mesh)
- its readout circuit comprising three MOSFETS (using TCAD's SPICE/BSIM3 compact model converted from Spectre BSIM3V3 models from the fab's PDK)

In the middle of a transient simulation, I need to modify the Physics section to account for some TCAD limitations, then to continue the same transient simulation with the new Physics section. The simulation is then divided in two parts whereby I run the first part, use the Save command to save the internal state into a file. Then, I use a different model (different Physics) and use the Load command as a start point to continue the transient simulation.

This method works fine when no SPICE/BSIM3 model is present in the circuit. However, it appears that the Save command does not save the internal conditions of the BSIM3 transistors so when I load them later on, it causes some node voltages to be stepped due to bad initial conditions at the beginning of the second transient segment.

It is possible to specify the initial condition when instantiating a SPICE/BSIM3 model through the parameter “ic” as follows: Vector of D-S, G-S, B-S (initial voltages double[3]). However, for some reason, even when entering the initial Vds, Vgs, and Vbs voltages as input (known from the end of the previous segment), the simulator does not solve the discontinuity problem. Actually, the “ic” parameter seems to produce no effect at all.

Would anyone share some ideas on how to solve the initial condition problem for the MOSFET in the middle of a transient simulation? Thanks.

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