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After checking the ADS component palette, I found there are some MOSFET models available but there are no NAND flash cell models.

How do I create one?

A NAND flash cell can hold different states (different I- V characteristics) depending on how it was operated that affect the Vth and IV characteristic. I should be able to apply operations that change the states as well just like normal NAND flash.

If in any case there is no direct path to create a floating gate MOSFET on ADS. Is there something that can achieve or mimic its behavior, maybe with the existing components that ADS provides?

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I don't know about ADS, but you can model stored gate charge with a capacitor, and model the charge/discharge path with a voltage-controlled switch. I suppose you could model the MOSFET with an actual MOSFET from the component library, but it's also possible to use more voltage-controlled switches to act as the drain-source channels of a push-pull output arrangement, under control of the voltage across the capacitor:

schematic

simulate this circuit – Schematic created using CircuitLab

enter image description here

When node STORE is pulsed, capacitor Cg charges to whatever voltage is present at DIN. Output DOUT is a fake MOSFET push-pull stage whose gates are some function of the potential at node GATE.

I really should have put a resistor in series with Cg, to keep things legal, but it works as a starting point for whatever model you eventually implement.

You could also use a voltage-controlled voltage source (VCVS) as the surrogate output, since this also effectively isolates the gate from the "channels". You might also model charge leakage with a few Terohms of resistance from GATE to ground.

As for multiple charge levels, I'm not sure how you'd model MOSFET behaviour on the channel side, but perhaps you can use components like VCVS, VCCS and VCS, and their configurable behaviour (like gain, switching threshold etc), to produce something that does what you want.

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