Geoffrey_Coram
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Boe - But that parasitic PNP is generally not included in the spice models.
Tommy - I find it odd that you got a notice that it was leaving the linearized region; do you also have an earlier warning that it is entering the linearized region? When it is in the linearized region, then the current is inaccurate (linearized instead of the true exponential). Of course, immediately after it exits the LR, it will still have a very large bulk current.
Whether you get latch-up or not depends on how the 3rd terminal of the BJT (the source of the MOS in this case) is connected. However, in the case that the drain-bulk current is linearized, that means that the junction is forward biased (p drain higher than nwell bulk). Usually, latch-up occurs when the junction is reverse-biased (for a PMOS, avalanche current from bulk to drain lowers the bulk potential such that the source-bulk junction is forward-biased).
The impact ionization current is responsible for sustaining latch-up, so you can't just measure the external base/bulk current and multiply this by beta. (BVceo is sometimes measured by detecting where the base current changes sign!)
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