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Current in bulk cascode device

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maxporter

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Hi all,

While simulating the output current of a 1uA current mirror I noticed a strange behavior. It’s a straight forward mirror: output is NMOS + cascode NMOS (biased at 950mV), 65nm PDK and 3V devices. Ideal supply and ground. While sweeping the output voltage I noticed an increase in current when Vout approaches Vsupply. Turns out a little bit current (7nA) is flowing in the bulk of the cascode. See screenshot: red is the output, green is Id of the NMOS current mirror (for both I changed the scale) and blue is the current in the bulk of the cascode. What effect am I seeing here? Thanks in advance.
current_mirror.png
 

Recall that the bulk of an NMOS is always connected to the lowest potential because we don't want the parasitic PN diodes to turn on. Now because the cascode device is at a higher potential, the parasitic PN diode is now reverse biased. What you are seeing is the reverse current of the parasitic diode and is temperature dependent.

Added note: It is actually the limiting factor for MOSFET based voltage references biased in subthreshold region.
 
Ok, reverse leakage current. Make sense. Thanks!
 

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