jfyan
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Re: inversion MOSFET against enhancement MOSFET
Hi Erikl,
yes, the serial connection of two caps will reduce the overall capacitance for small |Vpoly|, but with more negative Vpoly, the overall cap is still close to cox,because any small disturbance on Vpoly results in the change of "hole" only. this is because the amount of "hole" is much larger than the fixed charge. this means, the cap from junction is shorted.
thanks,
Jeff
erikl said:jfyan said:for poly over nwell cap, i think it is very close to MIS (metal-isolator-semiconduction) cap, and many references said that there is still a same capacitance (~cox) when Vpoly is enough lower than the voltage on nwell (low frequency).
here "enough" means, the generated "hole" is much more than doping level.
Sure, you're right, Jeff: a MOSCAP has a larger cap/area ratio than a MIS (or even a MIM) cap: the isolation layer of a MOSCAP (the gate oxide) is considerably thinner than the field oxide (FOX) of a MIS and still thinner than the (possibly high k) isolation layer of a MIM cap.
Your former question, however, was:
Here, my answer still is: no. If the poly is negative against the n-well (with a min. difference >≈ Vth), a p+ inversion layer will build up below the poly, creating a reversely polarized junction in the n-well in series with the gate oxide cap. Due to the rather low doping of the n-well, this reversely polarized junction produces a relatively wide depletion width, resulting in a low cap/area ratio of this junction cap. Being in series with the proper gate oxide cap, this low cap/area ratio junction cap reduces the overall cap/area ratio considerably (if not actually determines it).jfyan said:can i invert the polarity for the MOSCAP: can i tie the nwell to vdd, while poly to a lower voltage, is the same capacitance got?
Hence an inversion layer MOSCAP (n-well positive, poly negative) - due to the serial junction cap - will always show a smaller cap/area ratio than an enhancement (or: accumulation) MOSCAP (n-well negative, poly positive).
Hope I could explain this comprehensibly!?
erikl
Hi Erikl,
yes, the serial connection of two caps will reduce the overall capacitance for small |Vpoly|, but with more negative Vpoly, the overall cap is still close to cox,because any small disturbance on Vpoly results in the change of "hole" only. this is because the amount of "hole" is much larger than the fixed charge. this means, the cap from junction is shorted.
thanks,
Jeff