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This lecture (page 11) said that as Vgs increases further, the inversion layer forms and the channel to bulk capacitance is “shielded”. So the channel to bulk capacitance is 0.

Could anyone explain why? What does "shield" actually mean here?

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A capacitance is the result of a field between two plates.

For the MOSCAP in depletion the first plate is the gate and the second the bulk. As soon as the transistor gets into inversion a channel forms and acts like a third plate.

Since the channel is between the gate and the bulk the field lines originating from the gate will terminate on the channel. The bulk is effectively shielded and instead of a gate-bulk capacitance a gate-channel capacitance is seen.

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  • \$\begingroup\$ Thank you. In the depeletion region, there is no electric field so the capacitance is 0? Is this an theory? Also from the site below (page 4) says that in strong inversion the junction capacitor and Cox are in parallel. Could you explain why? aicdesign.org/SCNOTES/2010notes/Lect2UP060_(100324).pdf \$\endgroup\$ – anhnha Jul 3 '16 at 18:01
  • \$\begingroup\$ Which capacitance should be zero? The depletion region is of no importance since the channel screens it. The junction capacitance is something completely different, it would be important for a reverse biased junction. \$\endgroup\$ – Mario Jul 3 '16 at 18:09
  • \$\begingroup\$ Hi, is the channel to bulk capacitance called junction capacitance? \$\endgroup\$ – anhnha Jul 3 '16 at 18:19
  • \$\begingroup\$ Hi, is the channel to bulk capacitance called junction capacitance? If I am not mistaken. It is from the link above (page 3). \$\endgroup\$ – anhnha Jul 3 '16 at 18:25
  • \$\begingroup\$ The gate to bulk capacitance will be the total of gate to channel and channel to bulk (junction capacitor) in series. However, as the link below (page 3 and 4) the gate to channel and channel to bulk capacitors are in parallel in strong inversion. I dont see how they can be parallel here. Could you explain that? aicdesign.org/SCNOTES/2010notes/Lect2UP060_(100324).pdf \$\endgroup\$ – anhnha Jul 3 '16 at 18:30

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