Depletion region capacitance in MOSFET

[CoolDude420]

Homework Statement

Hi,

This isn't a homework question. I was reading the book "Design of Analog CMOS Integrated Circuits" by Behzad Razavi, in particular the section about the turn-on physics of a MOSFET (enhancement-mode)

Relevant Equations

N/A

I would like to ask about the behavior of a MOSFET as a capacitor, particularly at the onset of inversion/weak inversion - see Fig 2.6(c)

• At this stage in Fig 2.6(c), there is positive charge at the gate metal/poly contact, then an insulator oxide and then negative ions (positive holes have been repelled, this region is the depletion region) - this is the Cox capacitor in the schematic overlaying.
• My confusion is regarding the Cdep - depletion region capacitor. What are the two 'plates' or regions of charge accumulation that form the Cdep capacacitor? I assume one is the negative ions (sharing a plate with Cox) and the other is the repelled positive holes. But then, if you see my drawn schematic, that doesn't make sense, since the + charges are on top and negative ions should be on top?

Can someone explain to me how Razavi is extrapolating from the device physics that there is a Cdep capacitance? What are the two regions of charge (the "plates" in traditional capacitor sense) that form Cdep? Is one 'plate' shared with Cox? If it is, then my schematic of the + and - charge doesn't make sense anymore?

 

[Valkarie]

The Cdep capacitor refers to the depletion region of the MOSFET. The two plates of the capacitor are the gate metal/poly contact (which is positively charged) and the bottom of the depletion region (which is negatively charged). The depletion region is formed when negative ions (positive holes) are repelled away from the gate. This creates a barrier of negative charge between the gate and the source/drain, and this is what forms the depletion region capacitor. The positive charges are on top of the capacitor, while the negative ions are on the bottom.