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Applet Tutorial

In order to 'see' the equilibrium condition of MOS capacitor, do one of the two things: (1) click on "Form the Junction" button followed by "Equilibrate" button; or (2) mouse drag the M, O and S pieces together.

The equilibration process begins with charges around the O-Si interface: (Qox) + (Qch or Qb) = 0. This is purely an arbitrary situation that I viewed as the most 'natural' starting condition.  The equilibration process ends with the the condition: EFM in the gate metal = EFS in the Si.

Points to notice:

• EFM (the top of metal energy) moves toward EFS (the magenta color line in Si).
• A wire connection 'is needed' to equilibrate MOS because no current can flow through the oxide layer (insulator).
• Potential drop in Si occurs only for the depletion condition. For inversion or accumulation, potential drop in Si is neglible compared with the drop in the oxide layer.
• The kind of metal affects the EFM level.
• Type and doping level of Si affect the EFS level.
• Oxide thickness affects the amount of channel charge, Qch, needed for a corresponding oxide voltage drop, Vox.