CDG is very non-linear when:

  1. The channel is conducting and...
  2. The drain voltage drops below the gate voltage

This typically occurs at the miller plateau at turn-on. At turn off, the effect is seen as the VGS decreases from the maximum through the miller plateau.

The built-in MOSFET model extraction routines do not currently simulate this behavior and not all SPICE models model this behavior.

Reference materials: