2022-10-02 12:15 Status: # MOSFET Capacitances Most important parasitic component we have. ## Saturation  Because the channel is effectivelty connected to the source, we typically lump C(channel, gate) with C(source, gate) and C(bulk, channel) to C(bulk, source).  Symbolically:  Like a wheel. ## Issues In another region, the channel area changes shape so this model may not be valid any longer. Also $C_{DB}$ (the junction capacitance) is highly voltage dependent. Often the simple model is sufficient. ## Equations  $C_{gd}=C_{OV}W$ Where $C_{OV}$ is the overlap capacitance constant ($fF/\mu m$) $C_{gs}=C_{OV}W+C_{GC} = C_{OV}W+\frac{2}{3}WLC_{OX}$ 2/3 is the 'fudge factor and $C_{OX}$ is the oxide capacitance ($fF/\mu m^2$) $C_{db}=A_DC_{JO}+P_DC_{JOSW}$ Where $C_{JO}$ is the zero-bias junction capacitance ($fF/\mu m^2$) and $C_{JOSW}$ is the zero-bias side-wall junction capacitance where the edge effect of the drain (depends on the field oxide, don't count the edge near the large depletion region underneath the gate) ($fF/\mu m$) $C_{sb}=(A_S+\frac{2}{3}WL)C_{jo}+P_SC_{josW}$ [^1] --- # References [^1]: [vr-4602-wk03-sc04-capacitances](../../Spaces/University/ELEC4602%20–%20Microelectronics%20Design%20and%20Technology/Lectures/W2/vr-4602-wk03-sc04-capacitances.mp4)