Cox calculation
For example: the gate capacitance Cgg = Cox * W * L. W and L can be obtained directly from device parameters. Next, we need the specific Cox value for the MOSFET.
1. Dielectric constant
The dielectric constant is a coefficient representing insulating capability, denoted by ε, with units of F/m (farads per meter).
2. Relative permittivity
The relative permittivity εr is the ratio of a material's permittivity ε to the vacuum permittivity ε0, i.e. εr = ε / ε0. εr is dimensionless. The vacuum permittivity ε0 = 8.854E-12 F/m. For SiO2, εr = 3.9, so ε = εr * ε0 = 3.9 * 8.854E-12 F/m.
3. Cox calculation
Cox is the ratio of the dielectric permittivity ε to tox. The oxide thickness tox can be found in the process model file (.scs). For the HHG90 process example:
In the model file, for NCH5 toxn = 1.15E-08 m, for PCH5 toxp = 1.14E-08 m. With SiO2 permittivity ε = 3.9 * 8.854E-12 F/m,
Coxn = ε / toxn = 3 fF/um^2;
Coxp = ε / toxp = 3.02 fF/um^2.
CGB, CGS and CGD capacitances
In saturation, the largest capacitance in the circuit is CGS. The transistor gate input capacitance equals CGG ≈ CGS. Compared with the gate-drain capacitance CGD, CGS is larger by 2/3 (W L Cox). In cutoff, CGB = W L Cox; in the saturation and linear regions it is zero.
