The RC approximations in Section 5.5
and 5.6 also require an estimation of a
transistor's gate and drain capacitances.
The gate capacitance of a transistor consists of two parts: the
capacitance of the gate itself, and the capacitance of the polysilicon line
going into the gate. If 
is the effective length
of the transistor, 
is the length of the poly line
going into the gate, 
is the capacitance of the
gate per unit area, and 
is the poly line capacitance
per unit area, then a transistor of width W has a
gate capacitance of:
The same formula holds for both NMOS and PMOS transistors.
The value of depends on whether the transistor
is being used as a pass transistor,
or as a pull-up or pull-down transistor in a static gate.
Thus, two different values of are required.