6.1.2.2 Contributions to the Gate
Capacitance
From the dependence on the gate length the different contributions to
the total gate capacitance can be separated by simulation. We change the
capacitive coupling of the gate metal to the ohmic contacts and to the
semiconductor material by setting the permittivity of the passivation to
different values. e_{r} = 1 corresponds
to no passivation at all. The opposite extreme is represented by the case
e_{r} = 7 (space between contacts
completely filled with dielectrics). Figure
6.16 shows the simulated C_{G} as a function of L_{G}
for e_{r} = 0, 1 3 5 and 7. A
linear fit can be found for all simulated C_{G} versus L_{G}.
The separation of the lines also reveals a linear dependence on e_{r}.
Thus, C_{G} can be described by the expression
. 
(62)

From the separations of the straight lines in Figure
6.16, A_{2} can be deduced. This contribution is determined
by the device cross section above the semiconductor surface. In our case,
A_{2} » 66 fF/mm.
Finally, A_{3} is given by the slope of the lines. Apart
from some influences of minor importance (like lateral spacing of the three
contacts), this quantity is dominated by the sheet charge under the gate.
In our example, A_{3} = 2.4 nF/mm^{2} (for a gate
with L_{G} = 100 nm, this is equivalent to 240 fF/mm). This
last term in (62) is
the only one necessary for the function of the transistor, the other two
only reduce its performance.
Helmut Brech 19980311