6.2.3.2 Contributions to the Gate
Capacitance
In order to identify the different contributions to C_{G},
we use the procedure described in Section 6.1.2.
The capacitive coupling of the gate metal to the ohmic contacts and to
the semiconductor material is minimized by setting e_{r1}
= e_{r2} = e_{r}
= 1 (no passivation at all). The opposite extreme is represented by the
case e_{r} = 7. Figure
6.32 shows the simulated C_{G} as a function of L_{G}
for both cases. Again a linear fit can be found for C_{G}
versus L_{G} and also on e_{r}
which was demonstrated in Section 6.1.2:
. 
(62)

By extrapolation of the straight line for e_{r} = 1 to L_{G} = 0, the fringe capacitance C_{F} = A_{1} + A_{2} is obtained. In Figure 6.32, C_{F} 200 fF/mm. From the separation of the two straight lines A_{2} can be deduced. In this case, A_{2} » 85 fF/mm. Finally, A_{3} = 3.2 nF/mm^{2} is given by the slope of the two lines. (for a gate with L_{G} = 100 nm, this is equivalent to 320 fF/mm).
A_{1} and A_{3} depend on the epitaxial
layers, whereas
is given by the geometry of the contacts and the permittivity of the dielectric.
In the following the influence of
on the thickness of the passivation and the shape of the Tgate will
be investigated.
Helmut Brech 19980311