**Next:** 6.1.2.2 Contributions to the
Gate Capacitance **Up:** 6.1.2 RF Characteristics**
Previous:** 6.1.2 RF Characteristics

**6.1.2.1 Reduction of the Gate
Length**

When *L*_{G} is reduced, *f*_{T} will increase,
but a simultaneous and undesirable increase of the output conductance *g*_{0}
must be expected due to short channel effects such as increased real space
transfer. In Figure
6.14 the simulated parameter *g*_{0} (at the bias point
*V*_{DS} = 2 V, *V*_{GS} = 0.2 V) is shown as
a function of *L*_{G} and compared to the experimental DC
values of HEMTs
A and B
which differ only with respect to *L*_{G} but not in any other
geometrical dimensions. The simulation is able to reproduce *g*_{0}
realistically.

Figure 6.14 Measured (filled symbols) and simulated
(open symbols) DC output conductance *g*_{0} versus the gate
length at the bias point *V*_{GS} = 0.2 V and *V*_{DS}
= 2.0 V
As described in Section 5.3.1, the increase
of *g*_{m ext} is only small when *L*_{G} is
reduced. The capacitance *C*_{GD} is nearly independent of
*L*_{G}. The gate-source capacitance *C*_{GS}
is only partly dependent on *L*_{G}: fringe and other parasitic
contributions are independent of *L*_{G}, only the part due
to the gate contact area is length dependent. Therefore, the improvement
of *f*_{T} which can be achieved by a reduction of *L*_{G}
depends on the relative contribution of the parasitic (i. e. constant)
part of the capacitance to the gate capacitance *C*_{G}.

In Figure
6.15 the simulated *f*_{T} is shown along with the measured
and deembedded *f*_{T} according to the procedure described
in Section 3.2. Although only the simulation of
HEMT
A was fitted to the deembedded *f*_{T} also the *f*_{T}
of HEMT
B is simulated very well. The measured values in Figure
6.15 (which are taken from HEMTs
A and B)
are even below the ones calculated with complete T-gate structure and passivation
nitride due to the presence of parasitic pad capacitances as discussed
before.

Figure 6.15 Measured (filled squares) and simulated
f_{T} versus *L*_{G} for a passivated HEMT with e_{r}
= 7 (filled circles) and e_{r} = 0 (open
circles) at *V*_{GS} = 0.2 V and *V*_{DS} = 2.0
V
The dependence of *f*_{T} on *L*_{G} is shown
in Figure
6.15 for two examples: the theoretical limiting case of e_{r}
= 0 where the contribution of parasitics is reduced to the inevitable fringe
capacitances at the gate edge, and the case of the presence of a medium
with e_{r} = 7 (a silicon nitride
passivation layer, for instance) which fills the space between the Tgate
overhang and the semiconductor surface in the manner sketched in
Figure
6.6. Figure
6.15 demonstrates clearly that the improvement of *f*_{T}
achieved by a reduction of *L*_{G} depends strongly on the
relative contribution of parasitic capacitive couplings to the total gate
capacitance. The reduction of *f*_{T} in the presence of passivation
layers has been experimentally observed by Wu et al. [78].

To obtain an *f*_{T} of 100 GHz, a fully passivated device
of the general structure of HEMT
A must be either supplied with a gate length below 100 nm, or the gate
length is left as large as in HEMT
B (240 nm) but no passivation is allowed at all. The case of an unpassivated
device in air with e_{r} = 1
is close to the idealized case e_{r}
= 0 plotted in Figure
6.15. The two curves in the Figure are calculated under the assumption
of a constant interface charge density which is certainly an idealization
when the unpassivated case is considered.

**Next:** 6.1.2.2 Contributions to the
Gate Capacitance **Up:** 6.1.2 RF Characteristics**
Previous:** 6.1.2 RF Characteristics

*Helmut Brech*
*1998-03-11*