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6.2.3 RF Characteristics** Previous:** 6.2.3.3
Dependence of C_{G} on Passivation Thickness

**6.2.3.4 Dependence of C**_{G}
on T-Gate Cross Section

To investigate the influence of the gate cross section, different gate
geometries were simulated by varying the two characteristic dimensions
*d*_{G}_{ }and *d*_{T}. All calculations
were performed for *L*_{G} = 220 nm and *L*_{T}
= 820 nm except for the case of a rectangular gate as depicted in
Figure
6.36. One limiting case for the gate geometry is *d*_{G}
= 0 (as shown in Figure
6.33 and used for all simulations in Section 6.2
discussed up to here) whereas the other extreme is given by *d*_{G}
= *d*_{T} ("ideal" Tgate cross section).

Figure 6.36 Parameters to investigate the influence
of the gate cross section on C_{G}.
Figure
6.37 presents simulations of *C*_{G} and *f*_{T}
as a function of *d*_{G} for a completely passivated device
with *d*_{T} as a parameter. The closed circle in Figure
6.37 designates measurement and simulation of the particular HEMT shown
in Figure
6.33. Here, a distinct Tgate stem does not exist (*d*_{G}
= 0). According to Figure
6.37, the *f*_{T} of this device could be increased from
53 GHz to a value close to 60 GHz if the gate cross section could be improved
in a way that *d*_{G} changes from zero to 200 nm (*d*_{G}
= *d*_{T}, ideal Tgate). Further increase of *d*_{G}
and *d*_{T} would further improve *f*_{T}. Values
of about 65 GHz are expected for *d*_{G} = *d*_{T}
= 400 nm, without decreasing the gate length. In the case that *d*_{G}
= 0 and only *d*_{T} = 400 nm (gate stem sidewalls and device
surface enclose an angle of 53°), again *f*_{T}
60 GHz is expected, comparable to the case *d*_{G} = *d*_{T}
= 200 nm.

Figure 6.37 Simulated gate capacitance *C*_{G}
and current gain cut-off frequency *f*_{T} for *L*_{G}
= 220 nm and different gate cross sections (*V*_{DS} = 2.0
V, *V*_{GS} = 0.4 V). All calculations for *L*_{T}
= 800 nm.
Thus, it is of immense importance that the process technology is able
to realize a gate cross section where the gate stem sidewalls are really
perpendicular to the semiconductor surface (ideal Tgate). If the enclosed
angle is significantly smaller than 90° (Vshaped gate stem), *d*_{T}
has to be substantially greater compared to the perpendicular case if the
same *f*_{T} has to be realized. The multi-resist level approach
of most EBL processes facilitates the fabrication of such vertical Tgate
stems as shown in Figure
6.3. Gate technologies that use optical lithography are usually based
on narrowing the comparatively large resist openings by spacers. They often
have the property not to result in really vertical sidewalls as illustrated
in Figure
6.2. If such a technology is used for the fabrication of devices for
high-frequency application, it is important that at least the part of the
gate immediately adjacent to the semiconductor surface has perpendicular
sidewalls.

A rectangular gate shape shown in Figure
6.36 represents the limit for arbitrarily increased *d*_{T}
and *d*_{G}. The *C*_{G} of a fully passivated
device with a rectangular gate is indicated in Figure
6.37 by the upper bold line and the *C*_{G} for a device
without passivation by the lower bold line.

The upper limit in *f*_{T} is marked by the case of a rectangular
gate and no passivation which decreases the total *C*_{G}
to about 820 fF/mm and increases *f*_{T} to 88 GHz. Both rectangular
gate and no passivation are rather unpractical limits although they can
be realized as reported in [75].
The disadvantage of a rectangular gate is a significant increase in the
gate resistance *R*_{G}. This becomes especially important
for devices with long gate fingers such as power devices and for extremely
high frequencies of up to 100 GHz were the skin effect becomes significant.

Based on the results presented so far the following considerations on
the optimization of the RF performance of the HEMT investigated here can
be made. A reduction of the gate length from 220 nm to 120 nm would reduce
*C*_{G} by 320 fF/mm. The drawback would be a larger *g*_{0}.
As shown in Section 6.1.2 for the single heterojunction
HEMT *g*_{0} increases by 100 % to 30 mS/mm which is still
an acceptable value. In addition a single heterojunction HEMT is a worst
case assumption because it is expected that short channel effects are more
severe due to the lower barrier height under the channel compared to a
double heterojunction HEMT.

A realistic optimization of the Tgate shape with *d*_{G}
= 150 nm and *d*_{T} = 300 nm would yield a reduction of *C*_{G}
of 210 fF/mm. This reduces *A*_{2} of (62)
from 85 fF/mm to about 55 fF/mm such that a reduction of the passivation
thickness would have a reduced impact. For a realistic reduction from 700
nm to a residual passivation thickness of 200 nm a reduction in *C*_{G}
by about 70 fF/mm can be expected. These improvements together sum up to
a reduction *C*_{G} from 1.38 pF/mm to about 0.8 pF/mm which
translates into *f*_{T} = 92 GHz.

This shows that to practically increase *f*_{T} to significantly
over 100 GHz a higher *g*_{m} is necessary. This can be achieved
in first place by a smaller gate-to-channel separation. Unfortunately this
goes along with an increase in *C*_{G} due to an increase
in *A*_{3} of (62). But even
if the quotient of *g*_{m}/*A*_{3} would remain
constant this would still increase *f*_{T} due to the parasitic
capacitance which are unchanged by a reduction of *d*_{GC}.
A detailed investigation on the optimization of the RF performance of double
heterojunction HEMTs will be given in the following section.

**Next:** 6.3 Millimeter Wave HEMTs **Up:**
6.2.3 RF Characteristics** Previous:** 6.2.3.3
Dependence of C_{G} on Passivation Thickness

*Helmut Brech*
*1998-03-11*