220.127.116.11 Dependence on the Gate Length
The largest impact on the RF performance is again expected from a change
in LG. In Figure
6.44 the simulated fT versus LG
is shown for dGC = 10 nm and dGC =
13 nm corresponding to the devices shown in Figure
6.39 and Figure
6.40. Both characteristics are given for passivated and unpassivated
devices. For passivated devices with LG = 500 nm and
dGC = 10 nm fT = 36.3 GHz can be expected.
An even slightly larger value of 37.6 GHz is obtained for dGC
= 13 nm. fT is increased up to 78 GHz for LG
= 200 nm independent of dGC. If LG
is further reduced to 80 nm one obtains fT = 129 GHz
for the device with dGC = 10 nm compared to fT
= 118 GHz for dGC = 13 nm.
For unpassivated devices similar characteristics are obtained. For LG = 500 nm Figure 6.44 reveals fT = 41.7 GHz and fT = 44 GHz for dGC = 10 nm and dGC = 13 nm respectively. A cross over of the two characteristics can be observed for LG = 120 nm. For LG = 80 nm again the device with dGC = 10 nm exhibits the higher fT of 223.4 GHz compared to 216.4 GHz for dGC = 13 nm.
Although gm is increased substantially by a reduction
of dGC, fT is not increased significantly.
For devices with large LG, fT is even
reduced. This has to be attributed to a strong increase of CG.
Helmut Brech 1998-03-11