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6.2.3.1 Reduction of the Gate Length

The measured fT's were again corrected by the result of the deembedding procedure as described in Section 3.2. A comparison of the simulated fT's with the measurements for the two experimentally examined values of LG is shown in  Figure 6.31. The simulation of the device with LG = 220 nm was fitted to the measurements by a slight correction of the nominal given T­gate shape. Using the same T­gate shape also the simulation and measurement of the device with LG = 500 nm coincide very well as shown in  Figure 6.31. fT is reduced by both a linear increase in CG and a slight reduction of gm due to the reduction in LG. Both effects lead to a non­linear fT(LG) characteristics. fT is increased from 31 GHz for LG = 500 nm to 53 GHz for LG = 220 nm.
 

 
Figure 6.31 Simulated and measured current gain cut-off frequency fT at VDS = 2.0 V, ID = 160 mA/mm.
 

It has to be expected that the increase in fT saturates for extremely small LG because gm is not increased anymore or even reduced due to short channel effects. To improve fT further other contributions to CG have to be reduced. Therefore, the influence of CG on the shape of the T­gate and the passivation thickness will be investigated in the following.
 



next up previous contents
Next: 6.2.3.2 Contributions to the Gate Capacitance Up: 6.2.3 RF Characteristics Previous: 6.2.3 RF Characteristics

Helmut Brech
1998-03-11