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6.3.2.5 Dependence on the Passivation Thickness

In  Figure 6.49 fT and fmax are shown as a function of the thickness of the passivation dP. The ideal case was assumed that the DC characteristics of the device would not deteriorate by reducing or even removing the passivation.
 

 
Figure 6.49 fT and fmax versus dP. Only a moderate increase in fT and fmax is obtained for a reduction of dP from 700 nm to 200 nm. If the passivation is removed in regions with high electric field the increase in both fT and fmax is much more significant.
 

fT is increased from 104 GHz for a fully passivated device to fT = 163 GHz for an unpassivated one. fmax increases accordingly from 170 GHz to over 400 GHz. It was shown in Section 6.2.3.3 for the power HEMTs that the electric field is rather low at the top of the passivation and increases significantly towards the gate edge. Therefore only a minor improvement shown for a reduction in dP from 700 nm until the surface of the passivation reaches the semiconductor interface.
 

next up previous contents
Next: 6.3.3 Optimization of Millimeter Wave HEMTs Up: 6.3.2 RF Performance Previous: 6.3.2.4 Dependence of fT and fmax on LR

Helmut Brech
1998-03-11