6.3.2.4 Dependence of f_T and f_max on L_R

For further improvements of the RF characteristics of the millimeter wave HEMTs the dependence on L_R are investigated. In  Figure 6.48 f_T and f_max is shown for passivated and unpassivated devices. f_max was calculated with the same assumptions made in Section 6.1.2.3.
 

For passivated devices f_T is increased significantly from 91 GHz for L_R = 30 nm to 106 GHz for L_R = 90 nm. Only a minor reduction to f_T = 104 GHz for L_R = 170 nm can be observed. The improvement of f_max due to an increased L_R is even larger. The calculated f_max is 138 GHz and reaches a maximum of 194 GHz at L_R = 150 nm. Again the maximum of f_T is reached for smaller L_R than the maximum in f_max due to the larger impact of C_GD on f_max compared to its impact on f_T.

The maxima of both f_T and f_max are shifted to smaller L_R if the coupling between the contacts is reduced, and, even more important, the values are increased substantially. As discussed before, non passivated devices with the same DC characteristics as passivated devices mark a theoretical upper limit for the RF performance. But some of the improvements can again be obtained by thinning down the thickness of the passivation layers.
 

• 6.3.2.5 Dependence on the Passivation Thickness
• 6.3.3 Optimization of Millimeter Wave HEMTs
• 6.4 Comparison between Low Noise, Power, and Millimeter Wave HEMTs
• 6.5 Influence of Backside Doping on the C_G(V_GS) Characteristics