6.3.2.1 Dependence on the Gate Length

The largest impact on the RF performance is again expected from a change in L_G. In  Figure 6.44 the simulated f_T versus L_G is shown for d_GC = 10 nm and d_GC = 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 L_G = 500 nm and d_GC = 10 nm f_T = 36.3 GHz can be expected. An even slightly larger value of 37.6 GHz is obtained for d_GC = 13 nm. f_T is increased up to 78 GHz for L_G = 200 nm independent of d_GC. If L_G is further reduced to 80 nm one obtains f_T = 129 GHz for the device with d_GC = 10 nm compared to f_T = 118 GHz for d_GC = 13 nm.
 

For unpassivated devices similar characteristics are obtained. For L_G = 500 nm  Figure 6.44 reveals f_T = 41.7 GHz and f_T = 44 GHz for d_GC = 10 nm and d_GC = 13 nm respectively. A cross over of the two characteristics can be observed for L_G = 120 nm. For L_G = 80 nm again the device with d_GC = 10 nm exhibits the higher f_T of 223.4 GHz compared to 216.4 GHz for d_GC = 13 nm.

Although g_m is increased substantially by a reduction of d_GC, f_T is not increased significantly. For devices with large L_G, f_T is even reduced. This has to be attributed to a strong increase of C_G.
 

• 6.3.2.2 Contributions to the Gate Capacitance
• 6.3.2.3 Dependence of f_T on d_GC
• 6.3.2.4 Dependence of f_T and f_max on L_R
• 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