6.2.3.1 Reduction of the Gate Length

The measured f_T's were again corrected by the result of the deembedding procedure as described in Section 3.2. A comparison of the simulated f_T's with the measurements for the two experimentally examined values of L_G is shown in  Figure 6.31. The simulation of the device with L_G = 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 L_G = 500 nm coincide very well as shown in  Figure 6.31. f_T is reduced by both a linear increase in C_G and a slight reduction of g_m due to the reduction in L_G. Both effects lead to a non­linear f_T(L_G) characteristics. f_T is increased from 31 GHz for L_G = 500 nm to 53 GHz for L_G = 220 nm.
 

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

• 6.2.3.2 Contributions to the Gate Capacitance
• 6.2.3.3 Dependence of C_G on Passivation Thickness
• 6.2.3.4 Dependence of C_G on T-gate Cross Section
• 6.3 Millimeter Wave HEMTs
• 6.3.1 DC Characteristics
• 6.3.1.1 Dependence on the Gate-to-Channel Separation
• 6.3.1.2 Dependence on the Gate Length
• 6.3.2 RF Performance
• 6.3.2.1 Dependence on the Gate Length
• 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