6.2.3.2 Contributions to the Gate Capacitance

In order to identify the different contributions to C_G, we use the procedure described in Section 6.1.2. The capacitive coupling of the gate metal to the ohmic contacts and to the semiconductor material is minimized by setting e_r1 = e_r2 = e_r = 1 (no passivation at all). The opposite extreme is represented by the case e_r = 7.  Figure 6.32 shows the simulated C_G as a function of L_G for both cases. Again a linear fit can be found for C_G versus L_G and also on e_r which was demonstrated in Section 6.1.2:
 

.

(62)

 

By extrapolation of the straight line for e_r = 1 to L_G = 0, the fringe capacitance C_F = A₁ + A₂ is obtained. In Figure 6.32, C_F  200 fF/mm. From the separation of the two straight lines A₂ can be deduced. In this case, A₂ » 85 fF/mm. Finally, A₃ = 3.2 nF/mm² is given by the slope of the two lines. (for a gate with L_G = 100 nm, this is equivalent to 320 fF/mm).

A₁ and A₃ depend on the epitaxial layers, whereas  is given by the geometry of the contacts and the permittivity of the dielectric. In the following the influence of  on the thickness of the passivation and the shape of the T­gate will be investigated.
 

• 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