6.1.2.2 Contributions to the Gate Capacitance

From the dependence on the gate length the different contributions to the total gate capacitance can be separated by simulation. We change the capacitive coupling of the gate metal to the ohmic contacts and to the semiconductor material by setting the permittivity of the passivation to different values. e_r = 1 corresponds to no passivation at all. The opposite extreme is represented by the case e_r = 7 (space between contacts completely filled with dielectrics).  Figure 6.16 shows the simulated C_G as a function of L_G for e_r = 0, 1 3 5 and 7. A linear fit can be found for all simulated C_G versus L_G.
 

The separation of the lines also reveals a linear dependence on e_r. Thus, C_G can be described by the expression
 

.

(62)

 
By extrapolation of the straight line for e_r = 1 to L_G = 0, the fringe capacitance C_F = A₁ + A₂ » 243 fF/mm is obtained. Because the two-dimensional gate cross section is fully taken into account, this value is higher than the one given by Wasserstrom and McKenna [79] who use infinitely thin contacts in their model. In the simulation e_r can be even decreased to zero. The linear interpolation of C_G for e_r = 0 gives A₁ = 177 fF/mm. A₁ corresponds to the fringe capacitance purely attributed to the semiconductor which depends only on the epitaxial structure and the separations of source, gate, and drain contacts.

From the separations of the straight lines in  Figure 6.16, A₂ can be deduced. This contribution is determined by the device cross section above the semiconductor surface. In our case, A₂ » 66 fF/mm. Finally, A₃ is given by the slope of the lines. Apart from some influences of minor importance (like lateral spacing of the three contacts), this quantity is dominated by the sheet charge under the gate. In our example, A₃ = 2.4 nF/mm² (for a gate with L_G = 100 nm, this is equivalent to 240 fF/mm). This last term in (62) is the only one necessary for the function of the transistor, the other two only reduce its performance.
 

• 6.1.2.3 Reduction of the Recess Length
• 6.2 Power HEMT for 0.9/1.9 GHz and 40 GHz Applications
• 6.2.1 DC Characteristics
• 6.2.2 Dependence of Recess Geometry on the E-Field Distribution
• 6.2.3 RF Characteristics
• 6.2.3.1 Reduction of the Gate Length
• 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