6.1.1.2 Comparison of HEMT A and HEMT C

Initially, HEMT C was assumed to possess the same distance d_GC = 25 nm as HEMT A but only a slightly longer L_G = 190 nm and a shorter L_R = 60 nm. When these values are used in the simulation, this results in a too negative value of V_T and an overestimated I_D, as can be seen in  Figure 6.10. However, a reduction of d_GC by 1.7 nm leads to perfect coincidence of measured and simulated curves, as it is also shown in  Figure 6.10.
 

Variations of d_GC of such small size can easily occur in different technology runs. The agreement between measurement and simulation is even more evident for the transconductance shown in  Figure 6.11. Both the peak value g_{m max} = 580 mS/mm and its occurrence at V_GS = 0 V are simulated very well.
 

We now return to the initial simulation of HEMT C which was performed under the assumption that d_GC is equal to the value of HEMT A (d_gc = 25 nm). When this simulation of a "hypothetical" HEMT C (circles in  Figure 6.11) is compared to the simulation of HEMT A in  Figure 6.7, it is found that the hypothetical device has a g_{m max} that is about 25 mS/mm larger than the value of HEMT A though its gate is 20 nm longer. This must be a consequence of L_R which is 75 nm shorter and obviously overcompensates the small effect of the slightly longer gate.

From the previous comparison between HEMTs A and B (which share the same L_R), we can estimate that an L_G increase of 20 nm only causes a negligible decrease of g_{m max} of about 5 mS/mm. This independently confirms the conclusion drawn above: the reduction of L_R had a stronger impact on the transconductance of HEMT C than the slightly longer L_G.

A drawback of the shrinkage of L_R is the increase of C_G. This can be seen in  Figure 6.12 which shows the simulated gate capacitance C_G of HEMTs A and C. For HEMT C, the whole function C_G(V_GS) is shifted towards higher values by about 130 fF/mm as compared to HEMT A. As the shift is already completely present in the pinchoff region it can be entirely attributed to the increased coupling between the gate and drain contacts.
 

• 6.1.2 RF Characteristics
• 6.1.2.1 Reduction of the Gate Length
• 6.1.2.2 Contributions to the Gate Capacitance
• 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