6.2.3.3 Dependence of C_G on Passivation Thickness

As shown in (62) the second term of C_G depends linearly on e_r. Unfortunately, there exist no suitable passivation materials with e_r significantly smaller than 7. A way to practically reduce the average e_r in the space between semiconductor surface and contacts is to decrease the thickness of the dielectric coating such that this space is partially filled with air. This was done experimentally in [80] by selectively etching away part of the passivation.

The change in C_G strongly depends on the electric field E in the space where the passivation is removed, (i. e., e_r is changed from 7 to 1).  Figure 6.33 shows the spatial distribution of E for the HEMT with L_G = 220 nm at a bias V_DS = 2.0 V, V_GS = 0.4 V. The highest field appears at the drain-sided end of the gate footprint and decreases strongly towards the top of the gate contact. Therefore, only a moderate decrease of C_G is expected when only the top part of the passivation is removed.
 

This is quantitatively shown in  Figure 6.34. Simulations were performed for partially removed passivation, i. e. e_r2 =  7 and e_r1 = 1, where the interface between both regions is horizontal as indicated in  Figure 6.21. The capacitance C_G is reduced by about 120 fF/mm when the passivation is thinned to a residual thickness of 200 nm (passivation surface on a level with cap surface). The same decrease of C_G would be obtained by a reduction of L_G to about 40 nm, as can be deduced from  Figure 6.32. An even larger reduction of about 500 fF/mm is calculated for complete omission of the dielectric layer (case e_r  = 1 in Figure 6.32). For comparison, the term L_G A₃ in (62) has a value of about 700 fF/mm for the HEMT with L_G = 220 nm.

Among the various capacitances that characterize a transistor, we have up to now only considered C_G and its influence on f_T. However, as a figure of merit for RF performance, the maximum frequency of oscillation f_max is even more significant than f_T. Prerequisite for high f_max is a large ratio C_GS/C_GD.

In  Figure 6.34, the calculated quantities C_GS, C_GD and C_GS/C_GD are shown as a function of passivation thickness. With decreasing thickness, C_GD decreases more rapidly than C_GS, in our particular case especially for thicknesses below 200 nm. The ratio C_GS/C_GD accordingly rises dramatically.

f_max can be calculated according to (11) under the assumption that only C_GS and C_GD are changed by the passivation thickness. In reality the interface charge density on the semiconductor surface is strongly increased when the passivation gets very thin or is even removed completely. An increased interface charge density reduces the current for high V_GS and thus deteriorates g_m [80]. Additional drawbacks of removing the passivation completely are low yield in the manufacturing process and serious reliability problems.

In  Figure 6.35 the calculated f_T and f_max is shown. f_T increases strongly from 53 GHz to about 80 GHz but f_max even increases from about 140 GHz by a factor of 3. As a more practical case we can consider the previous case of a residual passivation thickness of 200 nm. It can be expected that f_T increases from 53 GHz to 60 GHz and f_max from 140 GHz to 180 GHz.
 

This result leads to two important conclusions: First, for high f_max, a thin passivation is very favorable, but, secondly, in this regime of small parasitic capacitances, C_GS/C_GD is subject to very rapid changes caused by very small process variations. Consequently, it is extremely difficult to achieve tight control over f_max, and hence the gain.
 

• 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