6.1.1.1 Comparison of HEMT A and HEMT B

Measured and simulated transfer characteristics of HEMTs A and B are shown in  Figure 6.7. Although the parameter fit procedure described above was only applied to HEMT A, also HEMT B is simulated precisely. Both measurements and simulations show that the drain current of HEMT A (L_G = 170 nm) is larger than the current of HEMT B (L_G = 240 nm) by about 40 mA/mm. As can be seen in  Figure 6.8, also the maximum transconductance of HEMT A is superior to that of HEMT B by approximately 20 mS/mm, again measured as well as simulated. These results reflect the faster carrier transport due to the shorter gate of HEMT A.
 

If the larger g_{m max} of HEMT A were not caused by accelerated transport but by a distance d_GC smaller than estimated in  Table 6.1, one would expect it to be correlated with a smaller I_D instead of a larger one. This comparison with experiment provides evidence that the simulation is able to model g_{m ext} and I_D of both HEMTs in a consistent and realistic manner.

However, there is also one small detail shown in  Figure 6.8 in which simulation and measurement do not agree completely: the gate-source voltage V_GS for which g_{m max} is measured for HEMT B is slightly more negative than the simulated one. The reason for this behavior is not clear. As the two HEMTs were not fabricated in the same lot, small differences in the semiconductor passivation interface states could occur.

On the other hand, the experimental observation that the transconductance of HEMT A decreases more rapidly with increasing positive V_GS than that of HEMT B is accurately reproduced by the simulation. As described in Chapter 5 the physical origin of this effect is the stronger real space transfer of electrons from the channel into the low-mobility barrier layer in the device with the shorter gate (HEMT A). The increase of real space transfer in short channel devices is also leading to a higher output conductance. This will be discussed in Section 6.1.2.
Figure 6.9 shows the simulated gate capacitances C_G of HEMTs A and B. For V_GS below pinchoff (i. e. V_GS < -0.9 V), C_G consists of the gate-drain capacitance C_GD and parasitic contributions including the fringe capacitances. These values are very similar for both transistors. When V_GS increases, the longer gate of HEMT B manifests itself in a stronger increase DC_G compared to HEMT A. For HEMT B, DC_GB » 520 fF, and for HEMT A, DC_GA » 350 fF. The ratio DC_GB /DC_GA » 520/350 » 1.48 is close to the ratio of the gate lengths L_GB/L_GA » 240/170 » 1.41, as expected.
 

• 6.1.1.2 Comparison of HEMT A and HEMT C
• 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