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6.3.1 DC Characteristics

The basic structure of the investigated HEMT is shown in  Figure 6.38. The epitaxial layers are similar to those of HEMTref. In the barrier below the channel a 5 nm thick layer with a doping concentration of 1.1*1018 cm­3 is used. The upper barrier layer contains a delta doping with a sheet concentration of 3.6*1012 cm­2. The T­shaped gate with a footprint length LG of 120 nm was manufactured by optical stepper lithography and a side wall spacer process as the previous described HEMTs. Source and drain contacts are self­aligned to the T­gate [72].
 

 
Figure 6.38 Schematic cross section of the investigated millimeter wave HEMT.
 

The verification of the simulation is based on two devices manufactured on the same wafer. It was intended to create the same recess depth for all devices. Inhomogenities in the etching process lead to some differences in dGC. The devices are simulated using the setup and model parameters obtained from HEMTref in Section 5.3.2. A HD model is used in the channel and supply layer and the DD model in the remaining semiconductor layers. The simulated transfer characteristics were fitted to the measurements of one device only by adjusting the given nominal values for the location and density of the delta doping in the supply layer, the interface charge density, and the gate-to-channel separation.
 

 
Figure 6.39 Simulated (lines without symbols) and measured (lines with symbols) transfer characteristics of two millimeter wave HEMTs with different recess depths.
 

The measurements of both devices are shown in  Figure 6.39. The simulation of the device with dGC = 13 nm was fitted to the measurement. Again the overestimation of the current near VT is significant. Between VGS = 0.4 V and VGS = 1.0 V simulation and measurement agree very well whereas for higher VGS the deviations already discussed in Section 5.3.2 and Section 6.2.1 occur. The second measurement can be simulated by a reduction of dGC to 10 nm. Although the deviations are somewhat larger than for the other simulation  Figure6.40 shows that the change in gm is simulated very realistically.
 

 
Figure 6.40 Simulated (lines without symbols) and measured (lines with symbols) transconductance of two millimeter wave HEMTs with different recess depths.
 



next up previous contents
Next: 6.3.1.1 Dependence on the Gate-to-Channel Separation Up: 6.3 Millimeter Wave HEMTs Previous: 6.3 Millimeter Wave HEMTs

Helmut Brech
1998-03-11