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6.3.1.2 Dependence on the Gate Length

In  Figure 6.42 VT is shown as a function of the gate length LG for dGC = 10 nm and dGC = 13 nm. In both cases only a moderate shift of about 1 mV has to be expected for DLG = 10 nm and LG > 200 nm. For LG = 100 nm and DLG = 10 nm the expected shift for is 8 mV. The stronger shift for small LG can be attributed to short channel effects which has also a significant impact on the transconductance as shown in  Figure 6.42. Below 200 nm the characteristics for both dGC saturate whereas a linear reduction of gm is shown for LG > 200 nm.
 

 
Figure 6.42 Simulated gm and VT versus LG for devices with dGC = 10 nm and dGC = 13 nm. Both the gm and fT characteristics are non linear due to short channel effects.
 

The most severe short channel effect is an increase of the output conductance g0. In Figure 6.43 the simulated g0 is shown as a function of LG. The measured value indicated by the asterisk is about two times higher. One reason for the discrepancy is the use of the DD transport model in the buffer layer. The second more important reason is that impact ionization is not included in the simulation. It was shown in Section 5.3.2.2 that impact ionization is already significant for VDS > 2.0 V for HEMTref. The millimeter wave HEMT presented in this section exhibits significantly smaller lateral dimensions thus an increase in g0 due to impact ionization has to be expected for even smaller values of VDS. Despite the underestimated magnitude of g0 the principle characteristics of g0 is simulated realistically. The output conductance is almost doubled if LG is reduced from LG = 140 nm to 80 nm.
 

 
Figure 6.43 Simulated g0 versus LG. g0 is underestimated due to a DD model in the buffer layer and disregarding impact ionization in the simulation. The principle dependence on LG is simulated very realistically.
 

The presented results on the DC performance shows an improvement of gm by more than 50 % compared to the gm of the power HEMT of Section 6.2.
 



next up previous contents
Next: 6.3.2 RF Performance Up: 6.3.1 DC Characteristics Previous: 6.3.1.1 Dependence on the Gate-to-Channel Separation

Helmut Brech
1998-03-11