220.127.116.11 Comparison of HEMT A and HEMT C
C was assumed to possess the same distance dGC =
25 nm as HEMT
A but only a slightly longer LG = 190 nm and a shorter
LR = 60 nm. When these values are used in the simulation,
this results in a too negative value of VT and an overestimated
ID, as can be seen in Figure
6.10. However, a reduction of dGC by 1.7 nm leads
to perfect coincidence of measured and simulated curves, as it is also
shown in Figure
Variations of dGC 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 gm max = 580 mS/mm and
its occurrence at VGS = 0 V are simulated very well.
We now return to the initial simulation of HEMT C which was performed under the assumption that dGC is equal to the value of HEMT A (dgc = 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 gm 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 LR 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 LR), we can estimate that an LG increase of 20 nm only causes a negligible decrease of gm max of about 5 mS/mm. This independently confirms the conclusion drawn above: the reduction of LR had a stronger impact on the transconductance of HEMT C than the slightly longer LG.
A drawback of the shrinkage of LR is the increase
of CG. This can be seen in Figure
6.12 which shows the simulated gate capacitance CG
A and C.
C, the whole function CG(VGS) 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.
Helmut Brech 1998-03-11