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Fig. 7.
Id-Vg characteristics of constant and graded doping
The variation of different device performance parameters with respect to channel
doping type are listed in Table 3. Because of the higher impurity scattering in graded
channel device, the conduction current in the Subthreshold region will come down
and hence threshold voltage will rise. Remarkable reduction in
I
off
is also observed in
both GC1 and GC2.
Table 3.
Device performance parameters with different doping types
C
gg
(F)
Constant 0.351 66.79 3.15E5 1.082E-4 5.990E-17
GC1 0.564 62.89 2.28E8 7.210E-5 5.203E-17
GC2 0.549 63.51 1.95E8 7.239E-5 5.274E-17
It can be certified that for deep sub-micron technology nodes, GC1 offers advan-
tages in analog performance than GC2, which is in closer agreement with the work
explained for long channel SOI devices in [13]. Employing lesser doping near the
Drain reduces the electric field and hence the total number of carriers generated by
impact ionization will also be reduced. Since the channel is too short, the confinement
of doping to the regions as depicted in Fig. 2 cannot be guaranteed as such in actual
fabrication scenario and the device may tend to behave like a heavily doped one
which puts limitation in adopting channel engineering for the deep sub-micron tech-
nology nodes. AC simulations at 50GHz show that gate capacitance
C
gg
is lowest for
GC1 than GC2 or constant doping conditions.
It is also worth noting that the effect of GC1 on the transconductance is not so
prominent. Since transconductance has a direct dependency on the mobility of carri-
ers, variation in mobility is also negligible. On the other hand, the ratio of
I
on
to
I
off
is
increasing by two orders of magnitude which is a striking advantage of the graded
channel devices for analog applications.
Doping Type
V
T
(V)
SS
(mV/decade)
I
on
/I
off
gm
(S)
