Biomedical Engineering Reference
Schematics of ( a ) a conventional nanowire FET and ( b ) a double gate nanowire FET
Fig. 5.16 ( a ) I D V G1 for various V G2 conditions and ( b ) impact of the change in V G2 on the
change in the threshold voltage (Copyright 2010 American Chemical Society)
There are two ways to drive the FET in the double-gate structure: the single-gate
(SG) mode and the tied double-gate (DG) mode [ 43 ]. In the SG mode, G1 is used as
a drive gate, and G2 is used as a supplementary gate to pin the channel potential at
a fixed voltage. In contrast, in the DG mode, G1 and G2 are electrically connected,
which implies that the same voltage is always applied to G1 and G2; that is, it has a
symmetrical bias (V G1 D
V G2 /.
The data measured in the SG mode (hollow circles) in Fig. 5.16 a show that
the drain current by G1 can be modulated according to the bias condition of G2.
The increment of the G2 voltage (V G2 / from a negative to positive value tends to
lower the V T value and degrade the subthreshold slope (SS), which is defined as
d.V G1 /=d. log I D /; that is, it becomes less steep. However, the characteristics of the
DG mode (filled squares) show a steeper SS than that of the SG mode due to the
greater control over current in the double - gate FET [ 43 ].
AsshowninFig. 5.16 b, when V G2 is larger than V T; DG (V T in the DG mode), V T
changes significantly in response to small changes in V G2 : However, changes in V T
are less sensitive to V G2 when V G2 is lower than V T; DG . Thus, the sensitivity in terms
of V T is less affected by the condition of V G2 .