Figures 3-2(a) and 3-2(b) show the transfer characteristics of NW-SONOS and
HM-SONOS devices in SG-mode, TG-mode, and DG-mode, respectively. SG-mode
denotes that the voltage applied to the side-gate is being swept, while a constant
voltage is applied to the top-gate. In the following discussion the constant voltage
stated above is set to zero if not specified. TG-mode is contrary to the bias
configuration for the SG mode, that is, a sweeping voltage is applied to the top-gate
while a constant voltage is applied to the side-gate. DG mode means the two gates are
tied together and applied with the sweeping voltage. Also shown in the figures is the
sum of the SG and TG modes for comparison purpose.
An obvious noticeable difference between Fig. 3-2(a) and 3-2(b) is the
off-current in SG-mode. One of the advantages pertaining to the HM-SONOS device
is to suppress gate-induced drain leakage (GIDL) induced in SG-mode [29]. GIDL
originates from reverse-biased junction in the overlap region between drain and gate,
which strongly depends on field strength wherein. In HM-SONOS, a hard-mask layer
is inserted between the side gate and drain, therefore the field strength is dramatically
weakened. As a result, the GIDL is suppressed dramatically, as shown in Fig. 3-2(b).
Such effect does not appear in TG-mode because the overlap region between the top
gate and the drain has nothing to do with the hard-mask layer. The results shown in
the figures confirm the expectation.
To more clearly illustrate the situation, the off-state leakage current of the
NW-SONOS and HM-SONOS devices operated under both SG mode and TG mode
measured at VG=-4V and VD=3V is expressed as a function of “gate width” in Fig.
3-3 and Fig. 3-4, respectively. The “gate width” refers to the planar width of the
side-gate pattern, as indicated in Fig. 3-5. In Fig. 3-3, we can see that the off-state
leakage of the SG mode is proportional to the gate width, indicating that the major
conduction occurs in the drain region overlapping the top of the side-gate, as shown in
Figs. 3-5(b) and 3-5(c). The figures in Fig. 3-5 are the cross-sectional views of the
device along the boundary between the drain junction and the NW channel (Line AB
in Fig.3-5(a)). Owing to the large voltage difference between the side-gate and the
drain, the thinner the gate dielectric, as well as the lighter the doping concentration at
the drain/gate dielectric interface, the bigger the GIDL leakage via band-to-band
tunneling with or without the assistance of traps in the junction [29]. For TG mode of
operation, the gate-width dependence in off-state leakage is lifted, as illustrated in Fig.
3-3. This is reasonable, since the flow paths of off-state leakage for TG mode (shown
in Figs. 3-5(d) and 3-5(e)) are actually not through the drain region over the top of
side-gate. Fig. 3-4 indicates that the off-state currents of the HM-SONOS under both
the TG and SG modes of operation are independent of the gate width. In this case, the
same reason as for the NW-SONOS holds for the TG mode, while suppression of the
leakage for the SG mode is due to the use of a hard-mask inserted between the
side-gate and the drain which tends to reduce the field strength and thus the GIDL is
hindered.
In Fig. 3-2, a larger on-current in TG-mode than the SG-mode can be explained
by a larger effective conduction width according to the TEM image, as well as the
much smaller S/D series resistance. This can also be understood with the aid of
schematic flow paths shown Fig. 3-5. As mentioned in Chap.2, a low-energy S/D
implant was performed in device fabrication. As a consequence, a large undoped
offset region actually exists between the n+ doping region and the channel region. In
the case of side-gated conduction, as shown in Figs. 3-3(b) and 3-3(c), the undoped
offset region (surrounded by the dashed line) is actually not gated by the side-gate.
This feature results in a large parasitic resistance and degrades the on-current of the
SG operation. In contrast, the offset region between the n+ doping region and the
top-gated channel region is actually gated by the top gate. Therefore the on-current is
not seriously affected.
Due to the dual-channel conduction in DG-mode, a larger on-current over TG-
and SG-mode can be obtained in Fig. 3-2(a) and 3-2(b). However, it has to be noticed
that the on-current in DG mode is much larger than the sum of SG and TG modes in
Fig. 3-2(a). This is an indication of the occurrence of volume inversion effect [30].
Such effect is due to the coupling of channel potential at different regions of a
multiple-gated device, and becomes profound as the channel body becomes ultra-thin,
such as the case of the NW-SONOS in the present study. The phenomenon is not
observed in Fig. 3-2(b), however, because of the much thicker body in HM-SONOS
devices, as shown in Fig. 2-3(b).