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understood as “on” (>200 nA) or “off” (<200 nA)
output states. A full exploration of ID(VSD, VGD)
reveals an “AND” functionality (e.g., output is “on”
only when both inputs are “on”) (Fig. 2C). Because
of the nonlinear character of the junction, the re-
sultant drain current when both VSD and VGD are
“on” is ~3 times the sum of the individual con-
tributions when only one input is “on”: ID(4 V,
4V)~3[ID(4 V, 0 V) + ID(0 V, 4 V)], which
yields a promising on-off current ratio.
Frequency response. One gauge of the per-
formance of a transistor is its ability to modulate
or amplify signals at high frequencies, as quan-
tified by the cutoff frequency fT.We characterized
the frequency dependence of the SketchFET de-
scribed in Fig. 2 using a heterodyne circuit that
incorporates the SketchFETas a frequencymixer.
The experimental arrangement is shown sche-
matically in fig. S6A.
The results of this heterodyne measurement
over a frequency range 3 kHz to 15MHz show that
the SketchFET operates at frequencies in excess of
5 MHz. In the measurement setup used, this fre-
quency ismost likely limited by the large (~megohm)
resistance of the three leads connecting to the de-
vice. The high mobility of the channel and the fact
that the I-V characteristics are far from saturation

n the conducting regime suggest that fT of the
SketchFET, without the large lead resistances,
could extend into the gigahertz regime.
Double junction. The fabrication of a second
family of structures begins by patterning the T-
junction, followed by two erasure steps in which
a negatively biased AFM probe (Vtip = –10 V)
scans across two of the leads (Fig. 3A). The result
is a device with two comparable tunneling gaps
separatedbyadistance l from the intersection.
The I-V characteristic of each junction is shown
in fig. S4B. The electrodes connected by these
two sections are labeled S1 and S2; the third
electrode is labeled as “drain” (D). Transport
experiments to measure the drain current as a
function of the voltages V1 and V2 applied to S1
and S2, respectively [ID(V1, V2)]. performed using
the methods described above. Positive values of
V2 have little effect on the I-V characteristic
between S1 and D (Fig. 3B), and vice versa.
Negative values of V2 can induce NDR in the
channel between S1 and D. A full exploration of
ID(V1, V2) reveals an “OR” functionality (e.g.,
drain output is “on”when either one of the source
inputs is on) (Fig. 3C), which is not surprising
given the topology of the junctions. We refer to
this structure as a double junction.
Negative differential resistance. A qualita-
tive explanation of the SketchFET NDR (Fig.
4A) originates from the fact that for a three-
terminal junction each nanowire exhibits a field
effect on the other two. When |VSD| is small,
conductivity between source and drain is greatly
suppressed; ID is mainly composed of current
from the negatively biased gate. Increasing VSD
will improve the conductivity between gate and
drain and will drive more negative gate current to
the drain, which manifests itself as NDR.When
|VSD| is large enough, the drain current ID is
dominated by current flowing from the source,
and the NDR vanishes.
For the double-junction structure, the origin
of the NDR (Fig. 4B) is less straightforward. To
study the nature of the coupling, we created a
family of double-junction structures and char-
acterized them for various distances l between
the junctions and the center of the T-intersection
(Fig. 4C). The normalized magnitude of NDR is
quantified as−ð∂ID=dV1Þ=ð∂ID=dV2Þ,whichcan
be visualized as the slope of contour lines in a
two-dimensional plot of ID(V1, V2). Smaller val-
ues of l resulted in stronger coupling between
the two junctions (Fig. 4D), manifested as a
larger NDR effect. The coupling strength—
given by the maximum NDR observed, SNDR ¼
max½−ð∂ID=dV1Þ=ð∂ID=dV2Þ
—is calculated as
a function of junction separation (Fig. 4E). An
approximately exponential decay of this cou-
pling strength is observed, with a fitted decay
length l0 =1.75 mm.
The long-range coupling of tunnel junctions
is consistent with the observation that the sheet
conductance of the nanowires is two orders of
magnitude larger than for unpatterned interfaces.
A possible explanation of where these extra elec-
trons come from, consistent with both observa-
tions, is sketched in Fig. 5. The writing process is
assumed to create positively charged regions
(e.g., oxygen vacancies) on the top LaAlO3
surface (Fig. 5A) (24). Directly below, at the
LaAlO3-SrTiO3 interface, electrons screen this
positive charge (Fig. 5B). These electrons can
come from two sources: either from the top
LaAlO3 surface, or from weakly bound donor
states (associated with defects in the SrTiO3)that
become ionized over a length scale x in the range
of several micrometers (Fig. 5C). This screening
is a type of lateral modulation doping that can
produce a considerably higher electron density
relative to planar unpatterned q-2DEGaswell as
a lateral potential profile much wider than the
real conductive nanowire region. Experiments
in which many parallel wires are connected
show saturation of the net conductance toward
the unpatterned q-2DEG value, again consistent
with this picture of lateral modulation doping.
The high conductance of the 12-nm wires,
produced by the large (~100 MV/cm) transient
electric field of the AFMprobe, is metastable and
prone to partial relaxation toward the unpatterned
q-2DEG value on a time scale that depends on
the ambient environmental conditions. Experi-
ments performed on a SketchFET stored under
vacuum conditions (fig. S8) show a nonexpo-
nential decay of the overall conductance (domi-
nated by that of the 12-nm leads) toward a
steady-state value that is comparable to the sheet
conductance of the unpatterned film. No discern-
ible degradation in the SketchFET switching
performance was observed over a 9-day period.
The extreme sensitivity of electron tunneling to
barrier thickness demonstrates that the SketchFET
and related structures are stable at length scales
that are small relative to their feature size (e.g.,
2-nm gap) and at time scales considerably longer
than the observation period.