Optoelectronics with Carbon Nanotubes

The decreased on/off ratio at a higher VDS is thought to be analogous to the higher off-
current for single-tube devices (see Figure III-1 and III-3, for example), which is due to the bias-
assisted tunneling though Schottky barrier at the injection site at “off-state” VTG. As for the
effect of the channel length, we observe that as LC increases from 4 to 6 μm, the split gate effect
becomes more pronounced and the attainable on-current decreases. This is similar in principle to
the observation of an increasing on/off current ratio and a decreasing on-current with increasing
LC in devices made out of the same film but equipped with a single global top gate 99 . The on-
current decrease is simply a function of overall channel resistance, which comes from diffusive
transport under percolation and a channel length greater than the electron scattering length. The
inferior on-off ratio of the shorter channel arises from the incomplete turn-off due to a small
residual amount of metallic tubes in the film. The effect and the probability of a (at least
partially) short-circuit created by metallic tubes decrease as the entire channel length is
increased, leading to a better off-state in Figure V-3 (b) than in (a).
Now we examine transport by applying a differential voltage between VTG1 and VTG2 to
see if they operate as p-n junction diodes. In Figure V-4 (a) and (b), we show the electrical
output characteristics of the same two devices (with different channel lengths) already shown in
Figure V-3 above. We swept VDS between -10 V and +10 V while keeping the split gate
voltages at specific values. First, we focus on the transport when it is forward-biased, i.e.
positive VDS in this case in the right half of each figure. By stepping up voltages of opposite
polarity to the two split gates from 0V/0V (black) to -10V/+10V (magenta), we observe that the
forward-biased transport becomes more efficient for a given VDS. It is also interesting that the
increase in current is more prominent for the longer-channel device as the split gate voltage
difference is increased. For example, compare the differences in IDS at maximum VDS between
the minimum split gate (0V/0V, black) and the maximum (-10V/10V, magenta) between the
panels. The 4 μm device shows only a factor of two increase in current from 16 μA to 32 μA
(Figure V-4 (a)), but the 6 μm devices sees an increase greater than an order of magnitude from
1.9 μA to 20 μA (Figure V-4 (b)). As we saw in the ambipolar top-gate sweeps of the same
devices, the longer diode has a better on/off ratio, which means that we can control the
accumulation of p- and n-doped carriers more effectively. Hence, it is reasonable that the
longer-channel device has a better diode behavior.
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