Page 56 - Organic Electronics in Sensors and Biotechnology

Scaling Effects in Organic Transistors and Transistor-Based Chemical Sensors 33

variation of the side current (collected at side-guarding electrodes)
corresponding to that of the drain current (Fig. 1.17b), which were
measured simultaneously. The side current response to the analyte
is markedly different compared to the drain current in Fig. 1.17b;
namely, the side current shows a decrease in response to the ana-
lyte whereas the drain current simultaneously shows an increase
upon exposure to the analyte. Since Fig. 1.17b and c was recorded
simultaneously and showed different kinds of sensing behavior, it
is evident that the side current is the spreading current traveling
outside the defined channel and the current collected simultane-
ously at the drain is the direct current through the nanoscale channel.
Therefore, utilizing the side-guarding function, we can eliminate
most of the contribution from the spreading current and detect the
true sensing response of a nanoscale channel. The experimental
findings obtained with this unique side-guard function are pre-
sented below for the chemical sensing responses dependent on
the scaling geometry, analyte delivery, as well as different type of
analytes.
We investigated the response of I (operated in saturation region)
ds
upon exposure to the saturated vapor of 1-pentanol, with a series of
channel length and varied grain sizes of pentacene under the same
experimental conditions. As shown in Fig. 1.18a, while the long-
channel-length devices all exhibited a decrease in drain current upon
delivery of the analyte, the small-channel-length devices showed an
increase. There are two mechanisms influencing sensor behavior: one
causes a decrease in current (dominant in large L devices) and the
other causes an increase (dominant in small L devices). There is a
crossover between these two types of response behavior which
depends on grain size, occurring in the interval of channel length
between 150 and 450 nm for ~80 nm grain size. Under the same
condition, when the average grain size of pentacene is increased to
250 nm, the sensors exhibits the crossover behaviors at larger channel
lengths (from 450 nm to 1 μm), as shown in Fig. 1.18b.
Figure 1.19 shows the SEM image (taken after all measurements)
for geometric relation of the same channel length (150 nm) with dif-
ferent grain sizes of the pentacene layer. Figure 1.20a and b is the sens-
ing responses of long-channel devices with pentacene grain sizes of
140 nm and 1 μm, respectively. For all devices with channel lengths of
2 μm or greater, I manifested decreasing responses upon analyte
ds
delivery. The amplitudes of decreasing signal for 2 μm channels were
smaller than those of longer channels. This effect is stronger with
larger pentacene grains (Fig. 1.20b). These results are consistent with the
reported work for sensing effects dependent on organic grain sizes
and channel lengths in large scale. 104, 106 The sensing responses shown
in Figs. 1.18 and 1.20 are reproducible for different devices with the
same channel lengths and grain sizes, leading one to conclude that