Page 54 - Organic Electronics in Sensors and Biotechnology

Scaling Effects in Organic Transistors and Transistor-Based Chemical Sensors 31

transistor channel and the sensing material which the analyte mole-
cules can directly access. The transient variation of drain current I in
ds
chemical sensing mode was examined under fixed gate (V ) and
g
source-drain voltage (V ) in air at room temperature, with saturation
ds
vapor of analyte molecules delivered to the proximity of organic-
semiconductor channel surface. Currents were measured throughout
the entire measurement period while the analyte was only delivered
for a certain period of duration, to facilitate a comparison of the drain
current behavior before, during, and after exposure to the analyte.
Before every sensing measurement, each device was tested with air
stream (the carrier to deliver various analytes), and no sensing or
kinetic effect on drain current was detected. The currents in the
absence of analyte continuously decrease over time due to bias stress
effect. This is a well-known effect in OFETs in which the current
slowly decreases with time at a fixed bias because the field-induced
carriers (holes) fall into deep trap states where they are less mobile,
which results in a reduction of drain current. 117, 118 The sensor response
is superposed on this background. After each sensing measurement,
the device characteristics were recovered by using a reverse-bias con-
figuration (high positive gate voltage with a small drain current flow-
ing for 1 min).
With the design of side-guard electrodes as described in Sec. 1.1.2,
we can investigate the transport behavior of OTFTs in truly nanoscale
dimensions excluding the spreading currents in large area, without
the difficulty to pattern the semiconductor layer into such a small size
as the channel length. More importantly, with these devices as chem-
ical sensors, the side-guarding function ensures that the active sens-
ing area is truly nansocale. This novel four-terminal geometric design
provides a powerful capability to investigate the sensing behavior
within the nanoscale active area through the defined channel without
any undesirable background from large-scale parasitic conduction
pathways around the channel. To investigate the sensing response
from the sensor active area in truly nanoscale geometries and the role
of the side-guarding electrodes in the sensing measurement, the drain
current transients upon exposure to 1-pentanol without and with
applying side-guarding function, as shown in Fig. 1.17a and b, respec-
tively, were recorded on a transistor with a 7 nm channel made by
trapping bridge and break junction technique. 119, 120 Surprisingly,
Fig. 1.17a and b manifested opposite sensing response directions.
Without the side-guarding function, the transient shown in Fig. 1.17a
represents the sensing behavior of the large area around the channel,
which is a decrease in drain current by 20 to 25%. With the side-
guarding function to eliminate most of the spreading current (two
side-guarding electrodes biased at the same potential as the drain),
the transient in the drain current (Fig. 1.17b) represents the true sens-
ing behavior of the nanoscale active area, which is an increase in
drain current by approximately a factor of 2. Figure 1.17c exhibits the