Odor sensor with organic transistor circuitry

Details Odor sensor with organic transistor circuitry Odor sensor with organic transistor circuitry

2001-02-26

2003-12-09

Kinkead, Arnold (Department: 2817)

Oscillators

Ring oscillators

C331S074000, C422S090000, C073S023340, C073S031060, C073S023210, C324S071500, C257S252000, C257S253000

Reexamination Certificate

active

06661299

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to circuitry employing organic transistors and, in particular, organic field effect transistors (OFETs) to detect chemical odors/vapors/gases (analytes).
Many different types of OFETs are known. By way of example,
FIG. 1
shows the structure of an OFET
10
having a semiconductor body region
12
with a source electrode
14
and a drain electrode
16
defining the ends of a conduction channel through the semiconductor body
12
. The OFET
10
also includes an insulator layer
18
and a gate (control) electrode
20
to which a voltage may be applied to control the conductivity of the semiconductor body region (i.e., the conduction channel). The OFET of
FIG. 1
is manufactured to have organic material in its semiconductor body region
12
that can absorb analytes and which, in response to the absorbed analytes, changes the conductivity characteristics of the conduction channel. As illustrated in
FIG. 1
, analytes (vapors/odors/gases) may flow over the OFET for a period of time. Ensuing changes in the conductivity of the OFET may be measured as shown in
FIG. 1A
by sensing the current (I
d
).
In known circuitry, the OFETs have been used as discrete devices. As shown in
FIG. 1A
, the source of an OFET may be connected to a first point of operating potential (e.g., VDD) and its drain may be connected via a load resistor RL to a second point of operating potential (e.g., ground potential). The gate of the OFET may be biased via resistors R
1
and R
2
to produce a desired operating direct current (d.c.) bias level within the source-drain (i.e., conduction) path of the OFET. The OFET may then be subjected to a flow of analytes which causes its conductivity to change. The corresponding change in conductivity of the OFET is then detectable by a circuit connected to the drain and/or the source of the OFET.
A problem with known OFETs is that their sensitivity to the analytes is relatively low. Also, known OFETs are subject to drift and threshold shift as a function of time, as shown in FIG.
2
A and
FIG. 2B
, respectively. In
FIGS. 2A and 2B
, it is seen that, for a fixed bias condition, source-to-drain current (I
d
) of an OFET changes (e.g., decreases) as a function of time. This is the case when there is no signal input (i.e., no odor), as illustrated by waveform A of FIG.
2
A and waveform portion C in FIG.
2
B. This is also the case following the application of an odor to the OFET, as illustrated in waveform B of FIG.
2
A and in waveform portion D in FIG.
2
B. That is, for a fixed bias condition, the current through the conduction path of the OFET changes (drifts) as a function of time. OFETs may also be subjected to hysteresis and offsets. As a result of these characteristics, it is difficult to use OFETs in known discrete circuits to differentiate an input signal from background conditions and to determine or measure the full extent of the input signal.
SUMMARY
Problems associated with the characteristics of OFETs, such as time-related drift, detract from their use as sensors and amplifiers of their sensed signals when the OFETs are used as discrete devices. Applicants recognized that OFETs should be incorporated in circuits specifically designed to overcome and/or cancel the problems associated with certain characteristics of OFETs such as their drift, threshold shift and hysteresis.
Circuits of various embodiments include at least one odor-sensitive organic transistor having a conduction channel whose conductivity changes in response to certain ambient odors.
In one embodiment, organic transistors are interconnected to increase their response to selected odor signals and such that the recovery of the organic transistors is enhanced and their drift is reduced. In a particular embodiment, the organic transistors are interconnected to form a ring oscillator whose frequency of oscillation changes sharply in response to an odor signal and in which the alternating signal applied to the gate electrodes of the organic transistors enhances their recovery and reduces their drift.


REFERENCES:
patent: 4344293 (1982-08-01), Fujiwara et al.
patent: 6484559 (2002-11-01), Dodabalapur et al.

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