Patent Publication Number: US-6661299-B2

Title: Odor sensor with organic transistor circuitry

Description:
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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawing, like reference characters denote like components; and 
     FIG. 1 is a highly simplified cross-section of a known OFET suitable for use in various embodiments; 
     FIG. 1A is a prior art circuit diagram of a discrete OFET circuit to sense and amplify signals resulting from odors; 
     FIGS. 2A and 2B are diagrams illustrating the drift in the current through the conduction path of an OFET as a function of time; 
     FIGS. 3A and 3B are the symbolic representation of P-type OFETs and P-type FETs, respectively, used in this application; 
     FIGS. 4A and 4B are the symbolic representation of N-type OFETs and N-type FETs, respectively, used in this application; 
     FIG. 5 is a schematic diagram of an OFET-differential amplifier combination embodiment the invention; 
     FIG. 6 is a schematic diagram of an OFET- amplifier combination embodiment; 
     FIG. 7 is a block diagram of a system employing OFETs in one embodiment; 
     FIG. 8 is a schematic diagram of a ring oscillator circuit employing OFETs in one embodiment; 
     FIG. 9 is a diagram of waveforms associated with one embodiment of the circuit of FIG. 8; 
     FIG. 10 is a schematic diagram of another ring oscillator circuit employing OFETs in one embodiment; 
     FIGS. 10A and 10B are schematic diagrams of cascaded inverters using OFETs for analyte sensing; 
     FIG. 11 is a block diagram of an odor sensor in one embodiment; 
     FIG. 12 is a cross section of an OFET suitable for use in circuits embodying the invention; 
     FIG. 13 is a drawing of various molecular structures of several materials used to make OFETs; and 
     FIG. 14 is a drawing of examples of molecular structures of odors to be sensed by circuits of various embodiments. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     In the discussion to follow, reference is made to organic field effect transistors (OFETs) which may be used to sense odors, vapors, chemicals and/or gases (analytes). These terms are used interchangeably and are intended to include each other in the specification and in the claims appended hereto. OFETs have been described in the literature and the teachings of these references as to the manufacture of these transistors and their reported characteristics are incorporated herein by reference. In the discussion to follow references to OFETs will also include organic thin film transistors (OTFTs) and any device having similar characteristics. 
     To better understand the discussion to follow and the drawings appended hereto, certain characteristics of OFETs will first be discussed. OFETs (or OTFTs) may be of N-conductivity type or P-conductivity type. To more easily differentiate the OFETs from known field effect transistors (FETs), the following symbology will be used in the appended drawings: (a) P-type OFETs will be shown as illustrated in FIG. 3A; (b) P-type FETs will be shown as illustrated in FIG. 3B; (c) N-type OFETs will be identified as shown in FIG. 4A; and (d) N-type FETs will be identified as shown in FIG.  4 B. The drawings for the OFETs include a rectangular box, with an X through the box, indicative of the semiconductor body with source and drain electrodes attached to the semiconductor body representing the ends of a conduction path (or channel) through the semiconductor body. The semiconductor body is electrically insulated from a gate (control) electrode, g, to which a bias voltage or a signal may be applied to control the conductivity of the semiconductor body. A P-type OFET is shown with an arrow pointing towards the body of the OFET and an N-type OFET is shown with an arrow pointing away from the body of the OFET. The semiconductor body defines the conduction path and the OFET includes source and drain electrodes defining the ends of the conduction path. OFETs, like FETs, are generally bi-directional conducting devices. Therefore: (a) the source of a P-type OFET or of a P-FET is defined as the one of the two electrodes connected to the semiconductor body whose potential is more positive than the other (drain) electrode; and (b) the source of an N-type OFET or of an N-FET is defined as the one of the two electrodes connected to the semiconductor body whose potential is less positive than the other (drain) electrode. 
     As noted above, OFETs are typically subject to several problems: 
     a) low sensitivity; (b) drift; (c) hysteresis; and (d) variation of their threshold voltage (V T ) as a function of time. 
     The various embodiments are directed to circuits that use OFETs but are configured to reduce or compensate for the problems associated with low sensitivity, drift, threshold shift and hysteresis. 
     FIG. 5 illustrates the use of an organic transistor, OFET M 2 , as a sensor of odors and as an amplifier of the signal sensed. In the discussion to follow, when an odor flows over an OFET and the OFET is turned-on and/or biased into conduction, the OFET may be said to be “sensing” or “sniffing” the odor and to be in a sensing or sniffing mode. OFET M 2  forms part of the input stage of a differential amplifier. The amplifier includes selectively enabled adaptive feedback circuitry, which enables the background, drift and threshold shift conditions to be effectively subtracted from the amplified output signal. When the amplifier circuit is in a standby non-sensing mode (i.e., not sensing or sniffing any odors, gases, chemical vapors etc.) negative feedback is used to cancel any drift or threshold shift due to OFET M 2 . When the circuit is in a sensing mode (i.e., “sniffing”) the feedback loop is opened with the circuit biased in a high gain state so it can respond quickly and with high gain to an input signal. 
     In the circuit of FIG. 5, transistor M 2  is an odor-sensitive P-type OFET. For ease of illustration the other transistors used in the circuit are non-odor sensitive FETs or OFETs. Transistors M 1 , M 2 , and M 5  form the input stage of a differential amplifier, with the source electrodes of M 1  and M 2  being connected to the drain of M 5 . Thus, M 1  and M 2  compete for the current from current-source M 5 . The source electrode of M 5  is connected to a point of fixed operating potential (i.e., VDD) and a bias voltage V B  is applied to the gate of M 5  causing a constant current, Io, to flow through the conduction path of M 5 . The current Io is equal to the sum of the current I 1  through the conduction path of M 1  and the current I 2  through the conduction path of M 2 ; that is, Io=I 1 +I 2 . The value of the current flowing through the conduction paths of M 1  and M 2  is a function of their relative gate voltages. The lower the gate voltage of M 1  with respect to M 2 , the greater is the fraction of the current Io that flows through it; similarly for M 2 . The drain of M 1  is connected to node A 1  and the drain of M 2  is connected to node A 2 . 
     The current I 1  flowing into node A 1  is mirrored via a current mirror circuit comprised of transistors M 3  and M 6  to produce a current I 6  flowing through the conduction paths of transistors M 6  and M 8 . For ease of illustration, assume that the current current I 6  which is equal to the current I 8  is also equal to the current I 1  ; i.e., I 6 =I 8 =I 1 . The current through M 8  is then mirrored via a current mirror comprised of transistors M 8  and M 9  to cause a current I 9  to flow through the conduction path of M 9  into node A 3 . The sources of M 8  and M 9  are connected to VDD volts and their gates are connected in common to the drains of M 8  and M 6 . When M 8  and M 9  have similar geometries, their drain currents will be substantially equal for the same gate-source bias conditions. In that case, the current I 9  flowing into node A 3  is equal to I 6  which is, in turn, equal to I 1 ; i.e., I 1 =I 6 =I 9 . 
     The drain current I 2  through M 2  flows into node A 2  and is mirrored via the current mirror transistor combination of M 4  and M 7 . The sources of M 4  and M 7  are connected to ground potential and the gate of M 7  is connected to the gate and drain of M 4 . When M 4  and M 7  are of similar geometries their drain currents will be substantially the same for like gate-source bias conditions. Thus, M 4  and M 7  function as a current mirror to produce a current I 7  through the conduction path of M 7  which is drawn out of node A 3 . In that case, the current I 7  is equal to the current I 2 . For the above conditions, if I 1  is equal to I 2 , the current I 9  flowing into node A 3  is equal to the current I 7  flowing out of node A 3 . Where M 9  and M 7  have essentially equal impedances for the corresponding bias condition, the voltage at node A 3  will be substantially equal to VDD/2 when I 1  is equal to I 2 . It should also be appreciated that, since M 7  and M 9  are effectively high impedance current sources, a small difference between the currents I 9  and I 7  results in a large voltage differential at node A 3 . Thus, the circuit has a very large open circuit (i.e., when there is no feedback) voltage gain. 
     Thus, the pair of currents I 1  and I 2  are mirrored via the current mirrors formed by M 3 -M 6 , M 4 -M 7 , and M 8 -M 9  and compared with each other to generate an output voltage at node A 3 . If the current through M 2  exceeds that through M 1 , then the node voltage A 3  is driven to a value near ground. If the current through M 1  exceeds that through M 2 , then the node voltage A 3  is driven to a value near VDD volts. Thus, transistors M 1 -M 9  implement a wide-output range differential amplifier with inputs given by the gate voltages of M 1  and M 2  and an output given by the voltage at node A 3 . 
     The gate of OFET M 2  is connected to a relatively constant bias voltage source V 2 . To better illustrate the operation of the circuit and the role of M 2  as a sensor, an input signal source  71  is shown connected between the source and gate of M 2 . The source  71  and its connections are shown with dashed lines since this source is internal to OFET M 2 . The Vin source  71  depicts the equivalent input signal due to a mobility or threshold voltage change in transistor M 2  when an odor is “puffed” onto (i.e., applied to) it. An odor “puffed” onto transistor M 2  is equivalent to the application of an input signal to its gate. When an odor to be sensed is applied to M 2 , the feedback loop is opened (i.e., P-type transistor M 12  is turned off by driving voltage V 12  to an active high state). Transistor M 2  is the only odor sensitive transistor in the circuit of FIG.  5 . 
     The gate of transistor M 1  is connected to the output (i.e., node A 5 ) of a low pass filter whose input is connected to the output (i.e., node A 3 ) of the differential amplifier to provide a negative feedback configuration. That is, output node A 3  of the amplifier is applied to the input of a source follower stage (i.e., the gate of M 10 ) comprised of transistors M 10  and M 11 . The source of M 11  is at VDD volts and a bias voltage V 11  is applied to the gate of M 11  to establish a current through M 11  and M 10 . The source of M 10  is connected to the drain of M 11  at an output node A 4  and the drain of M 10  is returned to ground potential. The conduction path of a transistor switch M 12  is connected between node A 4  and node A 5  which is connected to the gate of M 1 . A capacitor C 1  is connected between the gate of M 1  and ground potential. The low pass filter is implemented with transistors M 10 , M 11  and capacitor C 1 . When transistor M 12  is turned on closing the feedback loop (i.e. the circuit is not “sniffing”), the output at node A 4  of the source follower is connected to capacitor C 1  (via the low “ON” impedance of M 12 ) such that transistors M 10 , M 11  and capacitor C 1  implement a weakly nonlinear low pass filter. The time constant of the low pass filter may be controlled by altering V 11  or the value of the capacitance of C 1  or both. 
     When the feedback loop is closed (i.e., M 12  is turned on), any drift or change in the conductivity of M 12  is effectively cancelled because a conductivity change of M 2  causes a corresponding change in I 2 . The change in I 2  then causes a corresponding change in the voltages at nodes A 3  and A 4 . The change at A 4  is then applied via M 12  to the gate of M 1  with a magnitude and polarity to cause a change in I 1  which cancels or offsets the change in I 2  caused by M 2  (i.e., negative feedback tends to cause I 1  to equal I 2 ). Thus, when the negative feedback loop is closed, the gate voltage of M 1  adapts to compensate (or cancel) for long-term, time dependent, changes between the threshold or mobility of transistors M 1  and M 2  and automatically keeps the differential amplifier output at its balanced midpoint. 
     As noted above, when the circuit is not sniffing, the negative feedback is turned on and causes the circuit to adapt and compensate for any long-term differences between the transistor characteristics of M 1  and M 2 . The negative feedback is turned on by causing V 12  to go low (e.g., 0 volts) and transistor M 12  to be turned on. When M 12  is turned on, the feedback voltage applied to the gate of M 1  causes the currents I 1  and I 2  to be substantially equal. Assume that M 3 , M 4 , M 6  and M 7  are all made to the have the same geometry. Then, for I 1  equal to I 2 , the current I 2  is mirrored through M 7  so a current equal to I 2  is drawn from node A 3 . Concurrently, the current I 1  is mirrored through M 6  and M 8  and then mirrored via M 9  to produce a current equal to I 1  flowing into node A 3 . For I 1  equal to I 2 , the voltage at node A 3  will be equal to approximately VDD/2. The voltage at node A 3  is applied to the input of source follower stage M 10  to produce a similar output at node A 4 . When M 12  is turned on, the voltage at A 4  is applied, via the conduction path of M 12 , to the gate of M 1  and will tend to cause equal currents to flow through M 1  and M 2 . The high degree of feedback when M 12  is turned on ensures that any drift in M 2  gets compensated (i.e., the effect of the drift is effectively cancelled). Thus, when the circuit is ready to be used to sense (“sniff”) the presence of any vapors or chemicals, transistor M 12  is turned off by the application of a high voltage (e.g., VDD) to the gate of M 12 . When that occurs, the circuit is biased at an optimal operating point to respond to signals resulting from odors being “puffed” (applied) to the sensing OFET M 2 . 
     When the circuit is about to sniff or is sniffing, the feedback loop is opened (i.e., M 12  is turned off) and the circuit sits at its high-gain balanced midpoint, ready to amplify any odor-caused change in the current through M 2 . The voltage at A 3  serves as the output of the circuit with changes in the voltage of A 3  reflecting changes induced by the odor response of M 2 . 
     This is best illustrated as follows. When the circuit is ready to sniff the presence of an odor, the feedback loop is opened (i.e., transistor M 12  is turned off). When the loop is opened, capacitor C 1  is charged (and remains so for some time) to the voltage present at the output of the amplifier immediately before M 12  was turned off. Thus, the gate voltage of M 1  which represents one of the two inputs of the differential amplifier is held at a value representative of the gate voltage just before the feedback loop is opened. When the odor is applied, M 2  responds and its conductivity is modified by the chemicals present in the air or vapor being “sniffed”. If the conductivity of M 2  is decreased by the “input signal” then the current I 2  is decreased relative to the current I 1  and the voltage at node A 3  will rise sharply and quickly in response thereto. On the other hand, if the conductivity of M 2  is increased by the “input signal” then the current I 2  is increased relative to the current I 1  and the voltage at node A 3  will drop sharply and quickly in response thereto. In either case a good indication of the input signal condition will be produced at the output of the amplifier with the d.c. shift and drift substantially removed from the output signal. 
     FIG. 5 also shows a switching subcircuit formed by transistors M 12 , M 13 , and M 14 . The implementation of the low pass filter and the switching subcircuit are now briefly described. Transistor M 12  implements a switch that is turned on when the voltage V 12  is low. Normally, V 12  is driven low when the circuit is not “sniffing” and is driven high when the circuit is “sniffing”. The conduction channels of transistors M 13  and M 14  may be ratioed to have half the width (W) and the same length (L) as transistor M 12 . They help to alleviate charge injection problems caused by the switching voltage of M 12 . The charge injection is alleviated by having a signal complementary to V 12  drive M 13  and M 14 . The charge injection is dominated by the overlap capacitances of transistor M 12 ; the overlap capacitances of shorted-and-ratioed transistors M 13  and M 14  match those at the source and drain ends of M 12  and serve to cancel the effects of positive charge injection from M 12  with negative charge injection from M 13  and M 14 . 
     FIG. 6 shows a circuit in which an odor responsive P-type OFET M 21  is interconnected with a transistor M 11  to form a common-source amplifier. The output (node A 31 ) of the common-source amplifier is connected to the input of a source (voltage) follower stage comprised of transistors M 31  and M 41  whose output (node A 41 ) is selectively fed back to the gate of M 11  via transistor M 121 . As in FIG. 5, when odors/vapors are “puffed” onto OFET M 21  the feedback loop is opened, and the common-source amplifier amplifies the signal due to the odors/vapors. 
     In FIG. 6, the source of M 21  is connected to a power terminal  81  to which is applied VDD volts and its gate is connected to a constant bias voltage V 21 . The drain of M 21  is connected to the drain of an N-type FET, M 11 , at output node A 31 . The source of M 11  is connected to ground. Node A 31  is connected to the gate of source follower transistor M 31  whose drain is connected to terminal  81  and whose source is connected to terminal A 41 . Transistors M 11  and M 21  form a common-source amplifier with a control input being the gate voltage of M 11  and a signal input being the current through M 21  responsive to the odors/vapors puffed onto M 21 . The output of the common-source amplifier is the voltage at node A 31 . If the current through M 21  exceeds the current through M 11 , then the node voltage A 41  is driven near VDD. If, on the other hand, the current through M 11  exceeds that through M 21 , the node voltage A 41  is driven near ground. The output of the common-source amplifier is connected to the input of a source follower stage (the gate of M 31 ) whose output (A 41  at the source of M 31 ) is fed back to the gate of M 11  via switching transistor M 121 . 
     As in FIG. 5, an input signal source  71   a  is shown (with dashed lines) connected between the gate and source of M 21  to indicate the signal input function of the sensor, internal to M 21 , when an odor/vapor is puffed onto M 21 . That is, signal source  71   a  represents the effect of a mobility or threshold change on and within transistor M 21  when an odor is puffed onto it. Typically, the odor is puffed onto transistor M 21  when the feedback loop is open (i.e., M 121  is turned-off). Transistor M 21  is the only odor sensitive transistor in the circuit; any other organic transistor in the circuit is assumed to be odor insensitive. 
     Transistors M 31  and M 41  form a standard N-type FET source follower stage whose bias current is set by a voltage V 41  applied to the gate of M 41 . Transistor M 121  is turned on when the circuit is not sniffing. When M 121  is turned on the output of the source follower is tied to a capacitor C 11  such that transistors M 31 , M 41  and the capacitor C 11  implement a weakly nonlinear low pass filter. The time constant of the low pass filter may be controlled by altering V 41 , the capacitance of C 11 , or both. Transistor M 121  implements a switch that is turned on and off by a signal source  121  producing a voltage V 121 . 
     When the circuit is not sniffing, the source  121  applies a low voltage to the gate of M 121  to enable the negative feedback loop and cause the voltage at A 41  to be applied to capacitor C 11  and the gate of M 11 . Thus, during the non-sensing mode, the gate of transistor M 11  is connected to a low pass filtered version of the common-source amplifier&#39;s output in a negative feedback configuration. Consequently, during this mode, the gate voltage of M 11  constantly adapts to compensate for long-term changes in the threshold or mobility of transistor M 21  and keeps the output A 31  of common-source amplifier (M 11 , M 21 ) at its balanced equilibrium. 
     When the circuit is sniffing, the negative feedback is turned off and the circuit sits at its gain balanced equilibrium, ready to amplify any odor-caused change in the current through transistor M 21 . The output voltage at A 31  reflects changes induced by the odor response of M 21 . When the circuit is not sniffing, the negative feedback is turned on and causes the circuit to adapt and compensate for any long-term changes in the characteristics of OFET transistor M 21 . 
     FIG. 6 also shows a switching sub-circuit formed by transistors M 121 , M 61 , and M 71  which is active only when V 121  is active low. The implementation of the lowpass filter and the switching subcircuit are now briefly described. Transistors M 61  and M 71  are ratioed to have half the W and the same L as transistor M 121 . They help to alleviate charge injection problems caused by the switching voltage of M 121 . The charge injection is alleviated by having a signal (V 13 ) complementary to V 121  drive M 61  and M 71 . The charge injection is dominated by the overlap capacitances of transistor M 121 . The overlap capacitances of shorted-and-ratioed M 61  and M 71  match those at the source and drain ends of M 121  and cancel charge injection from M 121  with charge injection from M 61  and M 71 . 
     Features of the circuits of various embodiments, which were discussed above in FIGS. 5 and 6, are shown in FIG.  7 . FIG. 7 includes a high gain amplifier  91  responsive to signals from an OFET sensor that is integral to one of the amplifying devices in amplifier  91 . The output of the amplifier is selectively fed back by means of a switching network  92  and a low pass filter  93  to a control input of the amplifier  91 . The switching network is turned on and off as a function of a sniff signal circuit  94  which controls the application of chemical odors/vapors/gases (analytes) to the sensor contained within the high gain amplifier. During a non-odor-sensing period of time, the switching network  92  closes the negative feedback loop such that the low pass filter  93  is coupled between the output of amplifier  91  and an input of amplifier  91 . During an odor-sensing period of time, the switching network is open so that the feed back loop is removed from the circuit, and the high gain amplifier  91  amplifies signals resulting from the flow of odors over the OFET sensor. 
     To better explain other embodiments, assume, as shown in FIGS. 1A and 2B, that a discrete OFET is biased to conduct a source-drain current having a value I D1  prior to any odor being applied to the OFET, When an odor is “puffed” onto the OFET, the source-drain current changes from I D1  to I D2  in response to an odor (analyte) incident on the OFET from a time I 1  to a time I 2 . Thus, after an odor signal is applied to an OFET, the source-to-drain current changes. The change in the source-to-drain current persists even after the removal of the odor. When an odor is applied for a given time (e.g., 5 seconds), it normally takes a much longer period of time (e.g., one minute) for the OFET current to recover from the value of I D2  to a value approximately equal to that of I D1    
     After an OFET is subjected to an odor signal, applying an electrical bias cycle to the gate of an OFET facilitates its recovery to the condition existing prior to the application of the odor. That is, by applying an electrical signal to the gate of the OFET which goes positive and negative (or negative and positive) relative to the source (and/or drain) of the OFET, the OFET recovers more quickly and the degree of recovery is enhanced. Full recovery means the return of the drain current of the OFET to the level it would have had had an odor signal not been applied to the OFET. 
     In various embodiments, OFETs are operated so that they return more quickly to the existing operating condition extent immediately before the application of the selected odor (analyte) to the circuit. Ring oscillators employing OFETs to sense the presence of odors are very useful as sensing circuits for selected odors. 
     Ring Oscillator Sensors 
     Using OFETs in a ring oscillator circuit eliminates many of the above-discussed problems associated with OFETs. Referring to FIG. 8 there is shown five complementary inverters (I 1 -I 5 ) interconnected to form a ring oscillator. In FIG. 8 each inverter (I 1 -I 5 ) includes a P-type OFET (P 1 -P 5 ) and an N-type FET (N 1 -N 5 ). In one embodiment of the FIG. 8 circuit, the P-type OFETs were made of didodecyl α-sexithiophene (DDα6T) material and the N-type FETs were made of hexadecafluoro copper phthallocyanine (F 16 CuPc) material. In that embodiment, the material DDα6T was used to form the P-type OFETs because DDα6T is sensitive to the analyte octanol and esters such as allyl propionate which are the analytes selected to be sensed by the circuit. In contrast, the material F 16 CuPc was used to make the N-type OFETs, because it is insensitive to octanol and esters such as allyl propionate. Therefore, the N-type OFETs (N 1 -N 5 ) are insensitive or, at least, less sensitive to the selected analytes and could be replaced by standard N-type FETs. 
     The source electrodes of the P-type transistors (P 1 -P 5 ) are connected to a power terminal  81  to which is applied +VDD volts. The source electrodes of the N-type FETS (N 1 -N 5 ) are connected to a power terminal  83  to which is applied ground potential. The gate electrodes of the two transistors forming each inverter are connected in common and define a signal input terminal to the inverter. The drain electrodes of the two transistors forming each inverter are connected in common and define a signal output terminal of the inverter. Starting with the first inverter, the output of each inverter along the chain is connected to the input of the next inverter along the chain, except for the output of the last inverter (e.g., I 5 ) which is fed back to the input of the first inverter. Note that there is some capacitance, C, (which may be parasitic or discrete) associated with the input (gates) of each inverter. The combination of the effective output impedance of each inverter and the input capacitance of the next stage determines the time constants of each stage and the frequency of oscillation of the circuit. 
     In one embodiment, the oscillation frequency of the 5-stage all organic F 16 CuPc/DDα6T complementary ring oscillators ranged from a few Hz to several kHz. A selected analyte was “puffed” onto the ring oscillator circuit. The analyte reduced the conductivity of the P-type OFETs. In the discussion to follow, it is assumed that the conductivity of the OFET decreases when subjected to a gas (analyte). However, it should be understood that other OFETs have conductivities that increase when subjected to an odor (analyte). For OFETs whose conductivity increases in response to the presence of an odor, the circuit configurations discussed are also suitable. However, the response of the circuit would be the inverse of that described below (i.e., the frequency of oscillation would increase rather than decrease). 
     The mobility of the discrete F 16 CuPc transistors and of the DDα6T transistors, measured on devices fabricated alongside the ring oscillators, was approximately equal to ˜10 −2  cm 2 /V-s. The response of the circuit was measured with an oscilloscope with a high input impedance (50 M ohm) probe. The response of a particular circuit configured as illustrated in FIG. 8 is shown in FIG.  9 . The change in frequency as a consequence of the odor is clearly seen. Due to the decrease in the conductivity of the P-type OFETs, there is an increase in the RC time constants of the inverting stages. This causes the oscillation frequency to decrease very sharply. Note that FIG. 9 depicts the response of the circuit of FIG. 8 to an analyte “puffed” onto the circuit from time t 1  to time t 2  (approximately 5 seconds). As a result of the incident odor “puffed” onto the circuit the frequency changed from around 550 Hz to around 280 Hz. Thus a frequency change of nearly 50% was observed. A change in the amplitude of the oscillations was also observed (i.e., change in Vmax). The observed change is also much greater than the change observed in a discrete OFET in response to the same odor intensity. This demonstrates that a circuit of the type shown in FIG. 8 is a better odor/gas sensor than using a circuit using a single OFET. 
     Referring to FIG. 9, it is also seen that beginning at time t 2 , after the odor (analyte) is no longer applied to the circuit, the circuit begins to return to its condition prior to application of the odor (analyte). Applying an alternating signal to the gate of an OFET having a polarity to turn-it-on harder for a first time period and then having a polarity to turn-it-off for a second period of time, tends to enhance the recovery of the OFET to the state it had prior to the application of an analyte. This is in sharp contrast to the response of the discrete OFET shown in FIG. 2B, where the response of the discrete OFET does not begin to recover immediately after removal of the odor (analyte). 
     When the ring oscillator circuit of FIG. 8 is exposed to a selected analyte, the mobility of the material (DDα6T) is changed and the oscillation frequency changes. This provides a convenient means of measuring the presence and concentration of the analyte. By making the OFETs sensitive to certain particular analytes and not to others it is also possible to ascertain the presence of these certain analytes. However, usually odorant detection will be done by pattern recognition based upon the responses of several sensors. Therefore, it is generally not necessary to have sensors that respond only to a particular odorant. 
     In the circuit of FIG. 8 the P-type transistors P 1 -P 5  are OFETs formed on an integrated circuit (IC) by similar masking and processing steps. It is possible to obtain a still higher gain response by using OFETs of complementary conductivity as shown in FIG.  10 . FIG. 10 is another embodiment of an oscillator circuit in which complementary inverters are arranged such that in every other inverter (e.g., the odd numbered inverters) the P-type transistor is an odor-sensitive OFET and in the intermediate inverters the N-type transistor is an odor-sensitive OFET. The OFETs in the circuit of FIG. 10 are formed of materials which cause their conductivity to decrease when a selected analyte is puffed on the OFETs. Consequently, when an analyte is applied to the ring oscillator circuit of FIG. 10, the conductivity of OFETs P 1 , P 3  and P 5 A and OFETs N 2 A and N 4 A decreases. Therefore, each cascaded inverter is responsive to the presence of the analyte. In addition, the output of each inverter (e.g., I 1 ) is applied to the input of the next inverter (e.g., I 2 ) along the chain with a phasal relationship that results in the further amplification by the next inverter (e.g., I 2 ) of the signal from the preceding stage (e.g., I 1 ). For example, beginning with inverter I 1 , in response to an odor signal, the output of inverter I 1  produces a signal which is an amplified version of the response of OFET P 1 . Since the conductivity of P 1  decreases, in response to the odor, the effective impedance of P 1  increases and the current through P 1  decreases resulting in more time being required to charge the capacitance at the output node of inverter I 1 . The output of I 1  is applied to the input of inverter I 2 . By making N 2  an OFET whose conductivity also decreases (i.e., its effective impedance increases) in a similar manner to that of OFET P 1 , inverter I 2  functions to further amplify the response at the output of I 1 . This is evident from noting that as the effective impedance of OFET N 2  increases it causes the voltage at the output of I 2  to be discharged more slowly and hence the output of I 2  to decrease more slowly from its high state. Concurrently, the decrease in the voltage at the output of I 1  applied to the gate of N 2  also causes N 2  to conduct less. Hence, the condition at the output of I 2  is reinforced by the signal at the output of I 1 . In a similar manner to that just described, making P 3  an OFET and N 3  a regular FET ensures that the signals from the previous stages is amplified in phase with the signal generated by P 3  in response to its sensing an analyte. This same amplification of the sensed signal within a stage in cascade with the amplified signals of the previous stages occurs in inverter I 4 . 
     Different forms of the cascaded inverting stages using OFETs to sense odors of the type shown in FIGS. 8 and 10 may be used to practice the invention. An embodiment shown in FIG. 10A includes a first inverting stage comprising a P-type OFET, T 1 , and an odor-insensitive FET, T 2 . The source electrode of T 1  is connected to power terminal  81  to which is applied VDD volts and its drain electrode is connected to node  101 . A bias voltage VB is applied to the gate of T 1  to bias T 1  at a desired operating point. T 2  is shown as an N-type FET, but it may be a P-type FET or any load device. A control voltage, VC 1 , is applied to the gate of T 2  to control the conductivity of T 2  independently of the bias voltage applied to T 1 . The drains of T 1  and T 2  are connected in common at node  101  at which is produced the output of the first inverting stage. The source of T 2  is returned to ground potential. The second inverting stage includes a P-type OFET, T 3 , having its gate electrode connected to node  101 , its source electrode connected to node  81  and its drain connected to node  103 . A load, shown as a resistor RL, but which, in practice, may be a passive or active load, is connected between node  103  and ground potential. The signal generated at node  103  may be supplied to any suitable signal amplifying or processing circuit. 
     Another embodiment is shown in FIG.  10 B. the first inverting stage is similar to that of FIG.  10 A. However, the second inverting stage includes an N-type OFET T 3 A connected at its gate electrode to node  101 , at its source electrode to ground potential and at its drain electrode to output node  103 . A fourth transistor T 4  has its source-to-drain path connected between terminal  81  and node  103 . the gate electrode of T 4  is shown connected to a control voltage source VC 2  designed to control the conductivity (impedance) of T 4 . T 4  may be an active load (e.g., an N-FET, a P-FET, or an OFET) or it maybe replaced by a passive resistive load. 
     In FIGS. 10A and 10B, the inverting stages are cascaded to enhance signal amplification and increase the sensitivity of the odor sensitive transistors to the application of analytes (odors). The circuits of FIGS. 8,  10 ,  10 A and  10 B illustrate the use of multiple sensors (two, or more OFETs) that are connected in circuit to coherently amplify the effects of an analyte by acting synchronously. Thus, small changes produced in a single stage in response to a weak analyte concentration applied to the circuit are amplified over several stages leading to an improvement in signal to noise. 
     Referring to FIG. 11, there is shown an oscillator  110  coupled to a counter circuit  111  whose outputs are coupled to an alarm circuit  113 . The oscillator  110  may be any suitable oscillator using at least one odor-sensitive OFET for varying the frequency of oscillation in response to a selected odor. By way of example, oscillator  110  may be a ring oscillator as shown in FIGS. 8 and 10. Any suitable output signal of the oscillator can be applied to a counter  111  that tracks and calculates frequency of oscillations. If the frequency decreases below a predetermined level, for the condition where the response of the OFET to a selected odor causes the oscillator to decrease, the output of the counter  111  activates processing circuitry  113  and activates an alarm. Alternatively, if the frequency increases above a predetermined level, for a condition when the response of the OFET to a selected odor causes the oscillation to increase., the output of the counter  111  activates processing circuitry  113  and activates an alarm. 
     OFETS for use in circuits embodying the invention may be formed as shown in FIG.  12 . Note that a substrate  120  of standard Si electronics (both FET and bipolar) fabricated in a conventional manner known in the art as integrated circuit (IC) fabrication may be used. After the fabrication of the different levels of metallization needed for the Si circuitry, organic transistor sensor circuits are fabricated employing an upside-down approach. In the upside down approach, OFET circuits are formed by sequentially defining the interconnects, the gate metal level, a dielectric layer, source-drain metal level and the organic semiconductor layer. The organic semiconductor sees minimal post-deposition processing. The approach of FIG. 12 is different from typical circuit fabrication procedures where the transistor devices are first formed followed by the interconnections. However, any suitable fabrication scheme may be used to form circuits embodying the invention. 
     In FIG. 12, the fabrication of the organic FET circuits begins with the deposition (above the silicon circuitry) of a thick layer ( 122 ,  124 ) of SiO 2 (for isolation). The metal lines and vias (through-holes for the metal interconnects) are defined by photolithography and standard semiconductor processing techniques. The interconnection metal (AI) level is defined immediately above the substrate followed by the gate metal level (AI) and the source/drain metal level. The gate dielectric  124  may be a bilayer consisting of 20 nm of Si 3 N 4  and 10 nm of SiO 2 , with a capacitance of 200 nF/cm 2 . The interlayer isolation dielectrics  122  are SiO 2  or Si 3 N 4 . The organic semiconductors are deposited as a thin layer above and between the source (S) and drain (D) contacts. The S/D contacts are coated with a gold layer by electroless/immersion techniques to facilitate good ohmic contact with the organic semiconductors. The underlying Si electronics and the above-lying organic circuitry are electrically interconnected as required through dielectrics by forming vias. The organic transistor circuitry may include any combination of the circuits described above. The active organic semiconductor material is deposited over the pre-formed source-drain contacts and gate insulator may be any suitable material for the desired sensor selectivity. 
     The molecular structures of some materials used to form OFETs are shown in FIG.  13 . Exemplary materials for active semiconductor layers of P-type OFETs include: 
     a. didodecyl α-sexithiophene (DDα6T); 
     b. dioctadecyl α-sexithiophene; 
     c. copperphthallocyanine; 
     d. α-sexithiophene; 
     e. α,ω-dihexylsexithiophene; 
     f. poly(3-alkythiophene); 
     g. poly(3-hexylthiophene); and 
     h. poly(3-dodecylthiophene). 
     Exemplary materials for active semiconductor layers of N-type OFETs include: 
     a. hexadecafluorocopperphthallocyanine (F 16 CuPc); and 
     b. naphthalenetetracarboxylic diimide compounds. 
     These materials are listed by way of example only and any other suitable materials may be used. 
     The molecular structures of some odors used to test circuits embodying the invention are shown in FIG.  14 . However, it should be understood that any gas, chemical vapor, odor or analyte which causes a change in the conductivity of an OFET may be sensed by circuits embodying the invention. 
     The various embodiments shown herein are for purpose of illustration, and the invention may be practiced using any suitable circuit.