CHAPTER 1 Introduction
1.4 Overview of This Thesis
Introducing the principle of sensing method for ammonia sensors is in the first chapter. The ammonia sensors based on an organic diode with vertical nano-junctions (VNJ) are presented in the second chapter. There are the experimental data for the relation between concentration of ammonia and response of ammonia sensors. Then introducing the principle of readout system, which is proposed to acquire and process the signals, which are attributable to ammonia concentration in the third chapter. And the theory and framework of the readout circuit are described. In the fourth chapter, the simulation results and measurement data of the sensing circuit are shown and discussed. Finally, the results of the research are summarized, and moreover, the potential applications and future works are proposed in the fifth chapter.
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CHAPTER 2
Performances and Characteristics of Gas Sensors for Ammonia
2.1 Characteristics of Pentacene-based OTFT
Mobility of inorganic material is greater above three orders compared with mobility of organic material. In organic thin film material, pentacene thin film has the best mobility at present. The structure and electric characterization of the particular OTFT are introduced in this section.
2.1.1 Structure of Pentacene-based OTFT
The substrates of OTFT are made of the silicon material. The insulator on the second level and gate electrode is SiO2. PMMA [poly(methyl methacrylate)] was used as the buffer layer to improve electric characteristics of SiO2 dielectric surface [18]. The dipole moment of PMMA
PMMA UV-treated PMMA
Fig. 1. Functional end-groups of PMMA and UV-treated PMMA [18].
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end-groups are increased because part of the SiO2 with PMMA was irradiated by UV-light (shown in Fig. 1). After being deposited on the PMMA dielectrics, the pentacene film was evaporated to form the active layer. The source and drain contacts are made by depositing the gold film through a shadow mask. Fig. 2 shows the flowchart for the fabrication processes of the proposed pentacene-based OTFT.
2.1.2 Electric Characterization of Pentacene-based OTFT
Because the development of OTFT has not yet completed, modeling OTFT has not yet been realized. In order to analyze OTFT’s electric characterization in a wide range, the formula of electric characterization for metal-oxide-semiconductor field-effect transistors (MOSFETs) are used. When the OTFT are operated in triode region, the drain current (Id)of OTFT can be expressed as
Fig. 2. Fabrication and structure of pentacene-based OTFT [18].
8 where μ is the charge-carrier effective mobility, Cox is the gate oxide capacitance per unit area, W is the gate width and L is the gate length. Simplification of Eq. (1) shows Eq. (2) when Vd
<< (Vg-Vth), In saturation region, the drain current response of OTFT is defined as
2Because the mobility of MOSFET is much larger than the mobility of OTFT, the current of drain-source on OTFT is much less than that on MOSFET. In order to increase the output current of OTFT, enlarging the gate width and/or decreasing the gate length are one of the methodologies. The results can be used in increasing response at the same ammonia concentration.
2.2 Ammonia Sensing Responses of Pentacene-based OTFT
2.2.1 Primary Parameters of Pentacene-based OTFT
Turn-on current, threshold voltage (VTH), intrinsic mobility (μ) and sub-threshold slope (S.S.) are the significant variations of OTFT. When transistors are turned on, the maximum drain current is called turn-on current. When channel forms at the interface between the insulating layer and the substrate of the transistor, the gate voltage is defined as threshold voltage. The ability of driving charged particles under an applied electric field is intrinsic mobility. In order to make a low resistance conducting path between the drain and source, there are sufficient carriers in the channel. The device performance is better because of higher mobility. Sub-threshold slope is defined as
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l o gDS
1GS
SS I
V
. (5)
The magnitude of bias voltage is indicated in the equation. It is needed when increasing tenfold current in sub-threshold region. To summarize, this is an ability to control the on/off of gate channel. The smaller this parameter is, the better the efficiency of device is.
The above-mentioned parameters are different between STD (untreated pentacene-based)-OTFT and UV-OTFT, as shown in Fig. 3. The least change under four conditions is the S.S. transforms in the figure. Threshold voltage shift (Vth) and sub-threshold slope of STD-OTFT and UV-STD-OTFT are steady in nitrogen environment in addition to the variation of turn-on current (Id/Id0) and intrinsic mobility (μ/μ0). In ammonia surrounding, the Id/Id0, ΔVth, and μ/μ0 change obviously. Threshold voltage shift has contributed to more obvious changes in
Fig. 3. Parameters versus time plot of STD and UV-OTFT in nitrogen and ammonia conditions [18].
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sensing behavior comparing the other three factors. In contrast between STD-OTFT and UV-OTFT, the high efficiency is easy to perceive. Because of increasing dipole moment of PMMA end-groups by UV-light irradiates PMMA, the results indicate that promoting interaction between ammonia molecules and surface of OTFT.
2.2.2 Preconditions and Experiment Results
Other electric characterizations are revealed and demonstrate better sense response because of the ΔVth factor. They can be used well, for example current is experimented on some preconditions.
Ammonia is sensed by OTFT in linear region in this case. According to Eq. (1), when the voltage is biased fixedly on gate-source and drain-source, the drain current varies according to ΔVth. Because of Eq. (2), the variation of the drain current is proportional to ΔVth when overdrive voltage is higher than drain-source voltage. As previously mentioned, the concentration of ammonia is closely related to the ΔVth. Thus this study can extrapolate the positive correlation between the concentration of ammonia and the drain current at present.
Fig. 4 shows the drain current versus time for several ammonia concentrations individually in nitrogen environments. The ammonia sensing responses in nitrogen environment is revealed in the figure.
Fig. 4. The drain current versus time for several ammonia concentrations in nitrogen environment [18].
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2.3 Characteristics of VNJ-P3HT Diode
Only the concentration of ammonia larger than 100ppb can be detected by the Pentacene-based OTFT. A sensing system with high selectivity and the lowest detection lower than 50 ppb is required to detect the breath ammonia.
2.3.1 Structure of VNJ-P3HT Diode
In previous chapters, the structure of the Pentacene-based OTFT is complex. Analyzing by analog IC or other portable devices is difficult because the output current of the Pentacene-based OTFT is small, as shown in Fig. 4. Thus an organic diode with vertical nano-junctions (VNJ) produced by using low-cost colloidal lithography is developed [19]. The lowest detection lower than 20 ppb, real-time response, good enough selectivity, simple structure, high reproducibility and low production costs are advantages of the proposed ammonia sensor.
Fig. 5 shows the structure of VNJ-P3HT diode. The ammonia sensing layer is made of the P3HT [poly (3-hexylthiophene)]. The aluminum (Al) was used as cathode. High-density nano-pores on the cathode are produced by using the low-cost colloidal lithography to facilitate the interaction between the molecules of gaseous ammonia and P3HT film. The Indium tin oxide (ITO) film was deposited to be anode.
Cathode, Al P3HT
Anode, ITO
Fig. 5. The structure of VNJ-P3HT diode [19].
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2.3.2 Electric Characterization of VNJ-P3HT Diode
Without interacting with ammonia, Fig. 6 shows the output current of the VNJ-P3HT diode when the sensor is biased at 2V. And the output current of the VNJ-P3HT diode is called Is. Just before interacting with ammonia, the output current of the VNJ-P3HT diode is defined as initial current (I0). The initial current changes unexpectedly from time to time, as shown in Fig. 6. The range of the initial current is 10μA to 100μA roughly. The output current’s values of the VNJ-P3HT diode also change according to variation of the initial current. Thus it’s difficult to determine the concentration of ammonia just by the output current’s values of the VNJ-P3HT diode when initial current changes from time to time.
2.4 Ammonia Sensing Responses of VNJ-P3HT Diode
2.4.1 Primary Parameters of VNJ-P3HT Diode
Ammonia molecules react with these VNJ-P3HT diodes by diffusing into the P3HT film through the high-density pores, dedoping the P3HT film, and reducing the diode’s current.
The more ammonia is absorbed by sensor, the less current of output is produced. The value of current doesn’t change anymore until the current saturates. The current in steady state is called saturation current (Iss). Finally, the different specific gas concentration can be presented
0 1 2 3 4 5 6
13
by the ratio of saturation current to initial current (
0
Iss
I ). This study defines a response as the ratio of the VNJ-P3HT diode’s output current to the initial current (
0
Is
I ).
2.4.2 Preconditions and Experiment Results
The responses in 10ppb ~ 3ppm of ammonia are shown in Fig. 7. The output current of the VNJ-P3HT diode doesn’t change anymore because the responses are saturated after 200 seconds. The time of the initial current’s unexpected change from time to time is much longer than 200 seconds (as Fig. 6) and therefore it can be ignored. Thus the concentration of ammonia can be determined by the responses in steady state (saturation).
0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 1 6 0 1 8 0 2 0 0 2 2 0
Fig. 7. The original data of response (
0
Is
I ) in 10ppb ~ 3ppm of ammonia [19].
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Fig. 8(a) shows the current density as a function of applied bias voltage (J−V curves) of the VNJ-P3HT diode. The current density is proportional to the square of the applied bias voltage, indicating that the holes in P3HT follow the space-charge-limited conduction (SCLC).
That is to say, the carrier transport in most intrinsic or low-doped conjugated polymers follows the space-charge-limited current. Thus the injected charge carrier density is much higher than the background doping density in most volume of the sample (i.e., in the bulk region). Because the injected charges are unipolar carriers with very low mobility, they are considered as space charges. Finally, the injected charges are closer to the injection interface have a higher charge density.
The J − V curve of the VNJ-P3HT diode is shown by the red dashed line in Fig. 8(a) after injecting the 3 ppm ammonia (with a background of nitrogen) for 200 s. After contact with ammonia, the slightly right shift of the onset voltage indicates a slight increase of hole injection barrier. However, the main reaction is the current decline in the SCLC zone. The
(b) (a)
Fig. 8. (a) J−V curve of VNJ-P3HT diodes before (black solid curve) and after 200 s of 3 ppm ammonia sensing (red dashed curve). Green symbols represent the response (i.e., the current variation ratio) of VNJ-P3HT diodes. (b) The responses (measured at 2 V with a fixed sensing time as 200 s) to carbon dioxide (5%), nitric oxide (3 ppm), ethanol (100 and 1 ppm), acetone (1 ppm), and ammonia (3 ppm and 500 ppb). Blue bars, red bars, and green bars represent responses with backgrounds as pure nitrogen, dry air, and dry air passing through commercial ammonia filter (NiCl2·6H2O powders) [19].
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ratio of saturation current to initial current (
0
Iss
I ) is used to represent the response of the sensor.
The green symbols in Fig. 8(a) shows the response to the 3ppm ammonia 200 seconds exposure plotted as a function of applied bias. The peak of response is at around 0.8 V. But the current of diode at 0.8 V is too low to provide a good signal-to-noise ratio. A large and stable response as −0.6 is obtained when the applied bias voltage changes from 1.5 to 3 V.
Thus this study chooses 2 V as a fixed applied bias voltage to measure the response in the following works.
The relations are all the same in different concentration of ammonia according to [19].
The responses in steady state are same, which can determine the concentration of ammonia when the VNJ-P3HT diode is biased in 1.5~3V. Thus this study chooses the VNJ-P3HT diode as the sensor device at present. If this study tries to operate the diode sensor accurately, it needs to satisfy:
1. 1.5~3V should be applied to the bias of the sensor.
2. When initial current varies from 10~100μA, this study can perform response.
3. In order to present the specific gas concentration, use response in steady state.
4. 10ppb of ammonia can be detected by the sensor.
The responses of the VNJ-P3HT diode are also analyzed to several kinds of gases existing in human respiratory gas. The responses of the VNJ-P3HT diode (measured at 2 V with a fixed sensing time as 200 seconds) to carbon dioxide (5%), nitric oxide (3 ppm), ethanol (100 and 1 ppm), acetone (1 ppm) and ammonia (3 ppm and 500 ppb) are compared in Fig. 8(b). The responses with backgrounds as pure nitrogen, dry air, and dry air passing through commercial ammonia filter (NiCl2•6H2O powders) are represented by blue bars, red bars and green bars. Because of the greatly suppressed vapor pressure of ethanol (2 mmHg) and acetone (20 mmHg) at −20 °C, the response to ethanol and acetone in dry air are significantly suppressed. Thus the VNJ-P3HT diode has very small responses (0.009 to
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−0.017) to ethanol and acetone with dry air as the background. That is to say, cooling the tube at −20 °C before connecting to the sensor device (i.e., the sensor is at room temperature) leads to a suppressed humidity and suppressed concentrations of VOCs. The concentration of ammonia should not be significantly changed in our system because the boiling point of ammonia is −33.3 °C. On the other hand, the response to 3 ppm nitric oxide is positive and significant. Because of the oxidation (doping) of P3HT, the reaction is known and irreversible.
Fortunately, the concentration of nitric oxide is very low in our breath (<40 ppb).
The responses of a fresh VNJ-P3HT diode and of a 5 hours aged VNJ-P3HT diode to 400 ppb ammonia as a function of time are compared in breath ammonia testing. The aged sample was used to detect breath ammonia of rats for 12 times during 5 hours. The difference between response of the fresh sample and response of the aged sample is within 4% proving that the VNJ-P3HT diode is reliable enough for at least 5 hours of operation.
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CHAPTER 3
The Novel Sensing Circuit for Organic Gas Sensors
The instruments are usually used to measure the gas concentration sensed by VNJ-P3HT diode. There are few chip design studies which focus on the back-end read-out circuit because of the property of sensor device. The design of the proposed sensing circuit can provide more benefits for real-time detection of gas concentration compared with the measurement by traditional instruments. In order to detect ammonia concentration based on steady state current, the responses of VNJ-P3HT diode are measured. This study designs the sensing circuit to process current variation of VNJ-P3HT diode for varied ammonia concentrations. Because of using the ratio of saturation current to initial current to determining ammonia concentration, the sensing circuit solves the problem that the initial current of VNJ-P3HT diode are different from time to time. The real-time detection and convenient for diagnosis can also be achieved by the sensing circuit for biomedical usages. A readout circuit is needed to design as interface with the VNJ-P3HT diode at first. The output voltage which is converted by the output current of front-end sensors through the principle of a trans-impedance amplifier (TIA) can be processed by this circuit. The linear or other relationships between output signals of sensor circuit and gas concentration must be achieved in the next step. It is stand for that the gas concentration can be directly detected by output signals from back-end circuit. Thus the accurate and convenient medical measurement can be further achieved.
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3.1 Review of Past Sensing Circuits
3.1.1 A Readout Circuit for an OTFT Gas Sensor with a New Preamplifier
Fig. 9 shows that the previous readout circuit consists of a pre-amplifier, a sample and hold circuit and a differential current amplifier [20]. In order to convert the output current of a gas sensor into voltage, a pre-amplifier is composed of trans-impedance. According to distinct designing time instants in a target gas environment, the sample and hold circuit memorizes various voltages. The differential current is attributable to the voltages of sample and hold circuit using current mirror framework. Then a differential current amplifier amplifies the differential current. The differential current is converted into output voltage through the resistance load. In order to determine gas concentration database of pre-measured which are contrast, a micro-processor control unit (MCU) acquires and stores the outputs.
There are two major disadvantages in the previous readout circuit. First, the voltage value is recorded by a capacitor Chold of the sample and hold circuit. The voltage drop problem is serious because of the leakage current of the capacitor. Second, the value of initial
Fig. 9
.
Readout circuit for OTFT sensor [20].19
current changes unexpectedly from time to time in the previous chapter. But the design of previous sensing circuit is based on stable OTFT sensors, which means that the initial current of OTFT sensors doesn’t change from time to time. The output of the differential current amplifier changes and the gas concentration database is unreliable when the value of initial current changes.
3.1.2 A Front-end Readout Circuit Including an Analog Divider for an OTFT Gas Sensor
The previous readout system (shown in Fig. 10) consists of an analog IC, a micro-processor control unit (MCU) and Man-Machine Interface (MMI). The analog IC comprises further a pre-amplifier and an analog divider, which are used to be an interface with the OTFTs sensors and calculate the ratio of saturation current to initial current. The CPU of proposed MCU is LPC1768 by NXP. The AD/DA of MCU holds the input value of the analog divider and the
VDD
Fig. 10. The sensing system for OTFT sensor [21].
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ADC of MCU is used to calculate and store the output data of readout circuit. The MMI comprises further a Liquid Crystal Display (LCD) module and buttons. It is applied to be conveniently used by users and displays results of sensing system, which is controlled by the MCU.
The previous readout circuit also shows two major disadvantages. First, the system includes a MCU and an analog circuit, causing complexity to increase. Second, the design is not accurate, because it is limited by the resolution of the AD/DA converter.
3.2 Types of Divider Circuit
The analog signal processing circuits use the analog divider circuit widely, for example the filters, hearing-aid systems and logarithmic function generators. According to different circuit principles, many types of analog dividers are designed.
M1 M2
VDD
M3 M4
IX Ib IY
o u t
I
V1 V2
Fig. 11. Weak inversion divider circuit [22].
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3.2.1 Weak Inversion Divider
The divider and 1/x functions can be performed by a CMOS current-mode circuit in this study.
In order to meet the application of division, the design uses MOSFETs biased in weak inversion. Fig. 11 shows the proposed circuit. The drain current of a PMOS transistor in weak inversion is given by The output current in Fig. 11 is given by
1 2 Eq. (10) implements a current-mode divider circuit when the bias current I keep constant. b The input current (I , X I ) and output current (Y IOUT) are too small to be used for organic gas sensor because the MOSFETs are biased in weak inversion. The minimum output current of organic gas sensor is at least greater than 10 μA.
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3.2.2 Current-to-Voltage Mode Divider
Fig. 12 shows the proposed circuit. If both M1 and M2 are biased in the triode region without body effect, the source currents I and 1 I can be expressed as 2 threshold voltages of M1 and M2, respectively. The current mirror, M5 and M6, is used to copy the current I , so that 1
Fig. 12. Divider circuit for organic sensor [23].
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1 3 4
I I I . (13) The source voltages of M3 and M4 are equal (VSB3 VSB4 so VTn3 VTn4) and both of them are biased in saturation if they are matched (Kn3 Kn4). Because of the square-law characteristics of MOSFETs, the equation can be found as
1 A current-to-voltage-mode divider circuit is implemented in Eq. (16). The input current (I ) in can be up to 100 μA because the M1 and M2 biased in the triode region. Thus the
1 A current-to-voltage-mode divider circuit is implemented in Eq. (16). The input current (I ) in can be up to 100 μA because the M1 and M2 biased in the triode region. Thus the