Abstract:
A pulse generator circuit with ferroelectric memory element is disclosed that is optimized for printed, solution-processed thin film transistor processing. In certain embodiments, the circuit comprises dual thin film transistors that operate as a diode and resistor, respectively. Optionally, a third thin film transistor may be provided to operate as a pass transistor in response to an enable signal. The elements of the circuit are configured such that a rising pulse on an input node triggers an output pulse on an output node in the manner of a monostable multivibrator. The ferroelectric memory element is coupled to the output node such that a pulse on the output node may change a state of the ferroelectric memory element

Description:
BACKGROUND 
       [0001]    The present disclosure is related to digital electronic circuits, and more specifically to a pulse generator circuit optimized for printed solution-processed thin-film devices. 
         [0002]    A pulse generator, also called a monostable multivibrator or one-shot, is a common element of many circuits in a wide range of applications. One common use is for switch debouncing. Another is as a reset signal generator. The input to a one-shot is an edge or pulse and the output is a positive or negative voltage pulse. The duration and magnitude of the pulse can be fixed or settable and are generally constrained to fall within ranges specified for the application.  FIG. 1  shows a timing diagram of an example of a one-shot circuit. 
         [0003]    A common circuit implementation of a monostable multivibrator includes an inverter and a NAND or NOR gate.  FIG. 2  is an example of such an implementation. This circuit is a rising-edge-triggered negative pulse generator. In this circuit, input signal In is normally low, and inverter output D is high, making output signal Out high. When a rising edge is sent to In, output Out switches to low until the edge propagates through the inverter to D. Once D switches to low, Out returns to the high state. The duration of the pulse is determined by the propagation delay through the inverter. This can be controlled by the sizes of the transistors making up the inverter and NAND gate and/or by including additional delay elements, such as additional inverters or capacitors to the D signal. Of course, many other implementations are known in the art. 
         [0004]    There is a desire to produce devices such as the aforementioned monostable multivibrator using printed thin-film processes such as, but not limited to, those employing organic thin-film (OTF) material systems. OTF fabrication processes are much less mature than crystalline silicon technologies, and direct implementation of conventional complementary designs is challenging due to the relatively low yield, high variability, and instability of devices formed by printed OTF processes. For example, in certain of such process reliable fabrication of all integrated devices in a single process is problematic. Further, according to some OTF processes, only one or the other of N- and P-channel devices may be formed (not both simultaneously). In processes capable of producing both N- and P-channel devices, one device type often has significantly higher performance than the other. And, process-based limits on device size in combination with large parasitic capacitances limit design of devices having desired pulse widths. 
         [0005]    In light of these limitations, there is a need in the art for a device design capable of use within the context of OTF processes, such as circuit designs that include a minimum number of thin-film transistors (TFTs) and a single polarity. Such are disclosed herein. 
       SUMMARY 
       [0006]    Accordingly, the present disclosure is directed to circuit designs for converting an edge output by a sensor to a signal that addresses the difficulties of forming such circuits using solution-based and printed processes. The circuits provide a pulse sustained above a voltage threshold for a minimum time interval. Applications include clocking flip-flops, generating reset signals, polarizing a memory cell, and so on. The devices may be fabricated in an all-additive inkjet process. For the clarity and simplicity, processing using organic material (organic thin-film, OTF) is referred to below as an example of solution-based printed process, although it will be appreciated that many different types of solutions and materials, whether organic or inorganic, are contemplated herein and within the scope of the present disclosure. Therefore, OTF and organic material will be understood to be examples only, and the scope of the disclosure not limited thereby 
         [0007]    According to one aspect of the disclosure, an electronic pulse-generator circuit comprises: an input node; a capacitor communicatively connected to the input node and to a pulse line; a first thin film transistor having a channel formed of a first polarity organic semiconductor material, a first side of the channel communicatively connected to a first voltage source, a second side of the channel communicatively connected to the pulse line, the first thin film transistor configured to function as a diode; a second thin film transistor having a channel formed of the first polarity organic semiconductor material, a first side of the channel communicatively connected to the pulse line, a second side of the channel communicatively connected to a second voltage source (e.g., ground), the second thin film transistor configured to function as a resistor; an output node communicatively connected to receive a pulse from the pulse line; and a ferroelectric memory element communicatively coupled to the output node such that a pulse on the output node may change a state of the ferroelectric memory element. 
         [0008]    Implementations of this aspect may also include a third thin film transistor having a channel formed of the first polarity organic semiconductor material, a first side of said channel communicatively connected to the pulse line, a second side of the channel communicatively connected to the output node, and further comprising a gate communicatively connected to a first enable signal line. 
         [0009]    According to another aspect of the disclosure, the first polarity organic semiconductor material is p-type. 
         [0010]    Implementations may also include the first thin film transistor further comprising a gate communicatively connected to the pulse line at the capacitor and further wherein the second thin film transistor further comprises a gate communicatively connected to the second voltage source (e.g., ground). 
         [0011]    Other implementations of the present disclosure are also provided and discussed in greater detail, below. Thus, the above is merely a brief summary of a number of unique aspects, features, and advantages of the present disclosure. The above summary is not intended to be nor should it be read as an exclusive identification of aspects, features, or advantages of the claimed subject matter. Therefore, the above summary should not be read as imparting limitations to the claims nor in any other way determining the scope of said claims. 
         [0012]    It should be noted that the present disclosure focuses on solution-proceed devices. These are devices having one or more layers formed from the deposition of materials solution phase, which solidify (such as by evaporation). One form of solution processing is printing of patterned solution. Materials capable of solution processing may be either organic or inorganic. While this disclosure uses printed organic semiconductor technologies as an example, the invention is equally applicable to other process technologies. Furthermore, the invention can be implemented in other TFT technologies that are not solution based. Therefore, in this disclosure, references to solution processing, printing, and organic materials should be read as examples, not limiting distinctions. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    In the drawings appended hereto like reference numerals denote like elements between the various drawings. While illustrative, the drawings are not drawn to scale. In the drawings: 
           [0014]      FIG. 1  is a timing diagram of a positive-edge-triggered one-shot circuit with pulse duration, d, and voltage amplitude, v, as known in the art. 
           [0015]      FIG. 2  is an implementation of a monostable multivibrator including an inverter and a NAND or NOR gate as known in the art. 
           [0016]      FIG. 3A  is a schematic illustration of one embodiment of a pulse generator circuit for writing to a ferroelectric memory element, tailored for fabrication by organic thin-film processes, according to the present disclosure. 
           [0017]      FIG. 3B  is a schematic illustration of an equivalent circuit to that shown in  FIG. 3A  of an embodiment of a pulse generator circuit for writing to a ferroelectric memory element, tailored for fabrication by organic thin-film processes, according to the present disclosure. 
           [0018]      FIG. 4  is a schematic illustration of an alternate embodiment of a pulse generator circuit for writing to a ferroelectric memory element, tailored for fabrication by organic thin-film processes, according to the present disclosure. 
           [0019]      FIG. 5  is a plot of a simulation illustrating various voltages on lines as a function of time for the embodiment shown in  FIG. 4 . 
           [0020]      FIG. 6  is a plot of In and Out voltages for an actual example of the circuit of  FIG. 4  plotted against time. 
           [0021]      FIG. 7  is a schematic illustration of another alternate embodiment of a pulse generator circuit for writing to a ferroelectric memory element, tailored for fabrication by organic thin-film processes, according to the present disclosure. 
           [0022]      FIG. 8  is a schematic illustration of yet another alternate embodiment of a pulse generator circuit for writing to a ferroelectric memory element, tailored for fabrication by organic thin-film processes, according to the present disclosure. 
           [0023]      FIG. 9  is a schematic illustration of still another alternate embodiment of a pulse generator circuit for writing to a ferroelectric memory element, tailored for fabrication by organic thin-film processes, according to the present disclosure. 
           [0024]      FIG. 10  is a schematic illustration of a still further alternate embodiment of a pulse generator circuit for writing to a ferroelectric memory element, tailored for fabrication by organic thin-film processes, according to the present disclosure. 
           [0025]      FIG. 11  is a schematic illustration of an additional alternate embodiment of a pulse generator circuit for writing to a ferroelectric memory element, tailored for fabrication by organic thin-film processes, according to the present disclosure. 
           [0026]      FIG. 12  is a typical polarization hysteresis curve when a positive and negative electric field is applied across a ferroelectric capacitor. 
           [0027]      FIG. 13  is a block diagram illustrating a circuit, with an optional enable signal, for writing to a ferroelectric memory element according to an embodiment of the present disclosure. 
           [0028]      FIG. 14  illustrates an example schematic of an actual circuit of the type illustrated in  FIG. 13 . 
           [0029]      FIG. 15  is a schematic illustration of an alternate pulse generator circuit for writing to a ferroelectric memory element according to an embodiment of the present disclosure. 
           [0030]      FIG. 16  shows a plot of output pulses measured on eight samples of the circuit illustrated in  FIG. 15 . 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    We initially point out that description of well-known starting materials, processing techniques, components, equipment and other well-known details may merely be summarized or are omitted so as not to unnecessarily obscure the details of the present disclosure. Thus, where details are otherwise well-known, we leave it to the application of the present disclosure to suggest or dictate choices relating to those details. 
         [0032]    With reference to  FIG. 3A , there is shown therein a first embodiment of a pulse generator circuit  10  tailored for fabrication by organic thin-film (OTF) processes. Circuit  10  comprises an input In, and output Out, a capacitor  12 , a first thin-film transistor (TFT)  14 , and a second TFT  16 , each of the same polarity. Circuit  10  of  FIG. 3A  is intended to model a circuit  18  shown in  FIG. 3B , in that TFT  14  models resistor  20  and TFT  16  models diode  22 , with operation as follows. A rising edge on In of circuit  10  is passed though a high-pass filter made up of capacitor  12  and TFT  14  (e.g., resistor  20 ) to Out. Diode bias V b  is set to a voltage such that TFT  16  (e.g., diode  22 ,  FIG. 3B ) prevents Out from being negative. Voltage V b  is approximately one diode-drop above V I , e.g., above ground. Depending on the implementation, negative voltages may be allowed on signal Out, in which case V b  can simply be set to ground. Other values of V b  may also be acceptable, depending on the application. In some implementations, one or both of the TFTs  14 ,  16  can be replaced by their equivalent components,  20  and  22 , respectively, where diodes and resistors are available. Also capacitor  12  can be implemented in the OTF process (for example, with gate dielectric between gate and source/drain metal later with no semiconductor) or separately. 
         [0033]      FIG. 4  shows a first alternate embodiment of a pulse generator circuit  24  tailored for fabrication by organic thin-film (OTF) processes. Circuit  24  comprises an input In, and output Out, a capacitor  26 , a first thin-film transistor (TFT)  28  (acting as a resistor), and a second TFT  30 , each of the same polarity, similar to the embodiment illustrated and described above with regard to  FIG. 3A . Circuit  24  further comprises a pass TFT  32  connecting the output of the circuit to a load. A rising edge on In generates a pulse at P. The generation of a complimentary negative pulse with a falling edge is prevented by TFT  30 , which is sized much wider than TFT  28  to function as a diode. The dc value of P is given by V b  minus the threshold voltage of TFT  30  (V 30 ), and may be set to ground (but can be other values). In particular, if an additional voltage rail is not available or desired, V b  can be tied to ground and P clamped at −V 30 . A low signal on EnBar will pass the pulse P to Out through TFT  32 . (The on-characteristic of TFT  32  will further filter the pulse signal.) 
         [0034]    In principal, the sizes of TFTs such as  14  and  16 ,  28  and  30 , etc. are determined based on the source impedance seen at In. The transfer function from the source to P is given by 
         [0000]    
       
         
           
             f 
             = 
             
               
                 sCR 
                 x 
               
               
                 
                   sC 
                    
                   
                     ( 
                     
                       
                         R 
                         x 
                       
                       + 
                       
                         R 
                         s 
                       
                     
                     ) 
                   
                 
                 + 
                 1 
               
             
           
         
       
     
         [0000]    where R x  is the channel resistance of TFT  28  for a positive pulse and of TFT  30  for a negative pulse, R s  is the source impedance, C is the capacitance of capacitor  26 , and s is the Laplace parameter. The source impedance Rs is the output impedance of the circuit driving the pulse generator and is not part of the pulse generator circuit itself. At high frequency (s→∞), the transfer function is approximately 1 when R x &gt;&gt;R s  and approximately 0 when R x &lt;&lt;R s . TFT  30  acts as a diode in the sense that it is off during a positive pulse and on during a negative pulse. Thus, if R on  of TFT  28  is much greater than R s , and R on  of TFT  30  is much less than R s , positive pulses will be generated while negative pulses are shunted. In practice, the on-resistance of TFT  32  is usually non-negligible and will be accounted for in the transfer function analysis. 
         [0035]    The circuit described above may be fabricated on a flexible polyethylene naphthalate substrate by an additive OFT process that includes ink-jet deposition of the metal and semiconductor layers. The capacitor may be a parallel plate structure formed by the gate dielectric and the source-drain and gate metals. Other substrate materials and structures may be employed depending on the design criteria and application of embodiments of the present disclosure. 
         [0036]    Alternatively, if both n- and p-channel devices are available, a transmission gate can be used. By placing multiple pass transistors in parallel, the pulse can be selectively connected to one or more of several outputs. 
         [0037]    The sizes of TFTs  14 ,  16  of embodiment  10  ( FIG. 3A ) and TFTs  28 ,  30 ,  32  of embodiment  24  ( FIG. 4 ) are highly dependent on the application and implementation of the respective circuits, in particular on the characteristics of the TFTs themselves (threshold voltage, mobility, etc.), the process design rules (minimum channel length, available channel widths), and the load resistance and capacitance. Additionally, they are dependent on the desired pulse duration and shape. 
         [0038]      FIG. 5  shows In, EnBar, P, and Out values plotted against time in a simulation of circuit  24  ( FIG. 4 ) for a printed OTF process. In this simulation, a 20-volt input step, V b =0 volts, and a 10 pF capacitive load were used. A capacitance of capacitor  26  was 200 pF. Each of TFTs  28 ,  30 , and  32  had a channel length of 35 μm. The channel widths of TFTs  28 ,  30  and  32  were 200 μm, 6 mm, and 400 μm, respectively. In general, the length and width are process dependent. According to one embodiment, length may range from 2 μm to 200 μm, and widths may range from 100 μm to 1 cm. It will be noted that a rising pulse on In (at approximately 58 ms) triggers a pulse on Out when EnBar is low. 
         [0039]      FIG. 6  shows measured output of circuit  24  ( FIG. 4 ). Each of TFTs  28 ,  30 , and  32  had an approximate channel length of 35 μm. The channel widths of TFTs  28 ,  30  and  32  were approximately 200 μm, 6 mm, and 400 μm, respectively. The voltage V b  was 3.5 volts. Capacitor  26  had a value of 200 pF. Again, it will be noted that a rising pulse on In (at 40 ms) triggers an output pulse. 
         [0040]    Illustrated in  FIGS. 7 through 11  are various alternate embodiments of a pulse generator circuit tailored for fabrication by OTF processes. More specifically,  FIG. 7  includes TFTs  42 ,  44 , and  46  similar to those described with reference to  FIG. 4 . In addition, the circuit of  FIG. 7  includes an additional enable TFT  48 , enabled when a voltage En is high. In typical operation, signal En is the complement of EnBar at all times. When En is low (disabled), TFT  48  serves to aid in clamping Out near ground.  FIG. 8  is an alternate embodiment for filtering the pulse on Out in which TFT  42  from  FIG. 7  has been eliminated, leaving TFTs  52 ,  54 , and  56 . In function, TFT  56  serves in place of TFT  28  of  FIG. 4 . Here, Ena at the gate of TFT  56  is an analog voltage that sets the effective resistance of TFT  56 .  FIG. 9  is a modification of the circuit of  FIG. 4  including TFTs  62 ,  64 ,  66 , in which TFT  62  corresponding to the resistor ( 20  of  FIG. 3B ) has been relocated to the output side of TFT  66 . 
         [0041]      FIG. 10  is an illustration of a N-channel TFT implementation of the circuit illustrated in  FIG. 3A , including N-channel TFTs  72 ,  74 .  FIG. 11  is an illustration of a N-channel TFT implementation of the circuit illustrated in  FIG. 4 , including N-channel TFTs  82 ,  84 ,  86 . 
         [0042]    In the embodiments above, the output node Out may be communicatively connected to a memory cell to write or erase the cell. In one embodiment, the memory may be a ferroelectric memory, such as a ferroelectric capacitor. A ferroelectric capacitor comprises a ferroelectric material disposed between two electrodes. The electrodes act as the plates of the capacitor, and the ferroelectric material acts as the dielectric. We will assume for discussion that the capacitor is of the parallel-plate design, but many other configurations are possible and contemplated hereby. 
         [0043]    Ferroelectric materials are characterized by having remnant polarization after an electric field has been applied and removed. With reference to  FIG. 12 , a typical hysteresis curve  90  for the polarization when a positive and negative electric field is applied across the ferroelectric capacitor is shown. The capacitor is bi-stable, with two different polarization states possible when no electric field is applied. These polarization states can represent “0” and “1”, respectively for binary memory. 
         [0044]    The ferroelectric memory element (capacitor) is written by applying a voltage across its terminals. The voltage must be sufficiently high and applied for a sufficiently long duration that switching of the polarization state of the ferroelectric material is obtained. This may be accomplished using the pulse generator circuits disclosed above. For example,  FIG. 13  illustrates a block diagram of such a circuit  100 , with an optional enable signal, and  FIG. 14  illustrates an example schematic of an actual circuit  110  for this purpose. 
         [0045]    In  FIG. 13 , a pulse (Trig) may be received at the input node In, and circuitry described following produces a pulse on the output node Out, in order to change the state of ferroelectric memory element  102 , optionally when an enable signal is “high”. 
         [0046]    In  FIG. 14 , a pulse (Trig) is applied from the input node In. As previously described with reference to  FIG. 4  (as one example from the circuits disclosed above), a rising edge on In is passed though a high-pass filter made up of capacitor  112  and TFT  114  (e.g., acting as a resistor) to Out under control of pass TFT  118  and enable signal EnBar. Diode bias V b  is set to a voltage such that TFT  116  (e.g., acting as a diode) prevents Out from being negative. The pulse on Out is then applied to ferroelectric capacitor  120 , effectively changing its state. 
         [0047]    Ferroelectric memory  120  may, for example, be cross-bar memory cells, with printed metal electrodes and printed or spin-cast P(VDF-TrFE) based ferroelectric dielectric. Each memory cell may, for example, be on the order of 50 μm by 50 μm, and connected to the pulse generator circuit at Out by printed conductive silver ink. 
         [0048]    With reference to  FIG. 15 , another circuit  120  with ferroelectric memory cell is illustrated. Circuit  120  comprises TFTs  122 ,  124  configured to function as an inverter, and TFTs  126 ,  128 ,  130 ,  132  configured to function as a NOR gate. An additional TFT  134  acts as a pass transistor for enabling writing of the memory cell  136 . A positive pulse is generated at node X upon application of a negative edge at input node A. If the gating signal G is enabled (low), the pulse is propagated to the output Y. The duration of the pulse is determined by the propagation delay through the inverter and can be tuned by the sizing the inverter and its load (TFTs  126 ,  130 ). According to one embodiment, circuit  120  may be designed to achieve a minimum pulse width of 3 ms and pulse height of 16 V with a 20-V supply voltage. 
         [0049]      FIG. 16  shows a plot  140  of output pulses measured on eight samples (a-h) of the circuit illustrated in  FIG. 15  at node X with a supply voltage of 22 V. The pulse width ranges from 3-8 ms and the height from 16-20 V. The output pulse is triggered on the falling edge at node A, and the slight negative pulse on the rising edge is the result of charge injection. 
         [0050]    The physics of modern electrical devices and the methods of their production are not absolutes, but rather statistical efforts to produce a desired device and/or result. Even with the utmost of attention being paid to repeatability of processes, the cleanliness of manufacturing facilities, the purity of starting and processing materials, and so forth, variations and imperfections result. Accordingly, no limitation in the description of the present disclosure or its claims can or should be read as absolute. The limitations of the claims are intended to define the boundaries of the present disclosure, up to and including those limitations. To further highlight this, the term “substantially” may occasionally be used herein in association with a claim limitation (although consideration for variations and imperfections is not restricted to only those limitations used with that term). While as difficult to precisely define as the limitations of the present disclosure themselves, we intend that this term be interpreted as “to a large extent”, “as nearly as practicable”, “within technical limitations”, and the like. 
         [0051]    While examples and variations have been presented in the foregoing description, it should be understood that a vast number of variations exist, and these examples are merely representative, and are not intended to limit the scope, applicability or configuration of the disclosure in any way. Various of the above-disclosed and other features and functions, or alternative thereof, may be desirably combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications variations, or improvements therein or thereon may be subsequently made by those skilled in the art which are also intended to be encompassed by the claims, below. 
         [0052]    Therefore, the foregoing description provides those of ordinary skill in the art with a convenient guide for implementation of the disclosure, and contemplates that various changes in the functions and arrangements of the described examples may be made without departing from the spirit and scope of the disclosure defined by the claims thereto.