Abstract:
The use of an alternating current (ac) source to power logic circuitry can support satisfactory device performance for a variety of applications, while enhancing long-term stability of the circuitry. For example, when organic thin film transistor (OTFT)-based logic circuitry is powered by an ac power source, the logic circuitry exhibits stable performance characteristics over an extended period of operation. Enhanced stability may permit the use of OTFT logic circuitry to form a variety of circuit devices, including inverters, oscillators, logic gates, registers and the like. Such circuit devices may find application in a variety of applications, including integrated circuits, printed circuit boards, flat panel displays, smart cards, cell phones, and RFID tags. In some applications, the ac-powered logic circuitry may eliminate the need for ac-dc rectification components, thereby reducing the manufacturing time, expense, cost, complexity, and size of the component carrying the circuitry.

Description:
FIELD 
   The invention relates to logic circuitry. 
   BACKGROUND 
   Thin film circuit devices, including transistors, diodes, and the like, are widely used in a variety of modern electronic devices, including integrated circuits, printed circuit boards, flat panel displays, smart cards, cell phones, and radio frequency identification (RFID) tags. Thin film circuit devices are typically formed by depositing, masking and etching a variety of conducting, semiconducting and insulating layers to form a thin film stack. 
   Typically, thin film transistors are based on inorganic semiconductor materials such as amorphous silicon or cadmium selenide. More recently, significant research and development efforts have been directed to the use of organic semiconductor materials to form thin film transistor circuitry. 
   Organic semiconductor materials offer a number of manufacturing advantages for transistor fabrication. In particular, organic semiconductor materials permit the fabrication of organic thin film transistors (OTFTs) on flexible substrates such as thin glass, polymeric or paper-based substrates. In addition, organic semiconductor materials can be formed using low-cost fabrication techniques such as printing, embossing or shadow masking. Although the performance characteristics of OTFTs have improved with continued research and development, device performance and stability continue to present challenges. 
   SUMMARY 
   In general, the invention is directed to logic circuitry powered by alternating current (ac) power sources. The invention may be applied to logic circuitry incorporating thin film transistors based on amorphous or polycrystalline organic semiconductors, inorganic semiconductors or combinations of both. 
   The use of an ac power source to power thin film transistor-based logic circuitry can support satisfactory device performance for a variety of applications, while enhancing long-term stability of the circuitry. For example, when OTFT circuitry is powered by an ac power source, the OTFT circuitry may exhibit stable performance characteristics over an extended period of operation. 
   Enhanced stability may permit the use of OTFT circuitry to form a variety of thin film transistor-based logic circuit devices, including inverters, oscillators, logic gates, registers and the like. Such logic circuit devices may find utility in a variety of applications, including integrated circuits, printed circuit boards, flat panel displays, smart cards, cell phones, and RFID tags. 
   For some applications, ac-powered thin film transistor circuitry may eliminate the need for an ac to dc rectification block, thereby reducing the manufacturing time, expense, cost, complexity, and size of the component carrying the thin film transistor circuitry. The ac power source directly powers the logic gate circuitry. In particular, the ac power source applies an ac power waveform to one or more individual logic gates, instead of applying dc power to the logic gates via an ac-dc rectification block. 
   In one embodiment, the invention provides an electronic circuit comprising a first transistor and a second transistor arranged to form a logic gate, and an alternating current (ac) source to directly power the logic gate with an ac power waveform. 
   In another embodiment, the invention provides a method comprising directly powering a logic gate formed by at least a first transistor and a second transistor with an alternating current (ac) power waveform produced by an alternating current (ac) power source. 
   In an added embodiment, the invention provides a radio frequency identification (RFID) tag comprising a logic gate formed by at least a first transistor and a second transistor, and a radio frequency converter that converts RF energy to alternating current (ac) power, and directly powers the logic gate with the ac power. 
   In a further embodiment, the invention provides a radio frequency identification (RFID) system comprising an RFID tag including first and second transistors arranged to form a logic gate, a radio frequency (RF) converter that converts RF energy to alternating current (ac) power and directly powers the logic gate with the ac power, and a modulator that conveys information, and an RFID reader that transmits the RF energy to the RFID tag for conversion by the RF converter, and reads the information conveyed by the modulator. 
   In another embodiment, the invention provides a ring oscillator circuit comprising a plurality of transistors arranged to form a series of inverter stages, the inverter stages being coupled to form a ring oscillator, and an alternating current (ac) source to directly power the inverter stages in the ring oscillator with an ac power waveform. 
   The invention can provide a number of advantages. For example, ac-powered logic circuitry, and particularly OTFT-based logic circuitry, may exhibit increased stability over an extended period of time, relative to dc-powered thin film transistor circuitry. In the case of a ring oscillator, for example, ac-powered thin film transistor circuitry may maintain oscillation amplitudes over a longer period of time relative to dc-powered thin film transistor circuitry. 
   The availability of stable OTFT circuitry, in particular, may promote wider use of OTFT circuitry in a variety of applications, with more reliable performance, durability and longevity. Consequently, various applications for OTFT circuitry may benefit from manufacturing advantages associated with OTFT circuitry, such as the ability to form circuitry on flexible substrates, such as thin glass, polymeric or paper-based substrates, and use lower-cost manufacturing techniques. 
   As a further advantage, the use of ac power for the thin film transistor circuitry may eliminate the need for the ac-dc rectifier component otherwise required in some applications for delivery of de power to the circuitry. Accordingly, by eliminating the need for a rectifier component, the use of ac power may reduce the manufacturing time, expense, cost, complexity, and size of components carrying thin film transistor circuitry. 
   For RFID tags, as a particular example, the use of ac-powered thin film circuitry may substantially reduce the cost and size of the tag by eliminating the ac-dc rectifier component. In particular, by eliminating the need for a front-end rectifier block, ac-powered thin film logic circuitry can result in substantial cost and size savings in the design and manufacture of the RFID tag. 
   Additional details of these and other embodiments are set forth in the accompanying drawings and the description below. Other features, objects and advantages will become apparent from the description and drawings, and from the claims. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a circuit diagram illustrating an ac-powered inverter circuit. 
       FIG. 2  is a graph illustrating simulated performance of the inverter circuit of  FIG. 1 . 
       FIG. 3  is a circuit diagram illustrating an ac-powered inverter circuit based on complementary metal oxide semiconductor (CMOS) transistors. 
       FIG. 4  is a circuit diagram illustrating an ac-powered NAND gate circuit. 
       FIG. 5  is a circuit diagram illustrating an ac-powered thin film transistor-based NOR gate circuit. 
       FIG. 6  is a circuit diagram illustrating an ac-powered thin film transistor-based ring oscillator circuit with load capacitors. 
       FIG. 7  is a graph illustrating simulated performance of the ring oscillator circuit of  FIG. 6 . 
       FIG. 8  is a circuit diagram illustrating an ac-powered thin film transistor-based ring oscillator circuit without load capacitors. 
       FIG. 9  is a graph illustrating simulated performance of the ring oscillator circuit of  FIG. 8 . 
       FIG. 10  is a block diagram illustrating application of ac-powered thin film transistor circuitry in an RFID tag/reader system. 
       FIG. 11  is a circuit diagram further illustrating the RFID tag/reader system of  FIG. 10 . 
       FIG. 12  is a circuit diagram further illustrating a reader associated with the RFID tag/reader system of  FIG. 10 . 
       FIG. 13  is a graph illustrating simulated output of an RFID tag constructed using ac-powered thin film transistor circuitry. 
       FIG. 14  is a circuit diagram illustrating an ac-powered inverter circuit that drives a liquid crystal display element. 
       FIG. 15  is a circuit diagram illustrating an ac-powered inverter circuit that drives a light emitting diode (LED). 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a circuit diagram illustrating an ac-powered inverter circuit  10 . Inverter circuit  10  may include an ac power source  12  that supplies ac power to a logic gate in the form of an inverter  14  having a load transistor  16  and a drive transistor  18 . Each transistor  16 ,  18  may be a thin film field effect transistor, and may be based on an amorphous or polycrystalline inorganic or organic semiconducting material. As an example, organic semiconducting materials such as pentacene may be used to form OTFTs. As an alternative, circuit  10  may be formed by a combination of organic and inorganic semiconducting material, e.g., to form a complementary metal oxide semiconductor (CMOS) inverter circuit. For example, in some applications, inverter circuit  10  may be formed by an NMOS inorganic field effect transistor and a PMOS organic field effect transistor. When OTFTs are used, transistors  16 ,  18  may be especially adaptable to fabrication using low cost fabrication techniques, and may be formed on flexible substrates for some applications. 
   The ac power source  12  directly powers inverter  14  with an ac power waveform. The ac power is applied directly to inverter  14  in the sense that the inverter receives an ac power waveform instead of dc power produced by an ac-dc rectification component. In other words, inverter  14  operates in response to an ac power waveform. Accordingly, intervening circuitry may exist between ac power source  12  and inverter  14  provided that the inverter still receives an ac power waveform as operating power, rather than a dc power signal. In the example of  FIG. 1 , the ac power waveform is applied directly across the common gate and drain connection of load transistor  16  and the ground connection coupled to the source of drive transistor  18 . 
   The use of ac power source  12  to power thin film transistor-based logic circuitry, such as inverter  14  in  FIG. 1 , can support satisfactory device performance for a variety of applications, while enhancing long-term stability of the circuitry. For example, when inverter  14  is powered by ac power source  12 , the inverter may exhibit stable performance characteristics over an extended period of operation relative to dc-powered inverters, especially for organic semiconducting materials. Also, for some applications, ac operation of inverter  14  will eliminate the need for an ac-dc rectifier component to power the inverter. The ac power waveform supplied to inverter circuit  10  may have a variety of regular shapes, e.g., sinusoidal, square, or sawtooth-shaped. In addition, in some embodiments, the ac power waveform may have irregular shapes. Accordingly, the ac power waveform exhibits an alternating current flow but is not limited to any particular shape. Nevertheless, in many applications, the ac power waveform may be a sinusoidal waveform. 
   As shown in  FIG. 1 , the gate and drain of load transistor  16  are coupled to ac power supply  12 . The drain of drive transistor  18  is coupled to the source of load transistor  16 , and the source of the drive transistor is coupled to ground. A signal source  20  drives the gate of drive transistor  18 , e.g., with a logic signal. In response, inverter  14  produces an inverted output  22 , which may be output across a load capacitor  24 . Load capacitor  24  serves to filter out some of the ac voltage present at the output and provides for a cleaner output signal. The amount of filtering depends on the capacitance of load capacitor  24  and the frequency of the ac power. Load capacitor  24  may be formed by an input capacitance produced by gate/source overlap within a logic gate coupled to output  22  in the event inverter circuit  10  is coupled to drive one or more additional logic gates. 
   The gate/source overlap may be controlled during manufacture of a drive transistor  18  in a subsequent logic gate to produce a desired level of capacitance in load capacitor  24 . Alternatively, load capacitor  24  may be formed independently, particularly if output  22  does not drive another logic gate. In some embodiments, load transistor  16  may have a gate width to gate length ratio that is greater than or equal to a gate width to gate length ratio of the drive transistor  18 . In this case, direct current (dc) powering of the circuit would result in inferior operation of the logic gate, for NMOS or PMOS designs, because of the reduced gain. NMOS or PMOS ring oscillators based on this design, for example, would not operate if powered by direct current. An added benefit of having the gate width to gate length ratio of load transistor  16  greater or equal to the drive transistor  18  gate width to gate length ratio is that the total circuit area can be reduced. 
     FIG. 2  is a graph illustrating simulated performance of the inverter circuit  10  of  FIG. 1 . In particular, the graph illustrates signal voltage transitions over a period of time. For purposes of this simulation, transistors  16 ,  18  are modeled as PMOS organic field effect transistors. In  FIG. 2 , trace  25  is the input signal waveform applied to the gate of drive transistor  18  by signal source  20 . Trace  26  is the output signal waveform produced by inverter  14  at output  22 . In the example of  FIG. 2 , the input signal waveform transitions between a logic ‘0’ state  28  and a logic ‘1’ state  27 . In response, inverter  14  produces an inverted output in response to the input signal waveform, i.e., a logic ‘1’ state  32  and a logic ‘0’ state  30 , as shown in  FIG. 2 . Inverter  14  exhibits a propagation delay that is inversely related to the ac voltage applied to load transistor  16  and the mobility of the semiconductor material forming the inverter, and proportional to the parasitic capacitance within transistors  16 ,  18  and any external capacitance that may be independently added to inverter circuit  10 . The ac power source  12  may have a frequency characterized by a period that is less than the propagation delay time of inverter  14 . 
   In the example of  FIG. 2 , ac power source  12  produces a sinusoidal waveform having a frequency of 125 kHz and a peak-to-peak amplitude of 80 volts. Also, signal source  20  produces a square wave input signal waveform between approximately 0 and −15 volts, at approximately 100 Hz. Inverter  14  turns “on” in response to the ac power supply waveform applied by ac power source  12 , and serves to invert the input signal waveform applied by signal source  20 . The output  22  of inverter  14  may be applied to additional logic circuitry. In addition, a plurality of inverters  14  may be combined to form a variety of logic components, such as oscillators, logic gates, registers and the like. Although inverter circuit  10  is depicted in  FIG. 1  for use as a logic gate, the inverter circuit may be used as an analog amplifier in some cases. In addition, inverter circuit  10  can be used to drive a variety of loads, including display elements such as liquid crystal display (LCD) elements, or light emitting diodes (LEDs), including organic light emitting diodes (OLEDs). 
     FIG. 3  is a circuit diagram illustrating an ac-powered thin film transistor-based inverter circuit  14 ′ incorporating CMOS-based circuitry. As shown in  FIG. 3 , the source of p-channel transistor  16  is coupled to ac power supply  12 . An n-channel transistor  19  has a drain coupled to the drain of transistor  16 . In addition, the gates of transistors  16 ,  19  are coupled together and driven by a signal source  20 . Signal source  20  drives the gates of transistors  16 ,  19 , e.g., with a logic signal. The source of transistor  19  is coupled to ground. The source of transistor  16  and the drain of transistor  19  are coupled together to form the output  22  of inverter circuit  14 ′. In response to the logic signal, inverter  14 ′ produces an inverted output  22 . In some embodiments, a load capacitor may be coupled between output  22  and ground. Again, the load capacitor may be formed by the input capacitance of a subsequent logic gate coupled to the output of inverter circuit  14 ′. Alternatively, a load capacitor may be formed independently to provide the desired load capacitance for output  22 . 
     FIG. 4  is a circuit diagram illustrating an ac-powered thin film transistor-based NAND gate circuit  21 . As shown in  FIG. 4 , the gate and drain of load transistor  16  are coupled to ac power supply  12 . The drain of first drive transistor  18 A is coupled to the source of load transistor  16 . The drain of second drive transistor  18 B is coupled to the source of first drive transistor  18 A. The source of second drive transistor  18 B is coupled to ground. First and second signal sources  20 A,  20 B drive the gates of drive transistors  18 A,  18 B, respectively. In response, NAND gate  23  produces a logical NAND output  22 . Transistors  16 ,  18 A,  18 B form a NAND gate. NAND circuit  21  is operative in response to the ac power supply signal delivered directly to the NAND circuit by ac power supply  12 . In some embodiments, a load capacitor may be coupled across output  22 . The load capacitor may be formed independently or realized by the input capacitance of a logic gate driven by output  22  of NAND circuit  21 . 
     FIG. 5  is a circuit diagram illustrating an ac-powered thin film transistor-based NOR gate circuit  25 .  FIG. 5  represents another example of a thin film transistor-based logic circuit that operates with an ac power supply, in accordance with the invention. As shown in  FIG. 5 , the gate and drain of load transistor  16  are coupled to ac power supply  12 . Transistors  16 ,  29 A,  29 B form a NOR gate  27 . The drains of first and second drive transistors  29 A,  29 B are coupled to the source of load transistor  16 , and to output  22 . The sources of first and second drive transistors  29 A,  29 B are coupled to ground. First and second signal sources  31 A,  31 B drive the gates of drive transistors  29 A,  29 B, respectively. In response, NOR gate  27  produces a logical NOR output  22 . NOR circuit  25  is operative in response to the ac power supply signal delivered by ac power supply  12 . In some embodiments, a load capacitor may be coupled across logical NOR output  22 . The load capacitor may be formed independently or realized by the input capacitance of a logic gate driven by output  22  of NOR circuit  25 . 
     FIG. 6  is a circuit diagram illustrating an ac-powered thin film transistor-based ring oscillator circuit  33 . Ring oscillator circuit  33  is an example of another circuit that can be implemented using ac-powered logic gates, e.g., including inverter stages based on OTFTs. As shown in  FIG. 6 , ring oscillator circuit  33  includes an odd number of inverter stages arranged in series. In the example of  FIG. 6 , ring oscillator circuit  33  includes seven inverter stages  36 A– 36 G having, respectively, load transistors  34 A– 34 G and drive transistors  35 A– 35 G, respectively. Each transistor  34 ,  35  in ring oscillator circuit  33  is an ac-powered thin film field effect transistor. For example, ac power source  12  delivers ac power to the gates and drains of load transistors  34 . The source electrodes of drive transistors  35  are coupled to ground. 
   In the example of  FIG. 6 , each inverter stage  36  has an output coupled across a respective load capacitor  38 A– 38 G. For example, the output of inverter stage  36 A is coupled across load capacitor  38 B, and the output of inverter stage  36 G is coupled across load capacitor  38 A. Each capacitor  38  may be formed by the input capacitance produced by gate/source overlap within a drive transistor  35  of a subsequent inverter stage  36  that is driven by the output of a respective inverter stage. The output  40  of final inverter stage  36 G is coupled to the gate of drive transistor  35 A in first inverter stage  36 A to provide feedback. Like inverter circuit  10  of  FIG. 1 , ring oscillator circuit  33  of  FIG. 6  operates in response to the ac power supply waveform delivered by ac power supply  12 . During operation, ring oscillator circuit  33  provides a clock signal. For example, the output of each inverter stage  36  in ring oscillator circuit  33  can be tapped to provide a clock signal with a desired phase. 
     FIG. 7  is a graph illustrating simulated performance of the ring oscillator circuit  33  of  FIG. 6 . As shown in  FIG. 7 , ring oscillator  33  produces, as output  41 , an oscillating output waveform  41  characterized by high peaks  42  and low peaks  43 . In the example of  FIG. 7 , ac power source  12  produces a sinusoidal waveform having a frequency of 125 kHz and a peak-to-peak amplitude of 40 volts. Oscillating output waveform  41  in  FIG. 7  exhibits a frequency of approximately 300 Hz. In general, the output waveform produced by a ring oscillator circuit will have a frequency that is dependent on the number of inverter stages  36  and the propagation delays produced by the individual inverter stages. The propagation delay is inversely related to the ac power supply voltage applied to ring oscillator circuit  33  and the mobility of the semiconducting material, and proportional to any applicable parasitic or external capacitance present in inverter stages  36 . 
     FIG. 8  is a circuit diagram illustrating an ac-powered thin film transistor-based ring oscillator circuit  33 ′ without capacitors  38 .  FIG. 9  is a graph illustrating simulated performance of the ring oscillator circuit  33 ′ of  FIG. 8 . Ring oscillator circuit  33 ′ of  FIG. 8  conforms substantially to ring oscillator circuit  33  of  FIG. 6 , but does not include capacitors  38  at the outputs of respective inverter stages  36 . In the absence of capacitors  38 , the oscillating output waveform  41 ′, in  FIG. 9  including peaks  44  and  46 , reveals more of the 125 KHz ac power supply waveform. 
   Operation of thin film transistor circuitry, such as ring oscillator circuit  33 , also is possible with higher ac power supply frequencies. Functioning ring oscillator circuits that conform substantially to circuit  33  have been observed to operate, for example, with ac power supply frequencies on the order of 6 MHz. With increased semiconductor mobility, it may be reasonable to expect use of ring oscillator circuits as described herein with ac power supply frequencies of greater than 10 MHz. 
     FIG. 10  is a block diagram illustrating application of ac-powered thin film transistor-based circuitry in an RFID tag/reader system  55 . Use of ac-powered thin film transistor-based circuitry may be particularly desirable in an RFID tag for a number of reasons, as will be described. As shown in  FIG. 10 , system  55  may include a reader unit  56  and an RFID tag  58 . 
   Reader unit  56  may include a radio frequency (RF) source  62  and a reader  64 . RF source  62  transmits RF energy to RFID tag  58  to provide a source of power. In this manner, RFID tag  58  need not carry an independent power supply, such as a battery. Instead, RFID tag  58  is powered across a wireless air interface between reader unit  56  and the RFID tag. To that end, reader unit  56  includes an inductor  59  that serves, in effect, as an antenna to transmit and receive RF energy. 
   As further shown in  FIG. 10 , RFID tag  58  may include an ac power source  66 . As will be explained, ac power source  66  may serve to convert RF energy transmitted by reader unit  56  into ac power for delivery to thin film transistor circuitry carried by RFID tag  58 . RFID tag  58  may receive the RF energy from reader unit  56  via an inductor  67  that serves as a receiver. A capacitor  77  also may be provided in parallel with inductor  67 . RFID tag  58  further includes a clock circuit  68 , data circuit  70 , control logic circuit  72 , output buffer circuit  74  and modulation inverter  76 , one or more of which may be formed by an arrangement of thin film transistor circuitry. 
   Clock  68  drives control logic circuit  72  to output data from data circuit  70 , which may comprise a plurality of data lines carrying an identification code. Output buffer circuit  74  buffers the output from control logic circuit  72 . Modulation inverter  76 , in turn, modulates the buffered output for interpretation by reader unit  56  via inductor  67 . For example, modulation inverter  76  conveys the information by modulating the signal applied across inductor  67 . 
     FIG. 11  is a circuit diagram further illustrating the RFID tag/reader system  55  of  FIG. 10 . As shown in  FIG. 11 , RF source  62  may include an ac generator  71  that transmits an ac output signal via inductor  59 . For some applications, ac generator  71  may take the form of a sinusoidal current source with an output of approximately 0 to 5 amps at a frequency of approximately 125 kHz. 
   Inductors  59  and  67  form a transformer for electromagnetic coupling of RF energy between RF source and RFID tag  58 . Resistor  73  is selected to limit current. A capacitor  77  is placed in parallel with inductor  67  within ac power source  66  to form a parallel resonant tank that governs the frequency of the ac power source according to the equation: 
             f   =     1     2   ⁢   π   ⁢     LC           ,         
where L is the inductance of inductor  67  and C is the capacitance of capacitor  77 . With an inductance of 50 μH and a capacitance of 32 nF, inductor  67  and capacitor  77  generate a resonant frequency of approximately 125 KHz. Hence, in this example, the output of ac power source  66  is a sinusoidal waveform with a frequency of approximately 125 kHz. This waveform is then applied to clock circuit  68 , control logic  72 , data lines  70  and output buffer  74  as represented in  FIG. 11  by the terminals AC POWER and COMMON.
 
     FIG. 11  depicts an RFID tag  58  that carries an n-bit identification code. For ease of illustration, RFID tag  58  carries a 7-bit identification code specified by data lines  70 . In many applications, RFID tag  58  may carry a much larger identification code, e.g., 31-bit, 63-bit or 127-bit codes. In some embodiments, selected data lines  70  may carry information used for start bit identification, data stream synchronization and error checking. In the example of  FIG. 11 , clock circuit  68  is a ring oscillator formed by a series of seven inverter stages arranged in a feedback loop. 
   The ring oscillator of  FIG. 11  may be similar to ring oscillator  33  or  33 ′ of  FIGS. 6 and 8 . The outputs of two successive inverters are applied to a respective NOR gate provided in control logic  72 . In this way, seven NOR gates are used to generate a sequence of seven pulses within each clock cycle produced by the ring oscillator. Note that the number of NOR gates in control logic  72  may vary. Again, this arrangement could be extended, in principle, to larger numbers of bits, e.g., n=31, 63 or 127. 
   Switches shown in series with data lines  70  are connected to respective NOR gate outputs at one end. If a switch is closed, the respective data line couples the NOR gate output to ground If the switch is open, the NOR gate output is coupled as one of the inputs to a 7-input OR gate within control logic  72 . 
   In the example of  FIG. 11 , the switches for second and fourth data lines (from left to right) are closed. As a result, data lines  70  store the 7-bit identification code “1010111.” The switches can be made, for example, from metal lines that extend from the NOR gate outputs to ground. The electrical connections to ground can be intentionally broken or connected during manufacturing to produce, in effect, an open switch, and thereby encode a unique identification code into data lines  70  of RFID tag  58 . The electrical connections may be broken by a variety of manufacturing techniques such as, for example, laser etching, mechanical scribing, electrical fusing, or shadow masking. 
   The output of the 7-input OR gate in control logic  72  is applied to a cascade of buffer amplifiers in output buffer  74  to help match the output impedance of the logic circuitry to the input impedance of the modulation inverter  76 . The output of the buffer amplifiers in output buffer  74  is applied to the input of the modulation inverter  76 . Specifically, the signal TAG OUTPUT is applied to the gate of the drive transistor associated with modulation inverter  76 . Modulation inverter  76  then modulates the Q of the tank formed by inductor  67  and capacitor  77  to provide amplitude modulation of the carrier signal. In this manner, the received buffer output is conveyed to reader unit  56  so that the identification code can be read by reader  64 . In particular, reader  64  processes the signal received at L_tap via inductor  59 . 
     FIG. 12  is a circuit diagram further illustrating reader  64  associated with the RFID tag/reader system  55  of  FIG. 10 . Reader  64  receives, via L_tap, a signal containing the carrier signal, e.g., at 125 kHz, modulated by the TAG OUPUT signal, which may be on the order of 1 kHz, depending on the frequency of clock circuit  68 . A low junction capacitance signal diode  78  is used to demodulate the signal. A low pass filter section  80  removes the carrier frequency, and may include inductor  84 , capacitor  86 , resistor  88 , inductor  90 , capacitor  92  and resistor  94 . An amplifier stage  82  includes an amplifier  98  in a non-inverting configuration, with resistor  96  and feedback resistor  100  coupled to the inverting input. 
     FIG. 13  is a graph illustrating simulated output of an RFID tag constructed using ac-powered thin film transistor circuitry as shown in  FIGS. 10–12 . In particular,  FIG. 13  shows the transition of the signal TAG OUTPUT generated from output buffer  74 . As shown in  FIG. 13 , during the application of the ac power supply waveform to clock circuit  68 , control logic  72 , data lines  70 , and output buffer  74 , the circuitry is operative to produce a train of pulses in sequence with the clock circuit  68 . 
     FIG. 13  shows a transition between bit  0  ( 102 ), bit  1  ( 104 ), bit  2  ( 106 ), bit  3  ( 108 ), bit  4  ( 110 ), bit  5  ( 112 ) and bit  6  ( 114 ) of the identification code specified by data lines  70 . In particular, it can be seen from  FIG. 13  that the 7-bit code transitions from high to low in a pattern corresponding to the code 1010111. Accordingly, such a pattern can be readily resolved by reader  64  to determine the identification code carried by RFID tag  58 . 
     FIG. 14  is a circuit diagram illustrating an ac-powered thin film transistor-based inverter circuit  116  that drives a liquid crystal display element  118 . In the example of  FIG. 14 , inverter circuit  116  conforms substantially to inverter circuit  10  of  FIG. 1 . However, the output of inverter  14  drives a liquid crystal display element  118 . In particular, one electrode of liquid crystal display element  118  is coupled to the source of load transistor  16  and the drain of drive transistor  18 . The other electrode of liquid crystal display element  118  is coupled to ground. 
     FIG. 15  is a circuit diagram illustrating an ac-powered thin film transistor-based inverter circuit  120  that drives a light emitting diode (LED)  122 . Inverter circuit  120  conforms substantially to inverter circuit  10  of  FIG. 1 , but drives an LED  122 . The cathode of LED  122  is coupled to the source of load transistor  16  and the drain of drive transistor  18 , and the anode of the LED is coupled to ground. 
   The invention can provide a number of advantages. For example, ac-powered logic circuitry, and particularly OTFT-based logic circuitry, may exhibit stable performance over a longer period of time, relative to dc-powered thin film circuitry. Although dc-powered OTFT logic circuitry appears to undergo substantial changes in threshold voltage over time, the overall performance of ac-powered OTFT logic circuitry does not seem to change as quickly. Instead, ac-powered OTFT circuitry seems to be more stable over an extended period of time. 
   In the case of a ring oscillator, for example, ac-powered OTFT circuitry appears to maintain oscillation amplitudes over a much longer period of time relative to dc-powered OTFT circuitry. When OTFT-based ring oscillators are powered with dc power and monitored over time, the oscillation amplitude can exhibit a rather rapid decrease. When the same type of ring oscillator is ac powered, however, the rapid decrease does not occur. In particular, consistent oscillation amplitude has been observed for an ac-powered OTFT-based ring oscillator running continuously for over sixty hours, in contrast to a dc-powered OTFT-based ring oscillator which exhibited performance changes in less than ten minutes. 
   The availability of stable and reliable OTFT circuitry may promote wider use of OTFT circuitry in a variety of applications, with more reliable performance, durability and longevity. Consequently, various applications for ac-powered OTFT circuitry, including those described herein, may benefit from manufacturing advantages associated with OTFT circuitry, such as the ability to form circuitry on flexible substrates and use lower-cost manufacturing techniques. 
   As a further advantage, the use of ac power for the thin film circuitry may eliminate the need for the ac-dc rectifier circuitry otherwise required in some applications for delivery of dc power to the circuitry. Accordingly, by eliminating the need for rectifier circuitry, the use of ac power may reduce the manufacturing time, expense, cost, complexity, and size of components carrying thin film circuitry. 
   For RFID tags, as a particular example, the use of ac-powered thin film circuitry may substantially reduce the cost and size of the circuit by eliminating the ac-dc rectifier circuitry. In addition, the RFID tag may benefit from performance and reliability advantages associated with ac-powered OTFT circuitry, possibly creating new opportunities for application of RFID technology. For example, the increased reliability of ac-powered OTFTs may permit applications in which the RFID tag, in whatever form, is in more continuous or even persistent operation in conjunction with a reader unit. 
   Thin film transistors useful in forming ac-powered logic circuitry, as described herein, may take a variety of forms and may be manufactured using various manufacturing processes. For example, the thin film transistors may include organic semiconducting material, inorganic semiconducting material, or a combination of both. For some applications, organic and inorganic semiconducting materials can be used to form CMOS thin film transistor circuitry. Thin film transistors useful in forming ac-powered logic circuitry as described herein may include, without limitation, thin film transistors manufactured according to the techniques described in U.S. Pat. No. 6,433,359; U.S. patent application Ser. No. 10/012,654, filed Nov. 2, 2001; U.S. patent application Ser. No. 10/012,655, filed Nov. 5, 2001; U.S. patent application Ser. Nos. 10/076,174, 10/076,005, and 10/076,003, all filed on Feb. 14, 2002; and U.S. patent application Ser. No. 10/094,007, filed Mar. 7, 2002; the entire content of each being incorporated herein by reference. 
   Various modification may be made without departing from the spirit and scope of the invention. These and other embodiments are within the scope of the following claims.