# Patent

## Title
Electronic component comprising a modulator

## Abstract

An electronic component includes an RFID transponder, and also a security document comprises such an electronic component. The electronic component has a modulator for modulating a carrier signal, the modulator being constructed from at least two organic field effect transistors.

## Background
The invention relates to an electronic component comprising a modulator for modulating a carrier signal.
RFID transponders (RFID=Radio Frequency Identification) are increasingly being employed for providing merchandise, articles or security documents with information that can be read out electronically. They are thus being employed for example as electronic bar code for consumer goods, as luggage tag for identifying luggage or as security element that is incorporated into the binding of a passport and stores authentication information.
In this case, RFID transponders are usually constructed in the manner described in U.S. Pat. No. 5,528,22, for example.
The RFID transponder essentially comprises two components, an antenna and a silicon chip. Antenna and silicon chip are mounted on a common carrier substrate and electrically connected to one another by means of contact-connections. The RF carrier transmitted by a base station is fed back to the base station and in this case an additional item of information is modulated onto the fed back signal in accordance with a preprogrammed information protocol.
Furthermore, WO 99/30432 describes an RFID transponder having an integrated circuit constructed substantially from organic material, said integrated circuit providing the function of an ID code generator. The RFID transponder is constructed from electrical components based on conventional silicon technology, e.g. a rectifier diode, and from organic semiconductor components, the ID code generator IC (IC=Integrated Circuit).
Furthermore, DE 101 41 440 C1 describes an RFID transponder constructed—with the exception of the antenna—substantially from organic components.
In these RFID transponders, the carrier signal emitted by a base station is coupled into an antenna resonant circuit of the RFID transponder and the induced voltage is then rectified. The rectified voltage supplies a logic IC of the RFID transponder that drives a modulation transistor. The modulation transistor is driven by the logic IC with a binary signal containing a bit sequence, so that the attenuation of the resonant circuit is modulated according to the binary signal. The antenna's radiating behavior that changes as a result of this is detected by the base station and acquired as response signal of the RFID transponder.
The advantage of such an RFID transponder is that electronic components based on organic electronics (plastic electronics) can be produced at significantly lesser expense, such that RFID transponders can be used for extremely inexpensive applications. RFID transponders, for example as electronic radio labels, can thus form a replacement for the bar code.
However, organic circuits are significantly slower than conventional silicon based circuits. The basic building blocks of organic circuits are organic field effect transistors, so-called OFETs. These transistors are based on the principle of charge carrier accumulation rather than on the principle of charge carrier invasion, which results in a low switching speeds in comparison with the silicon transistors and a different switching behavior (e.g. unsuitability for AC voltage).
These properties discourage the use of such transistors in traditional multistep modulation methods.
It is an object of the present invention, then, to specify an improved device for effective and reliable modulation of a carrier signal, in particular for organic RFID transponders.
According to the invention, this object is achieved by an electronic component, in particular an RFID transponder, which has a modulator for modulating a carrier signal that is formed from at least two organic field effect transistors (72, 73).
In this case, the invention is based on the insight that, by means of two or more organic field effect transistors, novel modulators for modulating a carrier signal can be created which utilize the particular properties and the particular switching behavior of organic field effect transistors for the modulation of the carrier signal. The invention makes it possible, with extremely low component outlay, to realize powerful multistep modulation methods and thus to increase the amount of information that can be transmitted.
An organic field effect transistor, referred to as OFET hereinafter, has at least three electrodes, an organic semiconductor layer, and an insulating layer. The OFET is arranged on a carrier substrate, which may be formed as a polymer film. A substrate composed of an organic semiconductor forms a conductive channel, the end sections of which are formed by a source electrode and a drain electrode. The conductive channel is covered with an insulation layer, on which a gate electrode is arranged. The conductivity of the conductive channel can be altered by application of a gate-source voltage UGS between gate electrode and source electrode. In the case of organic semiconductors, this effect is based essentially on so-called hole conduction, “holes” in the crystal lattice that serve as charge carriers being densified if a drain source voltage UDS is applied between the source electrode and the drain electrode. The electrical conductivity consequently rises between the drain electrode and the source electrode, in which case the achievable reverse and forward resistances of an OFET subsequently also differ significantly from those of a transistor created using traditional silicon technology, on account of the different functional principle of an OFET.
The organic semiconductor layer comprises for example conjugated polymers, such as polythiophenes, polythienylenevinylenes or polyfluorene derivatives, which are applied from solution by spin-coating, blade coating or printing. So-called “small molecules”, i.e. oligomers such as sexithiophene or pentacene, which are vapor-deposited by a vacuum technique, are also suitable as organic semiconductor layer.
It is provided that the at least two OFETs form a variable load for modulation of the carrier signal. In this way, two or more bits can be transmitted simultaneously since even two OFETs of the same type can assume three states if they are connected in parallel. The first state is brought about by neither of the two OFETs being driven, with the result that a total resistance that is equal to half the reverse resistance of one OFET is formed. The second state is brought about by one of the two OFETs being driven. In this way, the resistance of said one OFET decreases to a forward resistance. The total resistance of the two OFETs is the product of both resistances divided by the sum of both resistances and, owing to the reduced resistance of said one OFET, is therefore lower than in the first state described above. The third state is brought about, then, by both OFETs being driven. The total resistance of the two OFETs is then equal to half the forward resistance of one OFET.
The gain in transmittable information that is achieved by means of the invention can be utilized in this case for increasing the data rate transmitted via the air interface, for synchronization purposes or for error detection/correction—depending on the coding system used.
The inventive circuit arrangement thus compensates for the disadvantage of the low switching frequency of OFETs by virtue of the simultaneously possible transmission of a plurality of bits and by virtue of the possibilities thus also opened up for parallel processing in an organic logic IC connected to the modulator.
Upon eliminating the disadvantages when using OFETs in RFID transponders, the advantages of OFETs are of particular significance. The fact that OFETs can be produced cost-effectively and in few work steps by means of a printing method means not only that the manufacturing costs are very low, but also that the development costs and the development time are reduced. It is also advantageous that the OFETs can be tailored individually to their purpose of use.
It is particularly advantageous to form the modulator with at least two differently formed OFETs. Thus, by way of example, two or more OFETs are connected up which have a different profile of the internal resistance and/or a different switching behavior. A bit in the code space can thus be assigned to the specific signal response of each of said OFETs to the change in the gate level from the low level to the high level (and also vice versa) itself. A superposition of these signal responses is obtained by connecting the different OFETs in parallel and/or in series, with the result that, when using n different OFETs, a character set of 2n different characters can be obtained. It is particularly advantageous in this case if the differing properties of the OFETs and also the interconnection of the OFETs are chosen such that the summationally superposed signal responses represent mutually orthogonal functions.
Although the decoding of the information coded into the carrier signal by means of such a modulator requires a complex evaluation circuit, which can be realized only with difficulty by means of an organic circuit, in the case of an RFID transponder the return response of the RFID transponder is evaluated by the base station, in which an evaluation circuit in traditional silicon technology is then preferably used.
In order to form a modulator with at least two differently formed OFETs, provision may be made for forming the OFETs with different geometries, for example forming them with different channel lengths and/or different channel widths.
These parameters can easily be altered and be optimized in few steps according to the trial and error method owing to the easy produceability described further above.
Provision may also be made for forming the OFETs with different semiconductor layers, in particular for varying the channel thickness or for using a semiconductor material having a different conductivity or doping. In this case, too, changes are possible at short notice because the starting substances are available as pastes or inks.
These described variations of the construction of the OFETs make is possible to form the OFETs with a different switching behavior and profile of the internal resistance.
It is particularly advantageous if the at least two different OFETs have different internal resistances in the turned-on (forward resistance) and/or in the turned-off state (reverse resistance) and the OFETs are connected up to one another in parallel connection and/or in series connection.
In this case, it may be provided that the OFETs in a parallel connection form the variable load. A parallel connection of two OFETs having different forward resistances can already form four states, to be precise, as explained thoroughly further above, through a change in the total resistance of the two parallel-connected OFETs. A respectively different state can then be formed by the optional driving of the two OFETs since the two OFETs are then formed with a different forward resistance.
It may furthermore also be provided that the two or more OFETs are modulated differently with different gate voltages since the forward resistance of the OFET is dependent on the applied gate voltage.
The OFETs may also be formed with a different switching behavior by changing the geometry and/or the semiconductor material. It is thus furthermore possible to use OFETs which exhibit different edge shapes in the event of a change in the input signal and the edge shapes of which differ in particular in terms of their edge steepness. For fast OFETs, an almost rectangular switching profile is characteristic, that is to say that when the OFET is driven, it switches without significant delay from the low level to the high level, and vice versa. Slow OFETS, by contrast, switch with a delay. That is tantamount to an inclined switching edge. A slow OFET therefore has a more greatly inclined switching edge than a fast OFET. By means of two or more OFETs, it is possible in this way to form a switching edge with changing inclination and, under certain circumstances, switching edges that change in regions. The information is thus coded for example into the edge steepness of the edges (edge steepness modulation).
Furthermore, it is also possible to combine the modulation schemes described above.
As already described above, besides a parallel connection of the OFETs, it may also be provided that the OFETs are arranged in a series connection. Given Appropriate design, OFETs can also have a finite conductance in the off state, so that the states described further above for the parallel connection can also be assumed in a series connection of OFETs.
In accordance with one preferred exemplary embodiment of the invention, the electronic component furthermore has an organic binary logic circuit, the gates of the OFETs being connected to respectively assigned outputs of the organic logic circuit and the OFETs being driven by the binary logic circuit for the simultaneous transmission of two or more bits by means of a multistage modulatiuon method. If the OFETs are formed by n different OFETs, then the OFETs will be driven by the binary logic circuit for the simultaneous transmission of 2n bits. In this way, by way of example, 8 bits can be transmitted simultaneously by means of 3 OFETs.
The modulator may be formed as a printed thin-film element on a carrier substrate, in particular a carrier film. Provision may be made for preferring carrier films because particularly robust RFID transponders which can be folded or bent in use can be produced in this way. Such a production process may be formed as a continuous roll-to-roll process.
However, it is also possible to provide a rigid substrate, for example a glass substrate.
In further embodiments, provision may be made for forming the modulator in optically transparent and/or optically invisible fashion. Such an embodiment may be particularly advantageous in conjunction with display elements or optical security elements.
It may be provided that the OFETs provided for the load modulation of the carrier signal are driven individually and/or jointly with an identical gate voltage corresponding to the high level of the logic assembly. In this way, the logic assembly for driving the modulator can be formed in a particularly simple manner.
It may also be provided that the OFETs for modulating the carrier signal are driven with different gate voltages. By way of example, it is thus possible to arrange a plurality of antennas designed for different frequency bands on an RFID transponder, which antennas are in each case connected to a rectifier and a capacitor for providing different supply potentials. The different supply voltage potentials provided are used for operating the logic assembly. Since different supply voltage potentials are available in the case of an electronic component of this type, it is possible in a simple manner for the OFETs used for modulating the carrier signal to be driven with different gate voltages.
The electronic component according to the invention may be for example part of a security document, ticket or merchandise label. Furthermore, it is also possible for an electronic component according to the invention not to provide the function of an RFID transponder, but rather any other function desired. Thus, it is possible for example, for the electronic component to comprise an oscillator that generates the carrier signal modulated by the modulator.
The invention will now be explained in more detail on the basis of a plurality of exemplary embodiments with the aid of the accompanying drawings.



In the figures:
FIG. 1 shows a block diagram of a transponder circuit;
FIG. 2 shows a basic circuit diagram of a transponder circuit;
FIG. 3 shows a basic illustration of a modulation diagram according to the prior art;
FIG. 4 shows a basic circuit diagram of a modulator for a first exemplary embodiment of the invention;
FIG. 5 shows a first modulation diagram, in particular of the modulator according to FIG. 4;
FIG. 6 shows a second modulation diagram, in particular of the modulator according to FIG. 4;
FIG. 7 shows a basic circuit diagram of a modulator for a second exemplary embodiment of the invention.



FIG. 1 shows a transponder 10 comprising a transponder circuit 12 and an antenna 14. The transponder circuit 12 is essentially formed from a logic component 12l and a modulation component 12m. The transponder 10 is preferably formed as a transponder with organic electronic components, such as OFETs for example. The modulation component 12m is referred to hereinafter as modulator.
The OFET is an organic field effect transistor having at least three electrodes and an insulating layer. The OFET is arranged on a carrier substrate, which may be formed as a film. A substrate composed of an organic semiconductor forms a conductive channel between a source electrode and a drain electrode. The conductive channel is covered with an insulation layer, on which a gate electrode is arranged. The conductivity of the conductive channel can be altered by application of a gate-source voltage UGS between gate electrode and source electrode. Consequently, the resistance between the drain electrode and the source electrode decreases after application of the gate-source voltage UGS and a current flow occurs between the source electrode and the drain electrode if a drain-source voltage UDS is applied. An OFET is therefore essentially a controllable resistor.
An item of binary information is stored in the logic component 12l of the transponder illustrated in FIG. 1, said information being transmitted to the modulation component 12m upon activation of the transponder 10, as a result of which the antenna 14 connected to the modulation component 12m emits a signal modulated with the binary information. Said signal can be evaluated in a receiver and provides information about the identity of the object connected to the transponder 10. The object may be for example an item of merchandise or a security document.
FIG. 2 then shows a circuit diagram of an exemplary embodiment of the transponder 10. Identical elements are designated by identical reference symbols. The antenna 14 forms together with a capacitor 16 an electrical resonant circuit 18 tuned to a carrier frequency. The resonant circuit 18 is connected to the input of a rectifier 20, as a result of which a DC voltage is provided at the output of the rectifier 20 if an electrical voltage is induced in the resonant circuit 18 by an electromagnetic field generated by an external transmitter. On the supply voltage side, the logic component 12l and the modulator 12m are connected to the output of the rectifier 20.
In this case, it is also possible to couple to the resonant circuit a bridge rectifier instead of a simple rectifier diode, and thus to use both half-cycles for the power supply of the logic component and of the modulator. Furthermore, it is also possible to arrange, between modulator and logic component, an additional storage capacitor protected against discharge with respect to the modulator by means of a diode, said storage capacitor effecting a reliable and constant power supply of the logic component.
Furthermore, it is possible, as indicated in FIG. 2, to arrange the modulator 12m downstream of the rectifier 20 or upstream of the rectifier 20 (represented by dashed lines).
The output of the logic component is connected to the input of the modulator 12m. In this way, the information stored in the logic component 12l is transmitted to the modulator 12m. 
The exemplary embodiment illustrated in FIG. 2 involves load modulation of the carrier signal, that is to say that the amplitude of the high-frequency carrier signal is modulated.
FIG. 3 then shows a modulation diagram according to the prior art. The signal level S can assume the values L for the low level and H for the high level, tantamount to the binary signals 0 and 1. FIG. 3 illustrates the binary character sequence 10101100, where a binary 1 is designated by 30h, a binary 0 is designated by 30l and two successive binary 1s are designated by 32h and two successive 0s are designated by 32l. In this way, a time corresponding to the number of binary characters or bits is required for the transmission of a binary character sequence.
FIG. 4 shows a first exemplary embodiment of a modulator 12m, formed as a parallel connection of two OFETs 42 and 44 having different characteristic curves.
The two gates of the OFETs 42, 44 are connected to the two outputs of the logic component 12l. The two sources of the OFETs 42, 44 are connected to a ground line GND, and the two drains of the OFETs 42, 44 are connected to an operating voltage UB.
FIG. 5 then shows a first modulation diagram that can be formed by means of a circuit arrangement according to FIG. 4. It may be provided that the two parallel-connected OFETs 42 and 44 are formed with a different resistance profile. For this purpose, it may be provided, for example, that the first OFET is formed with a first geometry, for example with a first channel cross-section, and the second OFET is formed with a second geometry, for example with a second channel cross-section. In this way, a different forward resistance is formed for the same gate-source voltage.
If, therefore, as illustrated in FIG. 5, firstly only the first OFET 42 is driven, a signal 50h having a signal level H1 is formed. In the case where only the second OFET 44 is driven, a signal 52h having a signal level H2 is formed. If both OFETs 42, 44 are driven, a signal 54h having a signal level H3 is formed. In the exemplary embodiment illustrated, the following relationship holds true between the signal levels H1 to H3:

L<H1<H2<H3.

If neither of the two OFETs is driven, a signal having a low signal level L is formed.
In this way, the amount of information that can be transmitted within a time period is increased since the number space that can be transmitted is now extended from 0, 1 to 0, 1, 2, 3. A transformation of the number system is therefore possible in this way. The use of more than two parallel OFETs may be provided in this case. By way of example, three parallel-connected OFETs may be provided in order to code an octal signal. If n parallel-connected OFETs are provided, 2n different signal levels can be transmitted.
In this case, it is advantageous for the forward resistances of the different OFETS to be chosen such that the 2n different signal levels succeed one another equidistantly. The resultant signal level in each case for the different switching states of the n different OFETs is in this case calculated from the sum of the conductances of the individual OFETs.
It may also be provided, however, that the first and the second OFET are formed with identical geometry and have different gate-source voltages applied to them, with the result that a different resistance is formed between drain and source electrodes of the two OFETs. Provision may also be made for operating a plurality of OFETs with different geometries and different gate-source voltages.
FIG. 6 then shows a modulation diagram with edge steepness modulation. Whereas in the exemplary embodiment according to FIG. 5 the information is coded into the amplitude value and the steepness of the edges is not accorded any significance, in the exemplary embodiment according to FIG. 6 the different rise times of different OFETs that result during the transition from the low level to the high level are deliberately utilized for information coding.
In order to form the edge steepness modulation illustrated in FIG. 6, it is provided that the two OFETS 42 and 44 according to FIG. 4 have a different switching behavior. This can be achieved for example by means of differently formed semiconductor material or by means of different channel lengths or a different channel width. If the first OFET 42 formed with a short rise time is driven, a high signal 60hhaving steep edges is formed. If the second OFET 44 formed with a long rise time is driven, a high signal 62h having shallow edges is formed. In the case where both OFETs 42, 44 are driven simultaneously, a high signal 64h results which has a first steep partial edge 64s and a second shallow partial edge 64f and a signal amplitude formed as the sum of the two signal amplitudes of the two OFETs 42, 44. As can be discerned in FIG. 6, the high signal 64h is formed with double amplitude since it results from the superposition of the high signals 60h and 62h. Four states of the carrier signal can once again be formed by means of two differently formed OFETs, the modulated carrier signal being redundant on account of the simultaneous formation of different amplitudes and different signal edges.
The demodulation of the signal can thus be based on the average edge steepness, on the edge steepness in the leading region of the edge and/or on the maximum or average amplitude value.
In accordance with the exemplary embodiment according to FIG. 5, in this case as well it is possible to provide more than two parallel OFETs having a different switching behavior. If n parallel-connected OFETs are provided, 2n different signal levels can be transmitted.
Furthermore, it is also possible to combine the exemplary embodiments according to FIG. 5 and FIG. 6 with one another and to connect up n different OFETS to one another in a parallel connection, which OFETs differ in each case among one another in terms of their forward resistance or their response characteristic.
FIG. 7 shows a second exemplary embodiment of a modulator circuit. The two OFETs 42, 44 are now connected in series. The OFET 42 is connected by the drain to the operating voltage UB and by the source to the drain of the OFET 44. The source of the OFET 44 is connected to the ground line GND. The two outputs of the logic component 121 are connected to the two gates of the OFETs 42, 44.
This circuit arrangement can likewise be used for generating the signal shapes illustrated in FIGS. 5 and 6 if the reverse resistances of the OFETs 42 and 44 are chosen to be correspondingly low. The circuit arrangement illustrated in FIG. 4 may be preferred, however, if only a low operating voltage UB is provided.
It is also possible to provide circuit arrangements which combine parallel connection and series connection with one another.

## Claims

1. An electronic component comprising:
an antenna, a capacitor, and a modulator for modulating a carrier signal and coupled in circuit with the antenna and the capacitor; and
a rectifier having an output;
the modulator being connected to the output of the rectifier, the modulator comprising at least two different organic field effect transistors each with different semiconductor layers wherein the at least two different organic field effect transistors have at least one of 1) different internal resistances in a turned-on or in a turned-off state or 2) a different switching behavior, and are connected in the electronic component to form a variable load for load modulation of the carrier signal.

2. The electronic component as claimed in claim 1 wherein the organic field effect transistors are connected in parallel and/or series.

3. The electronic component as claimed in claim 1 wherein the at least two different organic field effect transistors exhibit different edge forms when the input signal changes, which edge forms differ in edge steepness.

4. The electronic component as claimed in claim 1 wherein the at least two different organic field effect transistors are formed with different geometries.

5. The electronic component as claimed in claim 1 wherein the at least two different organic field transistors have semiconductor layers that differ in terms of at least one of their thickness, doping or conductivity.

6. The electronic component as claimed in claim 1 further including a binary logic circuit having outputs, wherein the at least two organic field effect transistors each have a gate, the gates of the organic field effect transistors being connected to the outputs of the binary logic circuit.

7. The electronic component as claimed in claim 6 wherein the binary logic circuit is an organic binary logic circuit.

8. The electronic component as claimed in claim 6 wherein the organic field effect transistors are driven by the binary logic circuit for the simultaneous transmission of two or more bits by multistage modulation.

9. The electronic component as claimed in claim 8 wherein the organic field effect transistors are formed by n different organic field effect transistors which are driven by the binary logic circuit for the simultaneous transmission of 2n bits.

10. The electronic component as claimed in claim 9 wherein the n different organic field effect transistors have different internal resistances in the turned-on state and the n different organic field effect transistors are connected in parallel.

11. The electronic component as claimed in claim 9 wherein the n different organic field effect transistors have different edge forms during state transition and the n different organic field effect transistors are connected in parallel.

12. The electronic component as claimed in claim 1 wherein the organic field effect transistors are formed by at least one of printed-on semiconducting polymers, conductive printing inks or metallic layers.

13. The electronic component as claimed in claim 1 wherein the electronic component comprises a multilayer flexible film body.

14. A security document comprising:
at least one of a valuable document, identity card, merchandize label or ticket; and
comprising an electronic component as claimed in claim 1.

15. An electronic component comprising:
an antenna, a capacitor, and a modulator for modulating a carrier signal and coupled in circuit with the antenna and the capacitor; and
a rectifier having an output;
the modulator being connected to the output of the rectifier, the modulator comprising at least two different organic field effect transistors, wherein the at least two different organic field effect transistors have semiconductor layers having respective different geometries, and having different internal resistances in a turned-on or in a turned-off state and are connected in the electronic component to form a variable load for load modulation of the carrier signal.

16. An electronic component comprising:
an antenna, a capacitor, and a modulator for modulating a carrier signal and coupled in circuit with the antenna and the capacitor; and
a rectifier having an output;
the modulator being connected to the output of the rectifier, the modulator comprising at least two different organic field effect transistors, each with different semiconductor layers having different geometries and wherein the at least two different organic field effect transistors having a different switching behavior, and are connected in the electronic component to form a variable load for load modulation of the carrier signal.