Megahertz operation of organic field-effect transistors based on poly(3-hexylthiopene)Veit Wagner, Paul Wöbkenberg, Arne Hoppe, and Jörg Seekamp Citation: Applied Physics Letters 89, 243515 (2006); doi: 10.1063/1.2405414 View online: http://dx.doi.org/10.1063/1.2405414 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/89/24?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Overestimation of the field-effect mobility via transconductance measurements and the origin of theoutput/transfer characteristic discrepancy in organic field-effect transistors J. Appl. Phys. 105, 024506 (2009); 10.1063/1.3029587 Interdependence of contact properties and field- and density-dependent mobility in organic field-effect transistors J. Appl. Phys. 105, 014509 (2009); 10.1063/1.3058640 Analytic device model for light-emitting ambipolar organic semiconductor field-effect transistors Appl. Phys. Lett. 89, 233519 (2006); 10.1063/1.2402942 Time dependent evolution of the carrier mobility in poly(3-hexylthiophene) based field effect transistors J. Appl. Phys. 100, 034508 (2006); 10.1063/1.2229979 Close look at charge carrier injection in polymer field-effect transistors J. Appl. Phys. 94, 6129 (2003); 10.1063/1.1613369

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Megahertz operation of organic field-effect transistors basedon poly„3-hexylthiopene…

Veit Wagner,a� Paul Wöbkenberg,b� Arne Hoppe, and Jörg SeekampSchool of Engineering and Science, International University Bremen,c� Campus Ring 8, 28759 Bremen,Germany

�Received 28 September 2006; accepted 6 November 2006; published online 15 December 2006�

Switching speed is crucial for many applications in organic electronics. The possibility to achievehigher frequencies will enable new fields of applications. The authors demonstrate high-frequencyorganic thin film transistors based on poly�3-hexylthiophene�. Transistors with submicron channellengths show unity-gain bandwidth of 2 MHz in air at low supply voltages of 10 V. For channellengths L below 500 nm deviations from ideal L scaling law are observed experimentally, which areattributed to contact effects. They present a model beyond the ideal scaling law to predict themaximum operational frequency based on transistor parameters, geometry, and contactresistance. © 2006 American Institute of Physics. �DOI: 10.1063/1.2405414�

Organic semiconductors are proposed for multitude ap-plications as there are displays, electronic paper, sensors, orradio-frequency identification �RFID� tags.1,2 While slowswitching frequencies of about 1 Hz are sufficient for elec-tronic paper applications several kilohertz bandwidth is al-ready required for, e.g., flat panel back plane applications.Even higher switching speeds will enable further new appli-cations such as high data rate RFID tags and applicationsrequiring complex logic calculations.

Organic semiconductors in thin film transistors are real-ized either as polymer layers or as film of small organicmolecules. In this letter we use the polymer regioregularpoly�3-hexylthiophene� �P3HT�, which shows ordering phe-nomena and high mobility values of up to 0.2 cm2/ �V s�.3,4

Fastest P3HT device reported so far is a seven stage ringoscillator at 192 kHz operated at 60 V.2 Instead of such com-plex circuitry this work focuses on single devices. Our tran-sistors are manufactured in bottom contact configuration onhighly n-doped Si�100� wafers covered by a thermally grown50 nm thick SiO2 layer, which acts as gate insulator. Sourceand drain electrodes consist of 17 nm gold and a 3 nm tita-nium adhesion layer. They are patterned lithographically bye-beam lithography to achieve submicron structures down to120 nm �Fig. 1�. This process could be replaced by printingtechniques to enable efficient large scale production.5 Thedevice is exposed to hexamethyldisilazane vapor prior tospin coating of the P3HT material �as received, 0.1 wt % inchloroform� for proper surface termination of the channelarea. The spin coating is done in a class 100 wetbench atambient conditions. Measurements are performed in air atroom temperature within one day after device preparation toavoid degradation effects.6 For submicron structures and15 nm thick P3HT films mobilities of about �0.3–1��10−2 cm2/ �V s� and threshold voltages around +8 V arefound.

To compare different transistors with respect to fre-quency response the bandwidth has to be measured in a con-

sistent way. In literature the carrier channel transition time �or response time to a gate voltage step is used,2 which areusually difficult to access in high accuracy. Therefore in thisletter the well-defined unity-gain bandwidth will be consid-ered for this purpose. Typically the FET input current �gatecurrent� induced by an applied ac gate voltage increases lin-early with frequency due to the finite gate capacitance,whereas the FET output current �drain current� is mainlygiven by the transistors dc characteristics at not too highfrequencies. For the unity-gain bandwidth the upper fre-quency limit �frequency of transition� fT is reached if inputand output ac levels exhibit the same amplitude �unity-gaincondition�. Beyond this frequency fT the transistor is not ableto drive a second transistor of the same kind and usually nocircuitry of such transistors can operate beyond fT. The mea-surement setup is sketched in Fig. 2. Experimentally the gatevoltage is applied by an Agilent 33220A signal generator, theconstant drain voltage by a programmable voltage source,and the input and output current ac components are measuredby a SR830 lock-in amplifier, respectively. The frequencyscan is done computer-controlled by reprogramming the sig-nal generator. A corresponding measurement of input �gate�and output �drain� currents versus frequency are shown inFig. 3 in a log-log plot. The transistor shown has a channellength of L=2 �m and channel width of W=20 mm. The

a�Author to whom correspondence should be addressed; electronic mail:v.wagner@iu-bremen.de

b�Present address: Imperial College London, UK.c�Jacobs University Bremen as of spring 2007.

FIG. 1. Electron microscope picture of a high W /L ratio submicron transis-tor structure with L=120 nm. Bright areas represent the interdigitated golddrain and source electrodes while dark areas are SiO2 in the channel area.

APPLIED PHYSICS LETTERS 89, 243515 �2006�

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linear increase of gate current, expected for capacitive cou-pling, is obvious, whereas the drain current amplitude re-mains almost constant up to the frequency of transition fT=8.6 kHz. Above fT capacitive coupling becomes dominantalso for the drain current, as is seen from the linear increaseat high frequencies and from an additional phase shift �latternot plotted�. The example in Fig. 3 proves that the draincurrent level for our P3HT transistors given by the dc re-sponse remains unaltered up to the frequency of transition fT,while the gate current is given by the gate capacitance. Theobserved slopes correspond to capacitances deduced fromthe actual geometry of the transistor, i.e., 200 and 90 pF.

To achieve very high frequencies fT small gate voltagemodulations should create huge drain current modulations,i.e., a large transconductance value is needed. Furthermore�parasitic� gate capacitance should be minimized to reducethe gate current level. The gate capacitance is given byCi�W�L+Ap� and the transconductance scales as �CiW /Laccording to the gradual channel approximation �GCA�.7

Here Ci denotes the gate capacitance per area and Ap para-sitic areas contributing to the gate capacitance in addition tothe channel area. Consequently for higher bandwidths weneed �a� high carrier mobility, �b� small channel length, and�c� small ratio of parasitic area versus channel area Ap / �W�L�, whereas changing the gate capacitance per area Ci, e.g.via the insulator thickness di, will not affect the upper fre-

quency limit of the transistor assuming the GCA is valid.Figure 4 shows the frequency response of a P3HT transistorwith L=480 nm and W=46.2 mm at Vgs=−4.5 V and Vds=−10 V with a 0.5 V peak-to-peak modulation at the gate. Thefrequencies of transition extrapolate to fT=2.0 MHz. If thedrain voltage is now reduced to Vds=−5 V the drain currentdecreases while the gate current remains unchanged �Fig. 4�and the frequency of transition extrapolates to fT=0.6 MHzonly. Both frequencies are beyond the upper frequency limit�=100 kHz� of the lock-in amplifiers in the measurementsetup. A 20 MHz oscilloscope was used to check that thegiven extrapolation is valid indeed. In summary these mea-surements demonstrate an organic field-effect transistorbased on the material P3HT with a bandwidth of fT=2.0 MHz at 10 V.

We expect to achieve even larger unity-gain bandwidthfor channel lengths L smaller than 480 nm. Quantitatively,assuming no parasitic capacitances and the GCA valid, fTcalculates to �see Eq. �5��

fT = 1/�2���V/L2, �1�

with V= �Vgs0−Vt� for the saturation regime. Note that thisfrequency is identical to the frequency obtained from thecarrier transition time ansatz2 except a factor 1 / �2��. How-ever, experimentally our submicron transistors do not followthis 1 /L2 scaling. We find higher frequencies than expectedfor larger channel lengths, e.g., fT=1.2 MHz instead of0.8 MHz for L=740 nm devices, while frequencies even be-low 2.0 MHz are found for smaller L, e.g., fT=0.64 MHz forL=250 nm devices. Since the gate insulator is sufficientlythin �di=50 nm� the GCA should remain valid. Therefore weattribute the limitation to residual contact resistances8–10 inour devices, as were observed previously in submicron P3HTtransistors.11–13 A more detailed model presented here calcu-lates the influence of contact resistance as well as parasiticgate capacitances on the transistors bandwidth.

For this we calculate the ac gate current ig as caused bythe total gate capacitance,

FIG. 2. Setup to measure unity-gain bandwidth. Vds is a constant voltagewhile the gate voltage Vgs consists of an ac input signal and a dc offset. Atthe frequency of transition fT the amplitudes of the input and output currentsare the same.

FIG. 3. �Color online� Input and output peak-to-peak currents vs modulationfrequency for a L=2 �m P3HT transistor with a channel width of W=20 mm. A frequency of transition around 10 kHz �fT=8.6 kHz� is ob-served for small voltages Vds=−5 V and Vgs=−2 V+sin��t��0.04 V.

FIG. 4. �Color online� Input and output peak-to-peak currents vs modulationfrequency for a L=0.48 �m P3HT transistor with a channel width of W=46.2 mm. Extremely high frequency of transition and current levels areobserved for low voltages Vgs=−4.5 V+sin��t��0.25 V and Vds=−10 V�squares�, −5 V �circles�.

243515-2 Wagner et al. Appl. Phys. Lett. 89, 243515 �2006�

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ig = j�Ci�WL + Ap�vgs, �2�

where vgs denotes the gate voltage modulation at angularfrequency �=2�f . For the drain current we obtain in satu-ration regime within the GCA under assumption of a contactresistance Rs at the source electrode,

Id = W/L�Ci/2�Vgs − Vt − RsId�2. �3�

For a gate voltage Vgs=Vgs0+vgs with a small modulation vgsaround an offset Vgs0 we obtain correspondingly a modulateddrain current Id= Id0+ id. Extracting from Eq. �3� the linearresponse in current due to modulation vgs yields

id = Rs−1�1 + �1 − �1 − 2Rs�Vgs0 − Vt��CiW/L�1/2�−1�−1vgs

�4a�

�vgsW/L�Ci�Vgs0 − Vt��1 − Rs�Vgs0 − Vt��CiW/L� , �4b�

where Eq. �4b� is an approximation valid for small Rs� ��Vgs0−Vt��CiW /L�−1. The combination of Eqs. �2� and�4b� results in the bandwidth prediction including contactresistance effects as

fT = 1/�2���V/L2�1 − RsWV�Ci/L�/�1 + Ap/�WL�� , �5�

with V= �Vgs0−Vt�. Equation �5� includes effects of parasiticgate capacitances �via Ap� as well as contact resistance ef-fects �via Rs�. As an example the width normalized contactresistance Rs�W has to stay below 7.5 k� mm to avoid10% bandwidth reduction, if we assume typical values forour transistors of V=10 V, L=0.5 �m, �=0.01 cm2/ �V s�,and Ci=67 nF/cm2. Obviously our devices show larger con-tact resistance than this low value.

In summary, an above megahertz operation submicronorganic field-effect transistor based on P3HT was presented,i.e., fT=2.0 MHz. To the best of our knowledge this is thehighest value reported for polymer transistors so far. Further-more we identified contact resistance as major problem to-wards even higher frequencies and presented a model de-scribing the influence of contact resistance and parasiticcapacitances on transistors unity-gain bandwidth.

The authors gratefully acknowledge financial support ofthe Deutsche Forschungsgemeinschaft �DFG� Project No.Wa 1039/2-2 within SP1121.

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