- Email: [email protected]
Novel applications of organic based thin film transistors
Microelectronics Reliability 40 (2000) 779±782
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Novel applications of organic based thin ®lm transistors Luisa Torsi Department of Chemistry, University of Bari, 4 via Orabona, I-70126 Bari, Italy
Abstract An overview of the main developments in the ®eld of organic based thin ®lm transistors is presented, and their operation is described, both when n-type and p-type channel organic transistor materials are employed as active layers. Perspective applications in gas sensors are discussed as well. Ó 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Polymers and organics in general, have been traditionally employed only as insulating materials until 1977, when the ®rst report on an electrically conducting polymer appeared [1]. Since then an intense activity leads to their successful use as active layers in devices such as light emitting diodes, thin ®lm transistors and many others. Several early reports on organic thin ®lm transistors (TFTs) appeared in the eighties (see e.g. Refs. [2±4]), but the idea that these devices may reach mobilities and the on/o ratios comparable to those of amorphous silicon consolidates only after the transistor action was demonstrated to occur in the molecular thin ®lms obtained by thermal evaporation of alphahexathyenilene (a-6T) oligomers [5]. A subsequent re®nement of the synthetic procedure and puri®cation of the products [6] as well as optimization of the device structure [7] allowed the routine fabrication of a-6T TFTs with mobilities of 0.01±0.03 cm2 /V s and the on/o ratios higher than 106 . More recently, p-channel TFTs materials such as pentacene, have been demonstrated to exhibit mobilities as high as 0.6 cm2 /V s [8,9], whereas n-channel materials such as C60 , as well as air more stable, 1,4,5,8naphthalene tetracarboxylic dianhydride (NTCDA) and copper hexadeca¯uorophthalocyanine (F16 CuPc) exhibit mobilities of 0.08 cm2 /V s [10], (0.001±0.003) cm2 /V s [11] and 0.03 cm2 /V s [12], respectively. After more than a decade from the ®rst demonstration of transistor action in organic materials, ®eld eect mobilities are still lower
E-mail address: [email protected] (L. Torsi).
than for amorphous silicon; nevertheless, organic TFT materials are considered appealing because of their unique property of being ¯exible [13] and processable at temperatures lower than those required in amorphous silicon technology. Both these elements make them compatible with a ¯exible plastic substrate, and applications are expected, in ®elds where ¯exibility as well as not-high TFTs switching speed are required.
2. Experimental A typical organic TFT structure is shown in Fig. 1a. A highly conductive Si wafer (resistivity 5±10 X cm) is used as substrate. For the a-6T TFTs, the gate dielectric is a SiO2 layer 300 nm thick (capacitance per unit area Ci 10 nF/cm2 ), whereas for the NTCDA, the gate oxide layer is composed of 200 nm of Si3 N4 plus 100 nm of SiO2 (Ci 20 nF/cm2 ). The silicon substrate with a gold ohmic contact functions as the gate, and the oxide as the gate dielectric. Gold source and drain contacts are photolithographically de®ned on the SiO2 such that W 250 lm is the channel width and L 4 lm and L 12 lm are the channel lengths for the NTCDA and a-6T TFTs, respectively. The chemical structures of the organic materials are reported in Fig. 1b. a-6T (synthesized as reported in Ref. [6]) and NTCDA (purchased from Aldrich and used without any further puri®cation) are sublimed under a vacuum of 10ÿ6 Torr directly onto the TFT substrate until a thin ®lm (about 50 nm) forms. Transistor I)V characteristics have been measured with a Hewlett Packard 4145B at room temperature and under vacuum, unless otherwise speci®ed.
0026-2714/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 6 - 2 7 1 4 ( 9 9 ) 0 0 2 8 4 - X
780
L. Torsi / Microelectronics Reliability 40 (2000) 779±782
Fig. 1. (a) Schematic of an organic TFT structure; (b) chemical structures of a p-channel (a-6T) and n-channel (NTCDA) organic TFT materials.
Fig. 2. (a) I±V characteristics of a L 12 lm channel a-6T and (b) L 4 lm NTCDA TFTs.
3. n- and p-channel organic TFTs Typical I)V characteristics for a-6T and NTCDA TFTs are shown in Fig. 2a and b, respectively. The ®rst set of curves show a linear regime at low drain±source voltages and a saturation regime at higher drain±source voltages. The saturation regime is also reached in the NTCDA TFT but at a comparatively lower gate±source voltages. This can be ascribed to short channel eects [14] occurring in the L 4 lm device. The a-6T TFT is a p-channel device, since negative gate biases are required to obtain increasing drain±source currents under negative drain±source voltages. Conversely, NTCDA TFT is an n-channel device. Organic TFTs are generally operated in the enhanced mode and in Fig. 3, the sketch of the energy band diagram evidences the formation of a two-dimensional [7] accumulation layer for both p and n-channel devices. Depletion-mode operation can be used to lower the o current for devices employing, not highly pure active layer materials. In fact, it is quite common that thermally evaporated organic thin ®lms are unintentionally p or n doped, and consequently, the source±drain current is not zero at zero gate bias. Only highly pure materials originate TFTs that are o already at zero gate bias [6]. Rapid thermal annealing of organic TFTs employing not highly pure materials has been demonstrated to lower the devices o current as well [15]. It is interesting to note that no inversion mode operation can be seen in polycrystalline organic TFTs [16]. The ®eld eect mobility (lFET ) for a-6T and NTCDA TFTs (see Fig. 2)
Fig. 3. Schematic energy-band diagram for a-6T (a) and NTCDA (b) TFTs. The left side shows the devices at no gate bias, whereas, on the right side, the p-channel device is negatively biased and the n-channel one is positively biased. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are shown for both materials as well.
L. Torsi / Microelectronics Reliability 40 (2000) 779±782
has been calculated in the saturated regime using the following equation [17]: IDS
WCi l VGS ÿ V0 2 ; 2L FET
where V0 is the threshold voltage and the other symbols have the usual meaning. An analytical model elaborated by Torsi et al. [14] can be used to extract mobility value also for short channel devices. The mobility for the a-6T TFT is comparable to that previously reported in the literature, whereas the NTCDA is about one order of magnitude lower. This can be explained considering that the substrate was not heated during the NTCDA deposition [11]. The switching speed for an organic TFT can be calculated, in ®rst approximation, following the equation: Ts
L2 : VDS lFET
Considering VDS 5 V, L 5 lm and lFET 10ÿ2 ± 10 cm2 /V s, Ts results to be 1±10 ls. In fact, this is the switching speed measured for an a-6T TFT [18]. This ®gure of merit along with on/o ratios better than 106 , have been induced to consider organic TFTs as interesting new devices for application in completely new products such as bendable plastic displays for futuristic TV sets and computers. This appealing perspective turned into a feasible scenario after the work by Dodabalapur et al. [18,19] in which it is demonstrated that a single heterostructure organic TFTs, can work both as an n- or a p-channel transistor. The employment of such devices in CMOS-like circuits will eventually allow one to exploit advantages such as low-power dissipation. The device structure is very simple, as can be seen in Fig. 4. Three layers of organic materials (a-6T, C60 and a-6T) are thermally evaporated without breaking the vacuum. The ®rst two layers are the active p- and n-type channel materials while the top a-6T ®lm acts, mainly, as a cap-layer protecting the highly moisture sensitive C60 . Other materials can be employed [20], as ÿ1
Fig. 4. Cross-sectional view of an a-6T/C60 heterojunction transistor.
781
long as their band alignment allows the formation of electron and hole accumulation layers, under appropriate gate bias; for the same reason the choice of the active layers thicknesses is crucial. In this device, both electrons and hole source±drain currents are measured with ®eld eect mobilities of about 10ÿ3 cm2 /V s [18]. In fact, when the gate is negatively biased with respect to the source, a current of holes ¯ows through the channel, whereas at a positive bias electrons are mainly accumulated at the interface with the gate dielectric. A further step towards low-cost, large area circuits with organic TFTs has been done thanks to the recent developments that comprise the fabrication of hybrid [21] as well as all organic [22] inverter circuits.
4. Organic TFT gas sensors Solid state semiconductor devices, such as Schottky barriers, MIS capacitors, MOSFETs and many others have been employed as gas sensing devices for more than two decades. The ®rst MOSFET device for hydrogen detection has been proposed in 1975 by Lundstrom et al. [23]. A schematic for the structure of such devices is shown in Fig. 5. Gas molecules interact with a sensitive gate material (palladium for hydrogen detection, for instance) inducing a change of the work function of the gate material that results in a shift of the device threshold voltage. Device operation temperature is usually higher than room temperature since catalytictype interaction between gate and volatile molecules are involved. Theoretical description of device operation mechanisms [24] as well as more complex device structure [25] have been proposed for the future. A dierent device operation is expected if an organic TFT is employed as gas sensor. This device con®guration allows the interaction of gases, as well as vapors, directly with the thin organic layer (see a schematic of this gas sensor structure shown in Fig. 6), producing changes of several device parameters. Few previous reports on the use of organic TFTs that have gas sensing devices have been already proposed [26±28], but they
Fig. 5. Schematic for the device structure of a gate gas sensitive MOSFET.
782
L. Torsi / Microelectronics Reliability 40 (2000) 779±782
Fig. 6. Cross-sectional view of an organic TFT gas sensor.
have never been exploited although they may present advantages such as the possibility of operating at room temperature. In fact, it is well known that most organic materials are gas sensitive already at room temperature, while inorganic metal oxides need to be heated up to 500±800°C. Moreover, the class of the organic materials that exhibits transistor action is becoming numerous allowing a wide choice for dierent organic materials/ volatile molecules interactions. This is important, considering that the revealing systems based on an array of dierent sensors, commonly called electronic noses, are considered a good way to obtain selectivity and sensitivity in gas detection. Further work is in progress in this new ®eld and interesting developments are expected. Acknowledgements Drs. A. Dodabalapur, H.E. Katz and Z. Bao of Bell Laboratories ± Lucent Technologies, Murray Hill (NJ), and Prof. L. Sabbatini and P.G. Zambonin of the Department of Chemistry ± University of Bari, are greatly acknowledged for useful discussions. Work partially carried out with the ®nancial support of ``Ministero della Ricerca Scienti®ca e Tecnologica'' (MURST) e Consiglio Nazionale delle Ricerche (CNR), Italy. References [1] Chiang CK, Fincher Jr. CR, Park YW, Heeger AJ, Shirakawa H, Lowis EJ, Gau SC, MecDiarmid AG. Phys Rev Lett 1977;39:1098.
[2] Tsumura A, Koezuka H, Ando K. Appl Phys Lett 1986; 49:1210±2. [3] Assadi A, Svensson M, Willander M, Inganas O. Appl Phys Lett 1988;53:195±7. [4] Madru M, Guillaud G, Al Saduon M, Maitrot M, Clarisse C, Le Contellec C, Andre JJ, Simon J. Chem Phys Lett 1987;142:103±5. [5] Horowitz G, Fichou D, Peng X, Xu Z, Garnier F. Solid State Comm 1989;72:381. [6] Kats HE, Torsi L, Dodabalapur A. Chem Mat 1995;7:2235. [7] Dodabalapur A, Torsi L, Katz HE. Science 1995; 268:270. [8] Lin YY, Gundlach DJ, Jackson TN. Science 1996; 273:879. [9] Liquindanum JG, Katz HE, Lovinger AJ, Dodabalapur A. Chem Mat 1996;8:2542. [10] Haddon RC, et al. Appl Phys Lett 1995;67:121. [11] Liquindanum JG, Katz HE, Dodabalapur A, Lovinger AJ. J Am Chem Soc 1996;118:11331. [12] Bao Z, Lovinger AJ, Brown J. J Am Chem Soc 1998; 120:207. [13] Garnier F, Hajlaoui R, Yassar A, Srivastava P. Science 1994;265:1684. [14] Torsi L, Dodabalapur A, Katz HE. J Appl Phys 1995;78:1088. [15] Torsi L, Dodabalapur A, Lovinger AJ, Katz HE, Ruel R, Davis DD, Baldwin KW. Chem Mat 1995;7:2247. [16] Horowitz G, Peng X, Fichou D, Garnier F. Appl Phys Lett 1990;67:528. [17] Sze S. Semiconductor devices physics and technology. New York: Wiley, 1985. p. 204±6. [18] Dodabalapur A, Katz HE, Torsi L, Haddon RC. Science 1995;269:1560. [19] Dodabalapur A, Katz HE, Torsi L, Haddon RC. Appl Phys Lett 1996;68:1108. [20] Dodabalapur A, Katz HE, Torsi L. Adv Mat 1996; 8:853. [21] Dodabalapur A, Baumbach J, Baldwin K, Katz HE. Appl Phys Lett 1996;68:2264. [22] Dodabalapur A, Liquindanum J, Katz HE, Bao Z. Appl Phys Lett 1996;69:4227. [23] Lundstrom I, Shivaraman MS, Svensson C. J Appl Phys 1975;46:3876. [24] Bergveld P. Sens Actuators 1985;8:109. [25] Blackburn GF, Levy M, Janada J. Appl Phys Lett 1983;43:700. [26] Laurs H, Heiland G. Thin Solid ®lms 1987;149:129. [27] Assadi A, Gustafsson G, Willander M, Svensson C, Inganas O. Synthetic Metals 1990;37:123±30. [28] Guillaud G, Simon J, Germain JP. Coordination Chemistry Reviews 1998;178±180:1433±84.
www.elsevier.com/locate/microrel
Novel applications of organic based thin ®lm transistors Luisa Torsi Department of Chemistry, University of Bari, 4 via Orabona, I-70126 Bari, Italy
Abstract An overview of the main developments in the ®eld of organic based thin ®lm transistors is presented, and their operation is described, both when n-type and p-type channel organic transistor materials are employed as active layers. Perspective applications in gas sensors are discussed as well. Ó 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Polymers and organics in general, have been traditionally employed only as insulating materials until 1977, when the ®rst report on an electrically conducting polymer appeared [1]. Since then an intense activity leads to their successful use as active layers in devices such as light emitting diodes, thin ®lm transistors and many others. Several early reports on organic thin ®lm transistors (TFTs) appeared in the eighties (see e.g. Refs. [2±4]), but the idea that these devices may reach mobilities and the on/o ratios comparable to those of amorphous silicon consolidates only after the transistor action was demonstrated to occur in the molecular thin ®lms obtained by thermal evaporation of alphahexathyenilene (a-6T) oligomers [5]. A subsequent re®nement of the synthetic procedure and puri®cation of the products [6] as well as optimization of the device structure [7] allowed the routine fabrication of a-6T TFTs with mobilities of 0.01±0.03 cm2 /V s and the on/o ratios higher than 106 . More recently, p-channel TFTs materials such as pentacene, have been demonstrated to exhibit mobilities as high as 0.6 cm2 /V s [8,9], whereas n-channel materials such as C60 , as well as air more stable, 1,4,5,8naphthalene tetracarboxylic dianhydride (NTCDA) and copper hexadeca¯uorophthalocyanine (F16 CuPc) exhibit mobilities of 0.08 cm2 /V s [10], (0.001±0.003) cm2 /V s [11] and 0.03 cm2 /V s [12], respectively. After more than a decade from the ®rst demonstration of transistor action in organic materials, ®eld eect mobilities are still lower
E-mail address: [email protected] (L. Torsi).
than for amorphous silicon; nevertheless, organic TFT materials are considered appealing because of their unique property of being ¯exible [13] and processable at temperatures lower than those required in amorphous silicon technology. Both these elements make them compatible with a ¯exible plastic substrate, and applications are expected, in ®elds where ¯exibility as well as not-high TFTs switching speed are required.
2. Experimental A typical organic TFT structure is shown in Fig. 1a. A highly conductive Si wafer (resistivity 5±10 X cm) is used as substrate. For the a-6T TFTs, the gate dielectric is a SiO2 layer 300 nm thick (capacitance per unit area Ci 10 nF/cm2 ), whereas for the NTCDA, the gate oxide layer is composed of 200 nm of Si3 N4 plus 100 nm of SiO2 (Ci 20 nF/cm2 ). The silicon substrate with a gold ohmic contact functions as the gate, and the oxide as the gate dielectric. Gold source and drain contacts are photolithographically de®ned on the SiO2 such that W 250 lm is the channel width and L 4 lm and L 12 lm are the channel lengths for the NTCDA and a-6T TFTs, respectively. The chemical structures of the organic materials are reported in Fig. 1b. a-6T (synthesized as reported in Ref. [6]) and NTCDA (purchased from Aldrich and used without any further puri®cation) are sublimed under a vacuum of 10ÿ6 Torr directly onto the TFT substrate until a thin ®lm (about 50 nm) forms. Transistor I)V characteristics have been measured with a Hewlett Packard 4145B at room temperature and under vacuum, unless otherwise speci®ed.
0026-2714/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 6 - 2 7 1 4 ( 9 9 ) 0 0 2 8 4 - X
780
L. Torsi / Microelectronics Reliability 40 (2000) 779±782
Fig. 1. (a) Schematic of an organic TFT structure; (b) chemical structures of a p-channel (a-6T) and n-channel (NTCDA) organic TFT materials.
Fig. 2. (a) I±V characteristics of a L 12 lm channel a-6T and (b) L 4 lm NTCDA TFTs.
3. n- and p-channel organic TFTs Typical I)V characteristics for a-6T and NTCDA TFTs are shown in Fig. 2a and b, respectively. The ®rst set of curves show a linear regime at low drain±source voltages and a saturation regime at higher drain±source voltages. The saturation regime is also reached in the NTCDA TFT but at a comparatively lower gate±source voltages. This can be ascribed to short channel eects [14] occurring in the L 4 lm device. The a-6T TFT is a p-channel device, since negative gate biases are required to obtain increasing drain±source currents under negative drain±source voltages. Conversely, NTCDA TFT is an n-channel device. Organic TFTs are generally operated in the enhanced mode and in Fig. 3, the sketch of the energy band diagram evidences the formation of a two-dimensional [7] accumulation layer for both p and n-channel devices. Depletion-mode operation can be used to lower the o current for devices employing, not highly pure active layer materials. In fact, it is quite common that thermally evaporated organic thin ®lms are unintentionally p or n doped, and consequently, the source±drain current is not zero at zero gate bias. Only highly pure materials originate TFTs that are o already at zero gate bias [6]. Rapid thermal annealing of organic TFTs employing not highly pure materials has been demonstrated to lower the devices o current as well [15]. It is interesting to note that no inversion mode operation can be seen in polycrystalline organic TFTs [16]. The ®eld eect mobility (lFET ) for a-6T and NTCDA TFTs (see Fig. 2)
Fig. 3. Schematic energy-band diagram for a-6T (a) and NTCDA (b) TFTs. The left side shows the devices at no gate bias, whereas, on the right side, the p-channel device is negatively biased and the n-channel one is positively biased. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are shown for both materials as well.
L. Torsi / Microelectronics Reliability 40 (2000) 779±782
has been calculated in the saturated regime using the following equation [17]: IDS
WCi l VGS ÿ V0 2 ; 2L FET
where V0 is the threshold voltage and the other symbols have the usual meaning. An analytical model elaborated by Torsi et al. [14] can be used to extract mobility value also for short channel devices. The mobility for the a-6T TFT is comparable to that previously reported in the literature, whereas the NTCDA is about one order of magnitude lower. This can be explained considering that the substrate was not heated during the NTCDA deposition [11]. The switching speed for an organic TFT can be calculated, in ®rst approximation, following the equation: Ts
L2 : VDS lFET
Considering VDS 5 V, L 5 lm and lFET 10ÿ2 ± 10 cm2 /V s, Ts results to be 1±10 ls. In fact, this is the switching speed measured for an a-6T TFT [18]. This ®gure of merit along with on/o ratios better than 106 , have been induced to consider organic TFTs as interesting new devices for application in completely new products such as bendable plastic displays for futuristic TV sets and computers. This appealing perspective turned into a feasible scenario after the work by Dodabalapur et al. [18,19] in which it is demonstrated that a single heterostructure organic TFTs, can work both as an n- or a p-channel transistor. The employment of such devices in CMOS-like circuits will eventually allow one to exploit advantages such as low-power dissipation. The device structure is very simple, as can be seen in Fig. 4. Three layers of organic materials (a-6T, C60 and a-6T) are thermally evaporated without breaking the vacuum. The ®rst two layers are the active p- and n-type channel materials while the top a-6T ®lm acts, mainly, as a cap-layer protecting the highly moisture sensitive C60 . Other materials can be employed [20], as ÿ1
Fig. 4. Cross-sectional view of an a-6T/C60 heterojunction transistor.
781
long as their band alignment allows the formation of electron and hole accumulation layers, under appropriate gate bias; for the same reason the choice of the active layers thicknesses is crucial. In this device, both electrons and hole source±drain currents are measured with ®eld eect mobilities of about 10ÿ3 cm2 /V s [18]. In fact, when the gate is negatively biased with respect to the source, a current of holes ¯ows through the channel, whereas at a positive bias electrons are mainly accumulated at the interface with the gate dielectric. A further step towards low-cost, large area circuits with organic TFTs has been done thanks to the recent developments that comprise the fabrication of hybrid [21] as well as all organic [22] inverter circuits.
4. Organic TFT gas sensors Solid state semiconductor devices, such as Schottky barriers, MIS capacitors, MOSFETs and many others have been employed as gas sensing devices for more than two decades. The ®rst MOSFET device for hydrogen detection has been proposed in 1975 by Lundstrom et al. [23]. A schematic for the structure of such devices is shown in Fig. 5. Gas molecules interact with a sensitive gate material (palladium for hydrogen detection, for instance) inducing a change of the work function of the gate material that results in a shift of the device threshold voltage. Device operation temperature is usually higher than room temperature since catalytictype interaction between gate and volatile molecules are involved. Theoretical description of device operation mechanisms [24] as well as more complex device structure [25] have been proposed for the future. A dierent device operation is expected if an organic TFT is employed as gas sensor. This device con®guration allows the interaction of gases, as well as vapors, directly with the thin organic layer (see a schematic of this gas sensor structure shown in Fig. 6), producing changes of several device parameters. Few previous reports on the use of organic TFTs that have gas sensing devices have been already proposed [26±28], but they
Fig. 5. Schematic for the device structure of a gate gas sensitive MOSFET.
782
L. Torsi / Microelectronics Reliability 40 (2000) 779±782
Fig. 6. Cross-sectional view of an organic TFT gas sensor.
have never been exploited although they may present advantages such as the possibility of operating at room temperature. In fact, it is well known that most organic materials are gas sensitive already at room temperature, while inorganic metal oxides need to be heated up to 500±800°C. Moreover, the class of the organic materials that exhibits transistor action is becoming numerous allowing a wide choice for dierent organic materials/ volatile molecules interactions. This is important, considering that the revealing systems based on an array of dierent sensors, commonly called electronic noses, are considered a good way to obtain selectivity and sensitivity in gas detection. Further work is in progress in this new ®eld and interesting developments are expected. Acknowledgements Drs. A. Dodabalapur, H.E. Katz and Z. Bao of Bell Laboratories ± Lucent Technologies, Murray Hill (NJ), and Prof. L. Sabbatini and P.G. Zambonin of the Department of Chemistry ± University of Bari, are greatly acknowledged for useful discussions. Work partially carried out with the ®nancial support of ``Ministero della Ricerca Scienti®ca e Tecnologica'' (MURST) e Consiglio Nazionale delle Ricerche (CNR), Italy. References [1] Chiang CK, Fincher Jr. CR, Park YW, Heeger AJ, Shirakawa H, Lowis EJ, Gau SC, MecDiarmid AG. Phys Rev Lett 1977;39:1098.
[2] Tsumura A, Koezuka H, Ando K. Appl Phys Lett 1986; 49:1210±2. [3] Assadi A, Svensson M, Willander M, Inganas O. Appl Phys Lett 1988;53:195±7. [4] Madru M, Guillaud G, Al Saduon M, Maitrot M, Clarisse C, Le Contellec C, Andre JJ, Simon J. Chem Phys Lett 1987;142:103±5. [5] Horowitz G, Fichou D, Peng X, Xu Z, Garnier F. Solid State Comm 1989;72:381. [6] Kats HE, Torsi L, Dodabalapur A. Chem Mat 1995;7:2235. [7] Dodabalapur A, Torsi L, Katz HE. Science 1995; 268:270. [8] Lin YY, Gundlach DJ, Jackson TN. Science 1996; 273:879. [9] Liquindanum JG, Katz HE, Lovinger AJ, Dodabalapur A. Chem Mat 1996;8:2542. [10] Haddon RC, et al. Appl Phys Lett 1995;67:121. [11] Liquindanum JG, Katz HE, Dodabalapur A, Lovinger AJ. J Am Chem Soc 1996;118:11331. [12] Bao Z, Lovinger AJ, Brown J. J Am Chem Soc 1998; 120:207. [13] Garnier F, Hajlaoui R, Yassar A, Srivastava P. Science 1994;265:1684. [14] Torsi L, Dodabalapur A, Katz HE. J Appl Phys 1995;78:1088. [15] Torsi L, Dodabalapur A, Lovinger AJ, Katz HE, Ruel R, Davis DD, Baldwin KW. Chem Mat 1995;7:2247. [16] Horowitz G, Peng X, Fichou D, Garnier F. Appl Phys Lett 1990;67:528. [17] Sze S. Semiconductor devices physics and technology. New York: Wiley, 1985. p. 204±6. [18] Dodabalapur A, Katz HE, Torsi L, Haddon RC. Science 1995;269:1560. [19] Dodabalapur A, Katz HE, Torsi L, Haddon RC. Appl Phys Lett 1996;68:1108. [20] Dodabalapur A, Katz HE, Torsi L. Adv Mat 1996; 8:853. [21] Dodabalapur A, Baumbach J, Baldwin K, Katz HE. Appl Phys Lett 1996;68:2264. [22] Dodabalapur A, Liquindanum J, Katz HE, Bao Z. Appl Phys Lett 1996;69:4227. [23] Lundstrom I, Shivaraman MS, Svensson C. J Appl Phys 1975;46:3876. [24] Bergveld P. Sens Actuators 1985;8:109. [25] Blackburn GF, Levy M, Janada J. Appl Phys Lett 1983;43:700. [26] Laurs H, Heiland G. Thin Solid ®lms 1987;149:129. [27] Assadi A, Gustafsson G, Willander M, Svensson C, Inganas O. Synthetic Metals 1990;37:123±30. [28] Guillaud G, Simon J, Germain JP. Coordination Chemistry Reviews 1998;178±180:1433±84.