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Geometrical effect in submicrometer channel organic field effect transistors
Thin Solid Films 518 (2009) 579–582
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Geometrical effect in submicrometer channel organic field effect transistors Touichiro Goto a,⁎, Hiroshi Inokawa b, Keiichi Torimitsu a a b
NTT Basic Research Laboratories, NTT Corporation, 3-1 Morinosato Wakamiya, Atsugi, Kanagawa, 243-0198, Japan Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Hamamatsu, 432-8011, Japan
a r t i c l e
i n f o
Available online 10 July 2009 Keywords: Short-channel effects Organic field effect transistors Semiconducting polymers Threshold voltage Carrier mobility
a b s t r a c t The electrical behaviors of submicrometer bottom-gate bottom-contact organic field effect transistors (OFETs) with submicrometer channel lengths and channel widths were investigated. Short-channel effects (SCEs) were observed for devices with shorter channel lengths and wider channel widths. The SCEs were effectively suppressed by reducing the channel width to 50 nm. The relationship between the drain current density and the drain voltage normalized by their respective channel lengths revealed that the drain current characteristics of shorter length channels fall into two types: parasitic contact resistances at lower drain voltage and SCEs caused by the space charge limiting current at higher drain voltages. The carrier mobility was also investigated, and found to be enhanced in the narrower channel width. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Organic field effect transistors (OFETs) using semiconducting polymers are attractive for sensor [1,2], light emitting diode [3] and electronic paper [4] applications because of their low manufacturing cost and compatibility with plastic substrates [5]. Although the OFET performance differs depending the semiconducting polymer that is selected, it is generally poorer than that of inorganic materials, such as polycrystalline or amorphous silicon, and oxide semiconductors. An effective way of improving OFET performance is to reduce the channel length and gate insulator thickness. With this downscaling, OFETs can operate at a higher frequency and a lower voltage [6]. In contrast, the suppression of the short-channel effects (SCEs) [7] will be an issue if proper scaling cannot be realized. To achieve effective downscaling for improvement of performance and OFET integration, it would be very helpful to gain a better understanding of the effect of the layout pattern on the SCEs. A recent study using carbon thin film transistors with uniquely shaped channel areas suggests that both channel length and channel width have a significant effect on the electrical properties of thin film FETs [8]. To assess the SCEs exactly, channel width should be taken into consideration in addition to channel length. Although carrier mobility is a commonly used parameter for assessing OFETs, when the channel width is narrow in comparison to channel length, carrier mobility does not exhibit exact values. An alternative parameter for assessing SCEs is the threshold voltage. Threshold voltage is a key parameter as regards charge transport and is indicative of carrier trap density [7,9]. When SCEs occur, the magnitude of the threshold voltage decreases greatly [9]. Therefore, we used the threshold voltage to assess the SCEs. This report describes ⁎ Corresponding author. Tel.: +81 46 240 2096; fax: +81 270 2364. E-mail address: [email protected] (T. Goto). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.07.021
the characteristics of OFETs with an unconventional submicrometer channel shape, and discusses the effects of the geometry on the SCEs.
2. Experiment The bottom-contact OFETs were fabricated by spin-coating semiconducting polymers on metal electrodes. The electrodes have nominal channel lengths (L) of 200, 500, 800 and 1000 nm and nominal channel widths (W) of 50, 100, 500 and 1000 nm on a p-type silicon substrate oxidized to a thickness of 120 nm which correspond to the capacitance per unit area of 28 nF/cm2. The oxide and the substrate serve as a gate insulator and a gate electrode, respectively. The metals and their thicknesses for the electrodes are Au (20 nm)/Ti (3 nm), and those for the contact pads for external connection are Au (200 nm)/Ti (3 nm), respectively. Fig. 1(a) is a cross-sectional view of the submicrometer OFETs and Fig. 1(b) is a schematic view of the metal electrodes around the channels. These electrodes have long facing sides in addition to parallel sides, and are unique in that the channel current flows in both the L × W region and the surrounding area. Therefore, a significant geometrical effect could be seen in the OFET characteristics. As a semiconducting polymer, we employed the well-studied poly (3-hexylthiophene-2,5-diyl) (P3HT) [10] with head-to-tail regioregularity, which has been shown to provide better carrier mobilities than the regiorandom configuration (Fig. 1(c)). P3HT is a soluble p-type semiconducting polymer and forms highly ordered and selfassembled films [11]. To produce well-ordered polymer chains in the films, the electrode devices were cleaned with acetone and treated with hexamethyldisilazane (HMDS). After the treatment with HMDS, the P3HT was solved in hot dichloroethene and spin-coated (20 s at 2000 rpm) [12].
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Fig. 2 shows four sets of Id–Vd characteristics of OFETs with (L, W) values of (200, 50) to (1000, 1000) for various gate voltages Vg. Here, we compare the characteristics for L = 200 and 1000 nm to reveal the SCEs. The characteristics for L = 1000 nm are typical FETs: an off-state (Vg = 0 V) leakage current of less than −100 fA at Vd = −20 V, an on/ off ratio more than 103 for Vg = − 40/0 V, and a clear saturation region. At (L, W) = (200, 1000), an off-state leakage current of −80 nA was observed, and the on/off ratio was only 12. Moreover, the saturation region did not appear clearly. These characteristics suggest the SCEs are conspicuous. At (L, W) = (200, 50), the off-state leakage current decreased to −70 nA and the on/off ratio increased to 25, indicating less severe SCEs. We also measured the depletion mode
(Vg = + 10 V) for L = 200 nm and observed leakage current at Vd = −20 V was slightly suppressed from −80 to −50 nA. Fig. 3 shows the Vth–Vd characteristics of four combinations of L and W. Vth is defined as the gate voltage extrapolated to Id = 0 A for Id–Vg characteristics in the linear region [13]. As shown in Fig. 3, the magnitude of Vth decreases as the magnitude of Vd increases. In particular, at (L, W) = (200, 1000), Vth varied about 12 V and that of the magnitude decreased to only −1 V as Vd varied from = − 5 to −20 V. Fig. 4 shows the Vth–L characteristics of W = 50 and 1000 nm for Vd = −5 and −20 V, respectively. With a fixed Vd, Vth in magnitude increased with increases in L. For Vd = −20 V, the Vth values at W = 1000 nm varied significantly by 9 V from L = 200 to 1000 nm, and the Vth value at L = 200 nm varied by about −1 V. In contrast, at W = 50 nm, the change in Vth was within 3 V. For Vd = − 5 V, the changes in Vth at W = 50 and 1000 nm were less than 3 V. These characteristics indicate the appearance of the SCEs because Vth changed greatly at shorter channel length regions and only slightly at longer regions. Figs. 3 and 4 also indicate that the narrow channel width (W = 50 nm) effectively suppresses SCEs. Torsi et al. suggest that when the magnitude of a trapped charge is high, the magnitude of the threshold voltage is large [9]. In light of their study, it is possible that Figs. 3 and 4 indicate that in a channel area with a wide channel width, the longitudinal electric field reduces the trapped charge because the electric field is evenly applied. As a result, reductions in the trapped charge cause reductions in the magnitude of the threshold voltage and the appearance of SCEs. On the other hand, in a channel area with a narrow channel width, the electric field is not applied evenly, and therefore the trapped charge does not decrease and threshold voltage does not change greatly. However, the relationship between the trapped charge at various channel widths and the threshold voltage requires further study. Although the absence of a saturation region and the appearance of a leak current at L = 200 nm in Fig. 2 indicates the existence of SCEs in a short-channel length and a wide channel width, the SCEs and Id–Vd characteristics are still not clearly understood. To determine the reason for these characteristics, we investigated the relationship between the drain current density (Id/W) and the drain voltage normalized by their respective channel lengths (Vd/L) [14]. Fig. 5 shows the Id/W–Vd/L characteristics at Vg = − 10 V for shorter channel length (L = 200 nm) and −40 V for longer channel length (L = 500, 800, 1000 nm, W = 50
Fig. 2. Id–Vd characteristics of submicrometer OFETs with channel lengths L and widths W; (L nm, W nm) = (a) (200, 50), (b) (1000, 50), (c) (200, 1000) and (d) (1000, 1000) for various gate voltages Vg.
Fig. 3. Vth–Vd characteristics at channel length L and width W; (L nm, W nm) = (a) (200, 50), (b) (1000, 50), (c) (200, 1000) and (d) (1000, 1000). The lines indicate least squares estimates of the characteristics.
Fig. 1. (a) Cross-sectional view of submicrometer OFETs. (b) Schematic view of the metal electrodes around the channel for submicrometer devices with nominal channel lengths (L) of 200, 500, 800 and 1000 nm and nominal channel width (W) of 50, 100, 500 and 1000 nm. (c) Structure of P3HT polymer.
The electrical characteristics of the submicrometer OFETs were measured in a vacuum chamber under 5 × 10 − 4 Pa at room temperature. 3. Results and discussion
T. Goto et al. / Thin Solid Films 518 (2009) 579–582
Fig. 4. Vth–L characteristics of (a) W = 50 nm and (b) W = 1000 nm for Vd = − 20 V, (c) W = 50 nm and (d) W = 1000 nm for Vd = − 5 V, respectively. The lines indicate least squares estimates of the characteristics.
and 1000 nm), respectively. Before Id/W enter saturation, longer channel lengths exhibit a higher current density for a given Vd/L, and the characteristics for each channel length are almost the same (Fig. 5 (a)). These characteristics suggest that the parasitic contact resistances dominate the Id–Vd characteristics. According to Blom et al. [15], the electrical properties of semiconducting polymers at high drain voltages exhibit a space charge limiting current (SCLC) relationship Id ∝ Vnd, where typically n ≧ 2. Fig. 5(b) shows n = 2.7 and 2.2 for (L, W) = (200, 50) and (200, 1000), respectively, and reveals that SCEs occur at short-channel
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lengths owing to SCLC. In Fig. 5(b), SCLC appears at Vd/L = 50 and 100 V/µm for W = 1000 and 50 nm, respectively and it means that the suppression of SCEs by reducing channel width is confirmed in terms of the drain current density. Therefore, the Id–Vd characteristics of OFETs have two regions. At lower drain voltages and higher gate voltages, the characteristics derive from the parasitic contact resistance, and at higher drain voltages and shorter channel lengths, they derive from the SCEs. These results agree with those reported by Austin et al. [14]. We also calculated the carrier mobility from the Id–Vg characteristics in the linear region [13]. Since some of the current flows outside the L × W region, we corrected the proportionality factor in the Id expression, W/L, based on a numerical simulation of the channel conductance assuming that the sheet resistance of the polymer film was uniform. A numerical simulation was first performed with the FastCap capacitance extraction program [16], and then the capacitance was converted to the conductance. Fig. 6 shows the carrier mobility µchannel length L characteristics for W = 50 and 1000 nm at Vd = −5 V. The carrier mobilities of our devices are in the same range as previous reports [11]. Although the W = 50 nm plots spread slightly, the carrier mobilities are almost independent of channel length. The carrier mobility values for W = 50 nm are about twice those for W = 1000 nm. This result may be due to the strong electric field at the channel edge for W = 50 nm, resulting in the efficient carrier injection and increase in the nominal mobility. 4. Conclusions We fabricated bottom-gate bottom-contact OFETs with a unique electrode arrangement, where current flows not only in the channel region but also in the surrounding area. The drain current–drain voltage characteristics and channel length dependence of the threshold voltage were evaluated in terms of short-channel effects. We found that the short-channel effects were significantly suppressed by reducing the channel width. We also concluded from the Id/W–Vd/L characteristics that the parasitic contact resistances dominate the Id– Vd characteristics at lower drain voltages and higher gate voltages, and the space charge limiting current characterized by SCEs at higher drain voltages and shorter channel lengths. In addition, we evaluated the carrier mobility with the correction of the current flowing outside of the L × W channel region. Although the mobility was almost independent of the channel length, the narrower channel width showed a nominally higher mobility. These results clearly indicated that the behaviors of the OFET may be significantly modified by the geometrical effects related to the electrode arrangement. Acknowledgements We thank Toshiaki Tamamura and Kazuhito Inokuma (NTT Advanced Technology Corporation) and Toru Yamaguchi, Junzo Hayashi and Katsuhiko Nishiguchi for support in the fabrication of
Fig. 5. Characteristics of the drain current density (Id/W) and the drain voltage normalized by the respective channel lengths (Vd/L). (a) L = 500, 800, 1000 nm, Vg = − 40 V and (b) L = 200 nm, Vg = − 10 V.
Fig. 6. µ–L characteristics of (a) W = 50 and (b) 1000 nm for Vd = − 5 V. The lines indicate average estimates of the plots.
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the electrode devices. We also thank Akira Fujiwara and Yukinori Ono for support in the electrical measurements of the OFETs and Yoshiaki Kashimura for constructive discussions. References [1] Z.T. Zhu, J.T. Mason, R. Dieckmann, G.G. Malliaras, Appl. Phys. Lett. 81 (2002) 4643. [2] B.K. Crone, A. Dodabalapur, R. Sarpeshkar, A. Gelperin, H.E. Katz, Z. Bao, J. Appl. Phys. 91 (2002) 10140. [3] H. Sirringhaus, N. Tessler, R.H. Friend, Science 280 (1998) 1741. [4] C.D. Sheraw, L. Zhou, J.R. Huang, D.J. Gundlach, T.N. Jackson, M.G. Kane, I.G. Hill, M. S. Hammond, J. Campi, B.K. Greenning, J. Frank, J. West, Appl. Phys. Lett. 80 (2002) 1088.
[5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
F. Garnier, R. Hajlaoui, A. Yassar, P. Srivastava, Science 265 (1994) 1685. J. Collet, O. Tharaud, A. Chapoton, D. Vuillaume, Appl. Phys. Lett. 76 (2000) 1941. L. Torsi, A. Dodabalapur, H.E. Katz, J. Appl. Phys. 78 (1995) 1088. H. Inokawa, M. Nagase, S. Hirono, T. Goto, H. Yamaguchi, K. Torimitsu, Jpn. J. Appl. Phys. 46 (2007) 2615. L. Torsi, A. Dodabalapur, L.J. Rothberg, A.W.P. Fung, H.E. Katz, Phys. Rev. B 57 (1998) 2271. T.A. Chen, X. Wu, R.D. Rieke, J. Am. Chem. Soc. 117 (1995) 233. Z. Bao, A. Dodabalapur, A.J. Lovinger, Appl. Phys. Lett. 69 (1996) 4108. H. Yang, S.W. LeFevre, C.Y. Ryu, Z. Bao, Appl. Phys. Lett. 90 (2007) 172116. T. Mori, J. Phys.: Condens. Matter 20 (2008) 184010. M.D. Austin, S.Y. Chou, Appl. Phys. Lett. 81 (2002) 4431. P.W. Blom, M.J.M. de Jong, J.J.M. Vleggaar, Appl. Phys. Lett. 68 (1996) 3308. K. Nabor, J. White, IEEE Trans. Computer-Aided Design 10 (1991) 1447.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Geometrical effect in submicrometer channel organic field effect transistors Touichiro Goto a,⁎, Hiroshi Inokawa b, Keiichi Torimitsu a a b
NTT Basic Research Laboratories, NTT Corporation, 3-1 Morinosato Wakamiya, Atsugi, Kanagawa, 243-0198, Japan Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Hamamatsu, 432-8011, Japan
a r t i c l e
i n f o
Available online 10 July 2009 Keywords: Short-channel effects Organic field effect transistors Semiconducting polymers Threshold voltage Carrier mobility
a b s t r a c t The electrical behaviors of submicrometer bottom-gate bottom-contact organic field effect transistors (OFETs) with submicrometer channel lengths and channel widths were investigated. Short-channel effects (SCEs) were observed for devices with shorter channel lengths and wider channel widths. The SCEs were effectively suppressed by reducing the channel width to 50 nm. The relationship between the drain current density and the drain voltage normalized by their respective channel lengths revealed that the drain current characteristics of shorter length channels fall into two types: parasitic contact resistances at lower drain voltage and SCEs caused by the space charge limiting current at higher drain voltages. The carrier mobility was also investigated, and found to be enhanced in the narrower channel width. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Organic field effect transistors (OFETs) using semiconducting polymers are attractive for sensor [1,2], light emitting diode [3] and electronic paper [4] applications because of their low manufacturing cost and compatibility with plastic substrates [5]. Although the OFET performance differs depending the semiconducting polymer that is selected, it is generally poorer than that of inorganic materials, such as polycrystalline or amorphous silicon, and oxide semiconductors. An effective way of improving OFET performance is to reduce the channel length and gate insulator thickness. With this downscaling, OFETs can operate at a higher frequency and a lower voltage [6]. In contrast, the suppression of the short-channel effects (SCEs) [7] will be an issue if proper scaling cannot be realized. To achieve effective downscaling for improvement of performance and OFET integration, it would be very helpful to gain a better understanding of the effect of the layout pattern on the SCEs. A recent study using carbon thin film transistors with uniquely shaped channel areas suggests that both channel length and channel width have a significant effect on the electrical properties of thin film FETs [8]. To assess the SCEs exactly, channel width should be taken into consideration in addition to channel length. Although carrier mobility is a commonly used parameter for assessing OFETs, when the channel width is narrow in comparison to channel length, carrier mobility does not exhibit exact values. An alternative parameter for assessing SCEs is the threshold voltage. Threshold voltage is a key parameter as regards charge transport and is indicative of carrier trap density [7,9]. When SCEs occur, the magnitude of the threshold voltage decreases greatly [9]. Therefore, we used the threshold voltage to assess the SCEs. This report describes ⁎ Corresponding author. Tel.: +81 46 240 2096; fax: +81 270 2364. E-mail address: [email protected] (T. Goto). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.07.021
the characteristics of OFETs with an unconventional submicrometer channel shape, and discusses the effects of the geometry on the SCEs.
2. Experiment The bottom-contact OFETs were fabricated by spin-coating semiconducting polymers on metal electrodes. The electrodes have nominal channel lengths (L) of 200, 500, 800 and 1000 nm and nominal channel widths (W) of 50, 100, 500 and 1000 nm on a p-type silicon substrate oxidized to a thickness of 120 nm which correspond to the capacitance per unit area of 28 nF/cm2. The oxide and the substrate serve as a gate insulator and a gate electrode, respectively. The metals and their thicknesses for the electrodes are Au (20 nm)/Ti (3 nm), and those for the contact pads for external connection are Au (200 nm)/Ti (3 nm), respectively. Fig. 1(a) is a cross-sectional view of the submicrometer OFETs and Fig. 1(b) is a schematic view of the metal electrodes around the channels. These electrodes have long facing sides in addition to parallel sides, and are unique in that the channel current flows in both the L × W region and the surrounding area. Therefore, a significant geometrical effect could be seen in the OFET characteristics. As a semiconducting polymer, we employed the well-studied poly (3-hexylthiophene-2,5-diyl) (P3HT) [10] with head-to-tail regioregularity, which has been shown to provide better carrier mobilities than the regiorandom configuration (Fig. 1(c)). P3HT is a soluble p-type semiconducting polymer and forms highly ordered and selfassembled films [11]. To produce well-ordered polymer chains in the films, the electrode devices were cleaned with acetone and treated with hexamethyldisilazane (HMDS). After the treatment with HMDS, the P3HT was solved in hot dichloroethene and spin-coated (20 s at 2000 rpm) [12].
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Fig. 2 shows four sets of Id–Vd characteristics of OFETs with (L, W) values of (200, 50) to (1000, 1000) for various gate voltages Vg. Here, we compare the characteristics for L = 200 and 1000 nm to reveal the SCEs. The characteristics for L = 1000 nm are typical FETs: an off-state (Vg = 0 V) leakage current of less than −100 fA at Vd = −20 V, an on/ off ratio more than 103 for Vg = − 40/0 V, and a clear saturation region. At (L, W) = (200, 1000), an off-state leakage current of −80 nA was observed, and the on/off ratio was only 12. Moreover, the saturation region did not appear clearly. These characteristics suggest the SCEs are conspicuous. At (L, W) = (200, 50), the off-state leakage current decreased to −70 nA and the on/off ratio increased to 25, indicating less severe SCEs. We also measured the depletion mode
(Vg = + 10 V) for L = 200 nm and observed leakage current at Vd = −20 V was slightly suppressed from −80 to −50 nA. Fig. 3 shows the Vth–Vd characteristics of four combinations of L and W. Vth is defined as the gate voltage extrapolated to Id = 0 A for Id–Vg characteristics in the linear region [13]. As shown in Fig. 3, the magnitude of Vth decreases as the magnitude of Vd increases. In particular, at (L, W) = (200, 1000), Vth varied about 12 V and that of the magnitude decreased to only −1 V as Vd varied from = − 5 to −20 V. Fig. 4 shows the Vth–L characteristics of W = 50 and 1000 nm for Vd = −5 and −20 V, respectively. With a fixed Vd, Vth in magnitude increased with increases in L. For Vd = −20 V, the Vth values at W = 1000 nm varied significantly by 9 V from L = 200 to 1000 nm, and the Vth value at L = 200 nm varied by about −1 V. In contrast, at W = 50 nm, the change in Vth was within 3 V. For Vd = − 5 V, the changes in Vth at W = 50 and 1000 nm were less than 3 V. These characteristics indicate the appearance of the SCEs because Vth changed greatly at shorter channel length regions and only slightly at longer regions. Figs. 3 and 4 also indicate that the narrow channel width (W = 50 nm) effectively suppresses SCEs. Torsi et al. suggest that when the magnitude of a trapped charge is high, the magnitude of the threshold voltage is large [9]. In light of their study, it is possible that Figs. 3 and 4 indicate that in a channel area with a wide channel width, the longitudinal electric field reduces the trapped charge because the electric field is evenly applied. As a result, reductions in the trapped charge cause reductions in the magnitude of the threshold voltage and the appearance of SCEs. On the other hand, in a channel area with a narrow channel width, the electric field is not applied evenly, and therefore the trapped charge does not decrease and threshold voltage does not change greatly. However, the relationship between the trapped charge at various channel widths and the threshold voltage requires further study. Although the absence of a saturation region and the appearance of a leak current at L = 200 nm in Fig. 2 indicates the existence of SCEs in a short-channel length and a wide channel width, the SCEs and Id–Vd characteristics are still not clearly understood. To determine the reason for these characteristics, we investigated the relationship between the drain current density (Id/W) and the drain voltage normalized by their respective channel lengths (Vd/L) [14]. Fig. 5 shows the Id/W–Vd/L characteristics at Vg = − 10 V for shorter channel length (L = 200 nm) and −40 V for longer channel length (L = 500, 800, 1000 nm, W = 50
Fig. 2. Id–Vd characteristics of submicrometer OFETs with channel lengths L and widths W; (L nm, W nm) = (a) (200, 50), (b) (1000, 50), (c) (200, 1000) and (d) (1000, 1000) for various gate voltages Vg.
Fig. 3. Vth–Vd characteristics at channel length L and width W; (L nm, W nm) = (a) (200, 50), (b) (1000, 50), (c) (200, 1000) and (d) (1000, 1000). The lines indicate least squares estimates of the characteristics.
Fig. 1. (a) Cross-sectional view of submicrometer OFETs. (b) Schematic view of the metal electrodes around the channel for submicrometer devices with nominal channel lengths (L) of 200, 500, 800 and 1000 nm and nominal channel width (W) of 50, 100, 500 and 1000 nm. (c) Structure of P3HT polymer.
The electrical characteristics of the submicrometer OFETs were measured in a vacuum chamber under 5 × 10 − 4 Pa at room temperature. 3. Results and discussion
T. Goto et al. / Thin Solid Films 518 (2009) 579–582
Fig. 4. Vth–L characteristics of (a) W = 50 nm and (b) W = 1000 nm for Vd = − 20 V, (c) W = 50 nm and (d) W = 1000 nm for Vd = − 5 V, respectively. The lines indicate least squares estimates of the characteristics.
and 1000 nm), respectively. Before Id/W enter saturation, longer channel lengths exhibit a higher current density for a given Vd/L, and the characteristics for each channel length are almost the same (Fig. 5 (a)). These characteristics suggest that the parasitic contact resistances dominate the Id–Vd characteristics. According to Blom et al. [15], the electrical properties of semiconducting polymers at high drain voltages exhibit a space charge limiting current (SCLC) relationship Id ∝ Vnd, where typically n ≧ 2. Fig. 5(b) shows n = 2.7 and 2.2 for (L, W) = (200, 50) and (200, 1000), respectively, and reveals that SCEs occur at short-channel
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lengths owing to SCLC. In Fig. 5(b), SCLC appears at Vd/L = 50 and 100 V/µm for W = 1000 and 50 nm, respectively and it means that the suppression of SCEs by reducing channel width is confirmed in terms of the drain current density. Therefore, the Id–Vd characteristics of OFETs have two regions. At lower drain voltages and higher gate voltages, the characteristics derive from the parasitic contact resistance, and at higher drain voltages and shorter channel lengths, they derive from the SCEs. These results agree with those reported by Austin et al. [14]. We also calculated the carrier mobility from the Id–Vg characteristics in the linear region [13]. Since some of the current flows outside the L × W region, we corrected the proportionality factor in the Id expression, W/L, based on a numerical simulation of the channel conductance assuming that the sheet resistance of the polymer film was uniform. A numerical simulation was first performed with the FastCap capacitance extraction program [16], and then the capacitance was converted to the conductance. Fig. 6 shows the carrier mobility µchannel length L characteristics for W = 50 and 1000 nm at Vd = −5 V. The carrier mobilities of our devices are in the same range as previous reports [11]. Although the W = 50 nm plots spread slightly, the carrier mobilities are almost independent of channel length. The carrier mobility values for W = 50 nm are about twice those for W = 1000 nm. This result may be due to the strong electric field at the channel edge for W = 50 nm, resulting in the efficient carrier injection and increase in the nominal mobility. 4. Conclusions We fabricated bottom-gate bottom-contact OFETs with a unique electrode arrangement, where current flows not only in the channel region but also in the surrounding area. The drain current–drain voltage characteristics and channel length dependence of the threshold voltage were evaluated in terms of short-channel effects. We found that the short-channel effects were significantly suppressed by reducing the channel width. We also concluded from the Id/W–Vd/L characteristics that the parasitic contact resistances dominate the Id– Vd characteristics at lower drain voltages and higher gate voltages, and the space charge limiting current characterized by SCEs at higher drain voltages and shorter channel lengths. In addition, we evaluated the carrier mobility with the correction of the current flowing outside of the L × W channel region. Although the mobility was almost independent of the channel length, the narrower channel width showed a nominally higher mobility. These results clearly indicated that the behaviors of the OFET may be significantly modified by the geometrical effects related to the electrode arrangement. Acknowledgements We thank Toshiaki Tamamura and Kazuhito Inokuma (NTT Advanced Technology Corporation) and Toru Yamaguchi, Junzo Hayashi and Katsuhiko Nishiguchi for support in the fabrication of
Fig. 5. Characteristics of the drain current density (Id/W) and the drain voltage normalized by the respective channel lengths (Vd/L). (a) L = 500, 800, 1000 nm, Vg = − 40 V and (b) L = 200 nm, Vg = − 10 V.
Fig. 6. µ–L characteristics of (a) W = 50 and (b) 1000 nm for Vd = − 5 V. The lines indicate average estimates of the plots.
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the electrode devices. We also thank Akira Fujiwara and Yukinori Ono for support in the electrical measurements of the OFETs and Yoshiaki Kashimura for constructive discussions. References [1] Z.T. Zhu, J.T. Mason, R. Dieckmann, G.G. Malliaras, Appl. Phys. Lett. 81 (2002) 4643. [2] B.K. Crone, A. Dodabalapur, R. Sarpeshkar, A. Gelperin, H.E. Katz, Z. Bao, J. Appl. Phys. 91 (2002) 10140. [3] H. Sirringhaus, N. Tessler, R.H. Friend, Science 280 (1998) 1741. [4] C.D. Sheraw, L. Zhou, J.R. Huang, D.J. Gundlach, T.N. Jackson, M.G. Kane, I.G. Hill, M. S. Hammond, J. Campi, B.K. Greenning, J. Frank, J. West, Appl. Phys. Lett. 80 (2002) 1088.
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