Enhancement-mode polymer space-charge-limited transistor with low switching swing of 96 mV/decade

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Enhancement-mode polymer space-charge-limited transistor with low switching

swing of 96 mV/decade

Yu-Chiang Chao, Hung-Kuo Tsai, Hsiao-Wen Zan, Yung-Hsuan Hsu, Hsin-Fei Meng, and Sheng-Fu Horng

Citation: Applied Physics Letters 98, 223303 (2011); doi: 10.1063/1.3586255

View online: http://dx.doi.org/10.1063/1.3586255

View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/98/22?ver=pdfcov Published by the AIP Publishing

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Enhancement-mode polymer space-charge-limited transistor with low

switching swing of 96 mV/decade

Yu-Chiang Chao,1 Hung-Kuo Tsai,2 Hsiao-Wen Zan,3,a兲 Yung-Hsuan Hsu,1

Hsin-Fei Meng,1,a兲 and Sheng-Fu Horng4

1

Institute of Physics, National Chiao Tung University, Hsinchu 300, Taiwan 2

Institute of Photonics Technologies, National Tsing Hua University, Hsinchu 300, Taiwan 3

Department of Photonics and Institute of Electro-Optics, National Chiao Tung University, Hsinchu 300, Taiwan

4

Department of Electrical Engineering, National Tsing Hua University, Hsinchu 300, Taiwan

共Received 18 December 2010; accepted 12 April 2011; published online 31 May 2011兲

In this letter, an enhancement-mode polymer space-charge-limited transistor was realized with a low switching swing of 96 mV/decade, a low operation voltage of 1.5 V, and a high on/off current ratio of 104_{. By investigating the influence of the device’s geometric parameters on the transistor} characteristics, a low switching swing was obtained by positioning the base electrode at the middle of the channel length and reducing the opening diameter. Simulations of the potential distribution at the central vertical channel verified that the base electrode has the best control over the magnitude of potential barrier, resulting in a low switching swing. © 2011 American Institute of Physics. 关doi:10.1063/1.3586255兴

Research on solution-processed organic transistors has increased because organic transistors can be fabricated on a flexible substrate, serving as the key components of low-cost and large-area electronic devices.1,2 Operation voltage, on/ off current ratio, and subthreshold swing are the three impor-tant parameters commonly used to evaluate an organic

field-effect transistor 共OFET兲. Although conventional OFETs

usually show high on/off current ratios, most of them still require a large operation voltage because of the long channel length and the low carrier mobility in organic materials. The subthreshold swing, which describes the ease of switching the transistor using the gate bias is usually high for OFETs. The degraded subthreshold swing, which mainly results from interface and bulk traps,3 limits the operation frequency and the output current at a low operation voltage. Organic verti-cal transistors have recently attracted significant attention be-cause of their low operation voltages.4–12 The organic static induction transistor共SIT兲 is a well-known vertical transistor, which can achieve a high on/off current ratio of 103 _{at low} operation voltage of 3 V.7The SIT also delivers a high output current density of about 50 mA/cm2_.6

Another vertical

tran-sistor is the polymer space-charged-limited transistor

共SCLT兲, which is a three-terminal device that functions in a similar way to the vacuum tube triode.9–12The device struc-ture is shown in Fig.1共a兲. The carrier holes are injected into the semiconducting polymer by an emitter, passing through the openings on the base, and finally being collected by the collector. The on and off states of the SCLT are controlled by the magnitude of the potential barrier constructed between the emitter and the collector at the vertical channel.11Both the geometric structure of the device and the voltages applied to the base and the collector are crucial for potential barrier control. Because the SCLTs and OFETs operate according to different operation principles, the term switching swing is used here for SCLTs, similar to the way subthreshold swing

is used for OFETs. It has been demonstrated that 1.5 V is large enough to operate a SCLT with a high on/off current ratio of 105and a low switching swing of 300 mV/decade.11 However, the parameters that seriously influence the switch-ing swswitch-ings were never systematically studied for any vertical transistor. In addition, the normally on characteristics of these vertical transistors were frequently observed. A non-zero voltage was required to turn off the transistor, which made power consumption inevitable. A normally off vertical transistor therefore needs to be realized as a power-saving device.

In this letter, an enhancement-mode SCLT with a nor-mally off operation is realized. A switching swing as low as

a兲_{Authors to whom correspondence should be addressed. Electronic} ad-dresses: [email protected] and [email protected].

P3HT Glass ITO Al Al PVP

_L

_BE

L

_B

L

D

Emitter

Base

Collector

(a)

(b)

(c)

FIG. 1.共Color online兲共a兲 Schematic device structure of SCLT. The opening diameter is denoted by D, the channel length is denoted by L, and the base to emitter distance is denoted by LBE. The scanning electron microscope images of共b兲 100 nm and 共c兲 200 nm PS spheres on poly共4-vinyl phenol兲 are also shown. The insets show enlarged images.

APPLIED PHYSICS LETTERS 98, 223303共2011兲

0003-6951/2011/98_{共22兲/223303/3/$30.00} 98, 223303-1 © 2011 American Institute of Physics This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

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96 mV/decade is demonstrated by enhancing the base control on the magnitude of the potential barrier. Simulations of the potential distribution at the central vertical channel provide a reasonable explanation and verify the experimental results. This switching swing is the lowest of various vertical tran-sistors and is close to the theoretical limit of ln 10⫻kT/q

= 60 mV/decade in a silicon metal-oxide-semiconductor

field-effect transistor at room temperature.13 The realization of a normally off vertical transistor with a low operation voltage, low switching swing, and high on/off current ratio makes a high-speed and low-voltage operation circuit pos-sible.

Various devices were fabricated with different geometric parameters, including opening diameter 共D兲, channel length 共L兲, and base to emitter distance 共LBE兲, to investigate the influence of the device’ s geometric parameters on the tran-sistor’s characteristics. Devices were prepared on indium tin oxide glass substrates treated by 150 W oxygen plasma共RF兲 for 30 min. A layer of cross-linkable poly共4-vinyl phenol兲共PVP兲 was spin coated and annealed at 200 °C for 60 min.

Methylated poly共melamineco-formaldehyde兲共Aldrich, MW

= 511兲 was used as a cross-linking agent. A layer of 20 nm poly共3-hexylthiophene兲共P3HT兲 was then coated on the PVP and annealed at 200 ° C for 10 min. Xylene was used to spin rinse the P3HT to remove the soluble part and the P3HT left on the PVP was estimated to be 15 nm thick. The substrate

was then submerged into dilute ethanol solution关0.4% poly-styrene 共PS兲 spheres兴 of 100 or 200 nm diameter negatively

charged PS spheres 共Fluka兲 for 1 min to adsorb the PS

spheres on the thin P3HT surface as the shadow mask. The substrate was transferred to a beaker of boiling isopropanol solution for 10 s and immediately blow dried. After deposit-ing 40 nm of Al as the metal base electrode 共LB= 40 nm兲, the PS spheres were removed by adhesive tape共Scotch, 3M兲 and the polymer at the sites without Al coverage was

re-moved by 150 W O2 plasma 共RF兲. Various thickness of

P3HT was spin coated onto the substrate from the chloroben-zene solution and Al was then deposited to complete the SCLT with an active area of 1 mm2. LBE was controlled by the thickness of PVP layer and D was controlled by the di-ameter of the PS spheres. L was determined by the thickness of the P3HT layer prepared with the same recipe on another glass substrate.

A reliable comparison between the various SCLTs could only be obtained while the geometric parameters were well controlled. L and LBEcan be easily controlled by controlling the thicknesses of the P3HT and the PVP layers, respectively. However, randomly occurring aggregates of PS spheres re-sult in irregular and large openings on the base electrode.12 Therefore, as a first step, aggregates of PS spheres were pre-vented by modifying our previous fabrication processes.12 Reducing the concentration of the PS sphere ethanol solution and the substrate submerging time helped to prevent the for-mation of aggregates as shown in Figs. 1共b兲 and 1共c兲. The opening diameter D is now well controlled and the transistor characteristics are suitable for comparison.

The subthreshold region of a OFET shows how the de-vice is turned on and off and the subthreshold swing represents the switching speed. Here, similar to the formula 关␦共log IDS兲/␦共VGS兲兴−1 conventionally used for extracting

the subthreshold swing in the OFET, the formula

关␦共log JC兲/␦共VBE兲兴−1 is adopted to obtain the switching swings of the SCLTs from the transfer characteristics while the collector-to-emitter voltage共VCE兲 is fixed at ⫺1.5 V. JC, and VBE are the output current density and the base-to-emitter voltage of the SCLT while IDSand VGSare the drain current and the gate bias of the OFET. The formula 关␦共log JC兲/␦共VBE兲兴−1 describes the control ability of VBE with respect to JC. The switching swings of various SCLTs with different D, L, and LBEare shown in Fig.2. At least two devices, with an on/off current ratio of 104_{, were used to} obtain the average switching swing. Figure 2共a兲 shows the switching swings of the SCLTs with various L_BEwhile keep-ing L and D fixed at 440 nm and 200 nm, respectively. The

100

200

300

80

100

120

140

160

180

200 Swi

tc

h

in

g

Sw

in

g

(m

V/decade)

L

_BE

(nm)

D=200nm L=440nm

(a)

(b)

100

200

80

100

120

140

160

180

200 S

witchin

g

S

wi

ng

(m

V/decade)

D (nm)

L_EB=100nm L=250nm

FIG. 2. 共a兲 The switching swings of the SCLTs with various LBEwhile keeping L and D fixed at 440 nm and 200 nm, respectively.共b兲 The switch-ing swswitch-ings of the SCLTs for various values of D while keepswitch-ing L and LBE fixed at 250 nm and 100 nm, respectively.

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 10-6 10-5 10-4 10-3 10-2 10-1 100 VCE(V) = S.S. = 96 mV/decade JC (m A /c m 2 ) V_BE(V) -1.0 -1.5 S.S. = 108 mV/decade D=100nm L=250nm L BE=100nm

FIG. 3. 共Color online兲 Transfer characteristics of SCLT with D=100 nm, L = 250 nm, and LBEof 100 nm.

223303-2 Chao et al. Appl. Phys. Lett. 98, 223303共2011兲

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lowest switching swing was achieved for the device with an LBE of 200 nm. This means that the base electrode has the best control over the output current when it is positioned at the middle of the current channel. For another set of devices with L = 250 nm and D = 100 nm, the lowest switching swing was also obtained when the base electrode was posi-tioned at the middle of the current channel. Figure 2共b兲 shows the switching swings of the SCLTs with various D while keeping L and L_BE fixed at 250 nm and 100 nm, re-spectively. A lower switching swing is obtained while reduc-ing the openreduc-ing diameter D. Both positionreduc-ing the base elec-trode at the middle of the current channel and reducing the opening diameter are essential to obtain a low switching swing. As shown in Fig. 3, a low switching swing of 96 mV/decade is achieved for the device with D = 100 nm, L = 250 nm, and LBE= 100 nm. This device shows a normally off operation with a low operation voltage, low switching swing, and high on/off current ratio, which makes it possible to be used in a power-saving and high-speed device.

The change in the switching swing with different geo-metric structures of the device can be explained by the varia-tion in the potential distribuvaria-tion at the central vertical chan-nel. Simulations based on the device structure shown in Fig. 1共a兲 are carried out with SILVACO TCAD ATLAS software.

Simulation parameters are shown in Ref. 14. Figure 4共a兲 shows the potential distribution profiles at the central vertical channel of the devices with different LBE. The VCEis fixed at ⫺1.5 V. When VBE= 1 V, VBE, and VCEconstitute a

poten-tial barrier and the device is turned off. The device with an LBE of 100 nm shows the highest potential barrier. As the device is turned on by VBE= −1 V, the lowest potential is also obtained in the device with an LBE of 100 nm. The potential can be modulated to the largest extent in the device with an L_BEof 100 nm. This means that when the base elec-trode is positioned at the middle of the channel length, the base electrode has the best control over the output current. The device with a smaller D is also able to control the po-tential to a larger extent as shown in Fig.4共b兲. The change in simulated potential barrier gives a reasonable explanation of the change in switching swing shown in Fig.2.

In conclusion, an enhancement-mode vertical transistor is realized with a switching swing as low as 96 mV/decade. Such a low switching swing is obtained by placing the base electrode at the middle of the channel length and reducing the opening diameter. Simulations of the potential distribu-tion confirm the experimental results. The demonstradistribu-tion of a device with a low switching swing, low operation voltage, and high on/off current ratio, illustrates a possible applica-tion for high-speed and power-saving logic circuits.

This work was supported by the National Science Coun-cil of Taiwan under Contract No. NSC 99-2628-M-009-001.

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14_{The SCLT characteristics simulation is made based on the device structure} shown in Fig.1共a兲. Insulator surrounds the grid is used to make the model more realistic. The highest occupied molecular orbital and lowest unoccu-pied molecular orbital levels of P3HT are 5.0 and 3.0 eV. The work func-tion of emitter and collector are 5.0 and 4.0 eV. The hole mobility and electron mobility are 10−4_{and 10}−6 _cm2_{/V s.}

(a) (b) -0.05 0.00 0.05 0.10 0.15 0.20 0.25 0.30 -1.5 -1.0 -0.5 0.0 0.5 Collector L_BE(nm) = On State VBE= -1V 50 100 150 200 P o tent ia l( V ) Position (m) 50 100 150 200 Off State V_BE= 1V L_BE(nm) = ITO Al L=250nm D=100nm Emitter VCE=-1.5V -0.05 0.00 0.05 0.10 0.15 0.20 0.25 0.30 -1.5 -1.0 -0.5 0.0 0.5 Collector P o tent ia l(V) Position (m) L=250nm L_BE=100nm On State VBE= -1V ITO Al Off State VBE= 1V D (nm) = D (nm) = 100 200 300 100 200 300 Emitter VCE=-1.5V

FIG. 4. 共Color online兲 The potential distribution profiles at the central ver-tical channel of the SCLT for 共a兲 various values of LBEand 共b兲 various values of D.

223303-3 Chao et al. Appl. Phys. Lett. 98, 223303共2011兲