Elsevier

Organic Electronics

Volume 11, Issue 12, December 2010, Pages 1935-1941
Organic Electronics

Fast polymer semiconductor transistor by nano-particle self assembly

https://doi.org/10.1016/j.orgel.2010.08.012Get rights and content

Abstract

Polymer semiconductor based field effect transistors promise low cost electronics over rigid and flexible large area substrates using fabrication techniques such as ink-jet printing. However, the low mobility of carriers in these semiconductors does not permit high performance electronics. A primary means to achieve higher speed in spite of the low carrier mobility is to reduce feature size, particularly channel length, of the field effect transistors. In this paper, we use the controlled coffee stain effect in a silver nano-particle colloid along with ink-jet printing to develop a process which helps reduce field effect transistor channel length to about 1–10 μm and improve transistor speed.

Introduction

Solution processed polymer semiconductor based field effect transistors (FETs) are of interest for applications in large area electronic systems. Ink-jet printing is one of the most favored techniques to fabricate solution processed electronics [1], [2], [3], [4], [5].

Low temperature fabrication of solution processed electronics promises low cost electronics over large areas on flexible substrates. However, polymer semiconductors typically have a low mobility, μ (≈0.001–1 cmsq/Vs) thereby resulting in poor performance of FETs.

The transconductance of a FET, gm is μCiW/L where Ci is the gate dielectric capacitance per unit area, W and L are the channel width and channel length, respectively. In order to improve the transconductance with poor mobility, one can increase Ciϵ/d by either using a dielectric with a large dielectric constant, ϵ, or reducing the thickness of the dielectric, d. Non-polar dielectrics used in solution processed electronics typically have a low ϵ25. Polar dielectrics have a larger ϵ, but the use of such dielectrics results in hysteresis during transistor operation. A reduction in d in order to increase Ci, has to contend with the possibility of pinholes and shorting between the gate and source/drain metals. The only remaining possibility of improving gm is by increasing the aspect ratio of the FET, W/L. Increasing channel width, W, is not a good option since it also increases the parasitic capacitances. Thus, decreasing the channel length L is the only passive option to improving transconductance in solution processed FETs. In order to decrease L we are dependent on the feature size of the fabrication technique. Typical feature sizes obtained with ink-jet printing are of the order of ≈40 μm with some self-assembly techniques being an exception [2]. Giving allowances for flows and overlaps, reliable electronic devices typically have larger feature sizes (approaching 100 μm). In order to obtain a 10-fold increase in the effective gm, the channel length of the FET must be ≈10 μm or less.

Using a colloidal suspension of conductive nano-particles, we illustrate the use of the coffee stain effect to pattern fine feature electrodes and polymer semiconductor based FETs of channel length ≈10 μm or smaller. The self-assembly technique discussed is used in conjunction with ink-jet printing to make possible smaller features.

Section snippets

Coffee stain phenomena

We first study the physics of the coffee stain phenomena and develop concepts which will be used to develop FETs [6], [7], [8]. Consider a small (influence of gravity is negligible) sessile droplet of colloidal suspension of nano-particles placed on a substrate. Imagine the colloid to partially wet the surface and have its boundaries pinned to the surface as shown in Fig. 1a. The contact angle, θ, of this droplet is governed by the Young–Drupe relationship, γlvcos(θ)+γsl-γvs=0 where γlv, γsl,

Experiment

We use the phenomena described above to self-assemble high transconductance solution processed field effect transistors. The technique proposed is inspired from the geometrical aspects of surface tension induced droplet behavior and many of the earlier self-assembly techniques [20], [21], [22], [23], [24]. The setup of the fabrication is demonstrated in Fig. 2a where a straight wetting edge is brought in contact with a colloid of silver nano-particles in de-ionized water resting on a wetting

Transistor fabrication and testing

In this section, we discuss the fabrication and test of a prototype small channel thin film transistor fabricated using a combination of printing and the coffee stain based self-assembly process discussed earlier. The aim of this section is to identify the typical performance of the prototype transistor. In order to fabricate the prototype transistor, we use a doped silicon substrate as the gate electrode, with oxide dielectric. The fabrication process for the prototype is illustrated in Fig. 3

Fast process flow

In the previous section, we discussed the fabrication of a prototype transistor using a doped silicon gate. The drain and source contacts were created using a combination of self assembly, ink-jet printing, and laser ablation. However, laser ablation is a rate limiting step. In this section we demonstrate how the self-assembly process to achieve short channel lengths can be used alongside ink-jet printing to achieve a faster process flow.

Fig. 6 shows the steps to fabricate a transistor without

Pitch variation

The pitch of nano-particle deposits decrease as the deposits get closer to the straight edge as shown in Fig. 2c. This aspect of the deposits can be used to self-assemble transistors of different aspect ratios for use in circuits. This variation in the pitch of the deposits is easily understood by analyzing an illustration of the boundary line shown in Fig. 7. Analyzing the cross-sectional profile of the puddle observed from the straight edge, shown in Fig. 2b and illustrated in Fig. 7 (puddle

Conclusion

In summary, we showed how fluid mechanical phenomena such as the coffee stain effect can play a constructive role in the development of better electronics with solution processed semiconductors. We demonstrated the use of the controlled coffee stain effect to self assemble the source–drain contacts of the transistor with <10 μm separation to enable small channel length, high transconductance devices. A prototype device was fabricated and tested to prove the concept.

It must be noted that the

Acknowledgment

The authors would like to acknowledge Dr. A. Pattekar for interesting discussions.

References (24)

  • J. Xu et al.

    Angew. Chem.

    (2007)
  • H. Klauk

    Organic Electronics: Materials, Manufacturing, and Applications

    (2006)
  • H. Sirringhaus et al.

    Science

    (2000)
  • B.-J. de Gans et al.

    Langmuir

    (2004)
  • S.K. Park et al.

    Appl. Phys. Lett.

    (2007)
  • A.C. Arias et al.

    Appl. Phys. Lett.

    (2004)
  • R.D. Deegan et al.

    Nature

    (1997)
  • R.D. Deegan et al.

    Phys. Rev. E

    (2000)
  • Y. Popov

    Phys. Rev. E

    (2005)
  • R. Zheng

    J. Eur. Phys.

    (2009)
  • P.G. de Gennes et al.

    Capillarity and Wetting Phenomena

    (2004)
  • D. Bonn et al.

    Rev. Mod. Phys.

    (2009)
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