Electrolyte-gated transistors

Electrochemical transistors

The electrical conductivity of PEDOT:PSS can be modulated by electrochemical (EC) switching, which in turn makes these materials suitable for use in electrochemical transistors [1]. By applying a positive potential to the gate (G) electrode, relative the source (S) electrode, the current between the source (S) and drain (D) electrodes can be modulated. In the EC-transistor a high current is delivered in its on-state upon applying a negative drain potential and zero gate potential, while the transistor channel is switched off by combining a negative drain potential by a positive gate potential. One of the major advantages o f the EC-transistor is the low operating potentials, typically around 0.5-2 V. Such low operating potentials, which originate from the fact that the transistor is gated by an electrolyte, are desired within the field of printed electronics since printed batteries potentially can be used for powering the devices. However, it has also been shown that the transistor can withstand voltages up to 70 V and currents up to several mA without device degradation.

The lateral design of an electrochemical transistor is illustrated in the figure above, while the switching behaviour of a printed electrochemical transistor is shown to the right.

[1] Nilsson, D., Chen, M., Kugler, T., Remonen, T., Armgarth, M. and Berggren, M., "Bi-Stable and Dynamic Current Modulation in Electrochemical Organic Transistors", Advanced Materials 14, 51-54, 2002. Abstract 

 Electrolyte-gated field-effect transistors

Together with the research group of Prof. Magnus Berggren at Linköping University we have developed the so-called electrolyte-gated OFET where the gate insulator in an OFET is replaced with a solid polyelectrolyte [2, 3]. This forms a robust transistor, with low-voltage (< 1V) operation, high current throughput and short switching time (~50 µs) for e.g. logic circuits and smart pixel devices. When a negative voltage is applied to the gate electrode, cations are

attracted to the negatively charged gate electrode, while anions migrate towards the electrolyte/organic semiconductor interface. This polarization results in the formation of electric Helmholtz double-layers along the two interfaces being in contact with the electrolyte. For the electrolyte/organic semiconductor interface this sheet of charges is balanced by holes injected into the organic semiconductor from the source contact, thus establishing the transistor channel. Courtesy of Linköping University.

In the photograph to the left the OFET operates at 1 V despite the 0.2 mm thick solid electrolyte drop. Here the gate electrode is the metal wire dipped into the drop. The independence of the operating voltage with the thickness of the insulating layer makes this new kind of transistor compatible with low-cost manufacturing techniques, such as printing. Courtesy of Linköping University.

[2] Said, E., Crispin, X., Herlogsson, L., Elhag, S., Robinson, N. D. and Berggren, M., "Polymer Field-Effect Transistor Gated Via a Poly(Styrenesulfonic Acid) Thin Film", Applied Physics Letters 89, 143507, 2006. Abstract

[3] Herlogsson, L., Crispin, X., Robinson, N. D., Sandberg, M., Hagel, O., Gustafsson, G. and Berggren, M., "Low-Voltage Polymer Field-Effect Transistors Gated Via a Proton Conductor", Advanced Materials 19, 97-101, 2007. Abstract