Field effect transistors (FETs) work by controlling the flow of electrons in a semiconductor such as silicon. A similar class of devices, called ionic FETs, operate by controlling the flow of ions in an electrolyte solution, and could allow unique functions such as DNA, protein and nanoparticle manipulation. To realize such applications, however, devices capable of ion control with molecular-scale feature dimensions need to be fabricated reliably.

To this end, Ki-Bum Kim and colleagues1 at Seoul National University, Korea, in collaboration with researchers at IBM's T. J. Watson Research Center in Yorktown Heights, New York, USA, have developed a technique for making ionic FETs that can be integrated with solid-state electronics.

“The concept of ionic FETs was proposed to manipulate ions and bio-molecules in an electrical way. In particular it is considered to be one of the most viable approaches for sequencing DNA on a solid-state chip,” says Kim. “But although it was proposed many years ago, making the devices has not been that easy.”

An ionic FET controls the flow of ions in an electrolyte from one side of a membrane to the other by stopping or allowing ion movement through ‘ion channels’, or tiny holes, in the membrane. The membrane consists of a conducting gate electrode sandwiched between gate oxide (dielectric) layers. When a voltage is applied between the gate electrode and a source electrode in the electrolyte reservoir, ions flow through the membrane holes into the drain reservoir. Unfortunately, conventional approaches to making holes of molecular dimensions are slow, unreliable and unsuitable for large-scale fabrication.

Fig. 1: Transmission electron microscopy image of an ionic FET membrane consisting of a uniform array of holes, each less than 10 nm wide.

To make their devices structures, Kim and his team used electron-beam lithography to pattern and then etch a uniform array of 70–80 nm-diameter holes in a thin, conducting titanium nitride film (gate electrode). The film was then gradually coated with a titanium oxide layer by atomic layer deposition. This process resulted in the uniform narrowing of the hole to less than 10 nm in diameter (Fig. 1), with the final diameter controlled by the size of the oxide molecules. This enabled the researchers to build an ionic FET that could control the flow of ions in a potassium chloride solution.

“The next step in our work will be to manipulate bio-molecular species such as DNA rather than just ions,” says Kim. “If we can detect or manipulate DNA, it will give a tremendous possibility for solid-state-device DNA sequencing.”