Researchers at the University of Cape Town in South Africa have produced fully printed field effect transistors (FETs) with performances comparable with silicon.
The transistors were fabricated by screen printing on plain paper substrates, under ambient conditions, allowing for a high degree of accuracy and reproducibility.
Applications for the technology include pixel switches in flexible and glass-based active matrix displays for mobile phones, e-readers and other consumer electronics and appliances, RFID logic, emerging flexible and organic solar cells used to charge portable electronics and for low-power devices where grid access is unavailable.
The technology could also be used to make sensors for medical diagnostics, smart packaging and other applications.
Results of the technology, developed by professors Margit Härting and David Britton were published in the May 2009 issue of Applied Physics Letters. They were also validated by the research group of Dan Gamota and Jie Zhang at Motorola Central Research Laboratories, before they left Motorola in 2009 to form the company Printovate.
Printing the layers
The heart of each transistor is a printed layer containing silicon nanoparticles that form the active semiconductor material.
The silicon nanoparticles are produced by a proprietary method of milling of bulk silicon feedstock, allowing simple production of both p- and n-type nanoparticles in large quantities.
According to their inventors the particles are stable in air, for periods longer than two years, and maintain the bulk semiconducting properties of silicon when mixed with a suitable binder to form an ink. Printed silicon devices, made with water-based acrylic inks, have also shown no change in characteristics when stored for periods in excess of six months.
The printing of the transistors is identical to any other four-colour printing process, and has similar registration tolerances.
To produce a bottom gate FET, a silver gate electrode is first printed directly onto the paper substrate in the required position, using one of several available screen printing products, such as DuPont Luxprint 5000 conductor.
The second 'colour' is a screen printable dielectric ink, which completely covers the gate region where the silicon is to be applied. The third 'colour' is the active semiconductor layer, which has to be registered so that it is printed completely on the insulator above the gate electrode.
Finally, the source and drain electrodes are printed with the same 'colour' as the final layer. For a top gate FET, the print sequence is simply reversed.
The Cape Town researchers are integrating the transistors with other components to show them in action, when they present at the Plastic Electronics Conference in Dresden.