Looks good on paper

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: Once in a while, someone takes a familiar material like glass and finds a new use for it. Glass had been around for ages but, in the 1950s, Basil Hirschowitz of the University of Michigan thought of using a fibre made of the stuff to transmit light. Fibre optics have since revolutionised both surgery (Hirschowitz’s original intention) and telecommunications (an unexpected bonus). Elvira Fortunato, Rodrigo Martins and their colleagues at the New University of Lisbon in Portugal believe they have found similarly a novel use for paper. Writing in IEEE Electron Device Letters, they describe how to use it to make a transistor.

Transistors are the workhorses of electronics. They are switches that employ one electric current to control the passage of another. Linked together on the surfaces of silicon chips, they form the ‘logic gates’ that do the calculations in computers, mobile phones, television sets and the other electronic gadgets which dominate the modern world. The bold proposal that Fortunato and Martins are making is to replace the silicon with cellulose, the main ingredient of paper.

The silicon in a transistor has two separate roles. One, when it is doped with small amounts of other elements, is as a semiconductor. This is a material that permits the limited movement either of electrons (which are negatively charged) or of positively charged ‘holes’ in the crystal lattice where an electron ought to be. Silicon’s other role, when it is pure, is as a dielectric—a material that can be penetrated by an electric field, but not an electric current. It is silicon’s role as a dielectric that Fortunato and Martins propose to replace.

The two researchers built their transistors by coating both sides of a sheet of paper with semiconductors made of oxides of zinc, gallium and indium, rather than silicon. They then deposited aluminium onto the coated paper to connect the resulting components together. One side of the paper carried the control currents while the other carried the output currents. The paper thus acted as the dielectric between the components of each transistor, as well as being the substrate for the circuit, in the same way that the base of a silicon chip acts both as substrate and as dielectric.