Silicon nanowires are one of the great hopes for electronic devices of the future. Unlike features carved using photolithography, nanowires are easy to make on a nanometre scale. Electronic engineers hope to use them for everything from optoelectronics to biochemical sensing.
But there’s a problem because at the nanometre scale, the electronic properties of silicon can depend on the precise location of only a few dopants. That’s difficult to control and causes wide variation in device performance.
Consequently, nobody has been able to make reliable diodes, transistors or logic gates out of silicon nanowires.
Today, Massimo Mongillo et amis at the Universite Joseph Fourier, Grenoble, in France demonstrate a way out of this conundrum. These guys show how to fabricate diodes and transistors from undoped silicon nanowires and how how such devices can be wired together to make logic gates.
Making electronic devices from undoped silicon has always been tricky because of a phenomenon known as a Schottky barrier which occurs at the junction between a metal and a semiconductor. The electrons in the metal tend to push away the electrons in the semiconductor allowing current to flow in one direction but not the other.
That sounds useful and indeed it is in certain circumstances when it can be used as a diode. The problem occurs when rectification is not required and the junction must work in both directions. The Schottky barrier prevents this.
Mongillo and co have got around this problem by carefully coating the nanowire with metal silicides at the junction with a metal contact which prevents the formation of unwanted Schottky barriers.
As a result, they’ve created a device that they operate as a bipolar transistor, a Schottky diode and a p-n diode. The gain on their transistor is significantly more than 1 meaning that the output from one can be used as the input of another without additional signal restoration. Finally they link a couple of these devices together with external leads to create a NAND logic gate.
They point out that it ought to be possible to make a logic gate from a single nanowire using a different arrangement of contacts, although it is not clear how scalable this approach would be.
Ref: arxiv.org/abs/1208.1465: Multifunctional Devices and Logic Gates With Undoped Silicon Nanowires