Barred and latched

SCIENCE & TECHNOLOGY
Computer circuits

Feb 3rd 2005
From The Economist print edition

A molecular replacement for the transistor?

SMALL may be beautiful, but it is also hard. This is the reality faced by those trying to make ever-smaller transistors, one of the key components of virtually all modern electronic devices. The laws of physics place a limit on how small they can become.

This, then, is a tricky problem for those hoping to build future generations of computer chips that are vastly more powerful than today's. (The slow speed of electrical signals means that smaller components make for faster computers.) So the search has been on for a new type of architecture that will work in the tiny dimensions of the future. In a paper published this week in the Journal of Applied Physics, researchers from Hewlett-Packard's laboratories in Palo Alto, California, suggest they may have cracked part of the problem.

To understand the problem at hand, one might imagine a transistor as a sandwich. An electrical signal will pass across the three layers of the sandwich only if a small current is applied to the filling. So a transistor can be switched on or off with this control current. The problem with such a design is that, as the sandwich shrinks, the gap between the top and bottom layers gets so small that current tends to flow spontaneously across it.

Hewlett-Packard's researchers have been tinkering in a new area known as molecular electronics. Instead of using light to etch a transistor circuit on a wafer of silicon, they propose to fabricate tiny wires. The molecular-scale alternative proposed to the transistor is called a “crossbar latch”. The latch works by running an electrical current down a signal wire, through two switches. Controlling the voltage (the quantity of energy possessed by electrons moving down the wire) on the switches allows the latch to either work as an amplifier, restoring a signal to full strength, or as an inverter, converting a “1” to a “0”.

Because components to do things like logical “AND” operations (which returns a logical “1” only if it has two “1”s as input) already exist on the nanoscale, Phil Kuekes, one of the paper's authors, says the crossbar latch is the final piece of the puzzle necessary to build a molecular scale computer—something Hewlett-Packard's researchers plan to attempt next. They are, of course, not the only ones trying. The race to shrink electronic circuits will continue, even if and when the transistor falls by the wayside.

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