A single-atom transistor created by very accurately positioning a phosphorus atom on a silicon surface is reported in a paper published online this week in Nature Nanotechnology. The work represents the smallest transistor created to date. The phosphorus atom sits between source and drain electrodes, which are less than 20 nanometres apart, and also between two gate electrodes, which are just over 100 nanometres apart. Michelle Simmons and co-workers applied a voltage across the source and drain electrodes, and another voltage across the gate electrodes, and measured the current through the phosphorous atom. They found that the current depends on the voltages in a way that is characteristic of a field-effect transistor. To make the smallest ever transistor the team developed a technique that allowed them to replace one silicon atom in a group of six with a phosphorous atom. This corresponds to an accuracy of better than half a nanometre. Despite the advances noted in this study, a number of challenges need to be overcome before single-atom transistors are ready to feature as everyday devices.