Rice University, Houston, Texas

A physicist foresees a new era in electronics.

A material's electronic properties depend largely on its density of mobile charge carriers (electrons and holes). The most common way of tuning that density is 'doping'. This involves carefully adding atoms or molecules that donate or take up electrons from the surrounding material. But doping comes with a downside: these added impurities themselves become charged, so they scatter mobile charge carriers and muddy the predictability of the material's electronic properties.

How to avoid doping? Look to Julius Edgar Lilienfield. In 1925, he proposed what is now called the 'field effect', in which the material of interest functions as one electrode of a capacitor. When a voltage is applied to the other electrode, equal and opposite charge densities accumulate on the sample material. The density of charge carriers can be varied as it is in doping, but not to the same extent. Nonetheless, the field effect has an everyday role in transistors — which are the fundamental parts of consumer electronics.

Another of Lilienfield's inventions, the electrolytic capacitor, holds the key to much higher field-effect charge densities, which could have dramatic consequences. Researchers at Tohoku University in Sendai, Japan, recently used a polymer electrolyte to achieve sufficiently large charge densities at a strontium titanate surface to generate superconductivity (K. Ueno et al. Nature Mater. 7, 855–858; 2008). This has been seen before in doped strontium titanate, but the electrolytic capacitor approach avoids the disorder inherent in doping.

By using mobile ions in an electrolyte to attract charges in the sample, this quirky capacitor can build up charge densities approaching those of chemically doped electronic materials such as high-temperature superconductors. This opens up the possibility of transistor-like devices that can work with very low voltages.