Most semiconductor devices consist of an insulating layer sandwiched between a semiconductor and a metal conductor. This is the well-known MOS - metal-oxide-semiconductor - structure. When a voltage is applied across the device to contacts on the semiconductor and metal layers, positive holes move from the semiconductor into the insulator, while electrons travel in the opposite direction. This produces a narrow region of charges at the boundary in which current can flow.

But imperfections are common in the amorphous insulators, such as silicon dioxide, used in most semiconductor devices. This leads to an uneven distribution of charge at the interface and reduces efficiency. To overcome this problem, McKee and co-workers developed crystalline materials made from various combinations of barium, strontium, titanium and oxygen that were almost defect-free. In contact with silicon or germanium layers, these produce a far more uniform region of charge. "The barium-titanium-oxygen-germanium structure may well be electrically perfect", says McKee.

Scientists have long searched for a way to reduce the number of imperfections in the insulating layers of MOS structures, but McKee's team is the first to experiment with a crystalline structure. "To our knowledge, this is the first demonstration of charge inversion for a gate oxide on germanium", he says.

The breakthrough should also allow physicists to control semiconductor interfaces on the atomic scale - a feat necessary for fledgling technologies such as ferroelectric lithography and quantum computing.