Russell Ohl’s specialized area of research was into the behavior of certain types of crystals. He worked on materials research in the 1930’s at AT&T's Bell Labs’ Holmdel facility, investigating diode detectors suitable for high-frequency wireless, broadcasting, and military radar. Ohl, in 1939, discovered the PN barrier (or as it became known, the “P–N junction”). It was the impurities which made some sections more resistant to electrical flow than others, and thus it was the "barrier" between these areas of different purity that made the crystal work. Ohl later found that super-purifying germanium was the key to making repeatable and usable semiconductor material for diodes. All diodes (incl. LEDs, laser diodes etc) are descendants of Ohl's work. His work with diodes led him later to develop the first silicon solar cells.