In the mid-1930s Russell Ohl, an electrochemist at Bell Telephone Labs in Holmdel, NJ, began investigating the use of silicon rectifiers as radar detectors. He found that increasing the silicon purity helped improve their detection ability. On 23 February 1940, he tested a small silicon slab that yielded strange, surprising results. When exposed to bright light, the current flowing through the slab jumped appreciably. He also noticed that different parts of the crystal yielded opposite electrical effects when tested with a "cat's whisker" style probe.
Ohl and colleague Jack Scaff found that a seam in the slab marked the separation of the silicon into regions containing distinct kinds of impurities. One impurity, the element phosphorus, yielded a slight excess of electrons in the sample while the other, boron, led to a slight deficiency (later recognized as "holes"). They called the regions n-type (for negative) and p-type (positive); the surface or "barrier" where these regions met became known as a "p-n junction." Light striking this junction stimulated electrons to flow from the n-side to the p-side, resulting in an electric current. Ohl had discovered the photovoltaic effect that powers today's solar cells (1954 Milestone).
William Shockley's conception of the junction transistor in 1948 (1948 Milestone) derived from Ohl's serendipitous 1940 discovery. The p-n junction became the most common form of rectifier used in the electronics industry and has since become a fundamental building block in the design of semiconductor devices.
Ohl, R.S. "Light-Sensitive Electric Device" U. S. Patent 2402662 (Filed May 27, 1941, Issued June 25, 1946).
Shaff, J. H. & Ohl, R.S. "Development of silicon crystal rectifiers for microwave radar receivers," Bell System Technical Journal Vol. 26 (1947) p. 1
Ohl, Russel S. Electrical Engineer, an oral history conducted in 1975 by Frank Polkinghorn, IEEE History Center, Rutgers University, New Brunswick, NJ, USA.
"Oral history interview of Russell Ohl by Lilllian Hoddeson," (August 19-20, 1976) The Niels Bohr Library & Archives and the Center for History of Physics, Divisions of the American Institute of Physics.
Riordan, Michael and Hoddeson, Lillian. "The Origins of the pn Junction," IEEE Spectrum (June 1997) pp. 46-51.
Riordan, M & Hoddeson, L. Crystal Fire: The Birth of the Information Age. (New York: W. W. Norton, 1997) pp. 88-98.