Experimental silicon P/N junction, 1940
Engineer Russell Ohl worked at Bell Labs during the 1930s. While researching semiconductors—materials whose ability to conduct electricity can be manipulated—he found positive (P) and negative (N) regions created by impurities in his silicon sample. The barrier between the regions, called a P-N junction, prevented electrons from moving—until he exposed the silicon to sunlight. Then electrons crossed the junction and generated a current, converting sunlight into electrical energy. The silicon rod mounted in this reflector contains a P-N junction across the center. Ohl’s discovery contributed to the invention of both solar cells and transistors.
Gift of Russell Ohl; Smithsonian photo