654877bd_erials_Science_and_Engineering__Significance
[Date] 1939 [Candidate Moment] Russell Ohl, George Southworth, Jack Scaff, and Henry Theuerer discover the existence of p- and n-type regions in silicon. [Significance] 
Provides a necessary precursor to the invention of the transistor eight years later.
Greatest Moments in Materials Science and Engineering
Significance