Overview

In the fall of 1948, Frank Isakson, head of the Physics Section of the Office of Naval Research, was a frequent visitor at the Naval Research Laboratory, where I was a member of the Crystal Branch. During one of our frequent discussions of projects of mutual interest, he informed me about the Navy's interest in developing an infrared (IR) photoconductor with a response beyond 7 ?m, the long wavelength limit of PbS films, an intrinsic photoconductor developed in Germany during World War II. The properties of the III V semiconductors were still unknown at that time. In the summer of 1949 I had the good fortune of being able to attend the annual Modern Physics Symposium at the University of Michigan, one of a series of symposia that started in 1928. The lecturers that summer were Luis Alvarez (High Energy Physics), Richard Feynman (Path Integral Method), Frederick Seitz (Solid State Physics) and Gordon B.B. Sutherland (Infrared Spectroscopy of Solids).

In his lectures on semiconductors, Seitz discussed the nature of the impurity levels in Si and Ge and summarized the thermal ionization energies of group III acceptors and group V donors that had been obtained by Pearson and Bardeen at Bell Telephone Laboratories ^[1] from data on the temperature dependence of the carrier densities derived from resistivity and Hall measurements. He also discussed their conclusions that the ionization energies of the group III acceptors (0.048 eV) and group V donors (0.045 eV) were in reasonable agreement with a simple...