An apparatus is described for studying high current density bombardment of semiconductors. It operates at voltages up to 30 kV and samples can be cooled to 28 K. The excitation density is high enough to induce lasing in many II‐VI and III‐V compounds at both 77 K and room temperature. The emission spectrum over a range of current densities is useful in characterizing the doping and impurities in semiconductors.

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