The III-V photocathode - A major detector development.

Description of new (III-V) photocathodes which show improvements in sensitivity of as much as ten to a hundred times over conventional cathodes in the near infrared and useful improvements at shorter wavelengths. The development stems from a combination of basic knowledge of the photoemission process, gained in the 1950s, and the advancing understanding of the technology of III-V materials, in the 1960s. The superior performance of these cathodes is due to the fact that the vacuum level at the surface lies below the bottom of the conduction band in the bulk of the material. Consequently, the threshold of response is set by the III-V bandgap. The bandgap (and the threshold of response) can be varied by alloying different III-V materials together. A reduction in thermionic emission is realized with these cathodes. At present no semitransparent III-V cathodes with comparably interesting performance are available. The problems hindering further improvements, as well as the problems of placing these cathodes in practical multipliers and image tubes, are discussed briefly.