Current status of negative electron affinity devices

The introduction of electron emitters utilizing negative electron affinity has greatly improved the performance of many conventional light-sensing devices. The unique properties of such emitters have also made possible devices which were heretofore not feasible. Since their arrival as laboratory curiosities about five years ago, these emitters have had a large impact in the area of low-light-level detection, particularly scintillation counting. Recent advances in materials technology and surface activation processes have brought negative electron affinity photocathodes to the market place for use as detectors for both the Nd and GaAs lasers. In both cases, the detectors are more than an order of magntiude more sensitive to the laser light than previous photocathodes, and the dark currents (thermionic emission from the cathodes which can be limiting in low-light-level use) are several orders of magnitude less. Several other applications of negative electron affinity are presently under development which may further affect photocathodes, photomultipliers, imaging devices, and even the time-honored thermionic cathode. The operating principles of this type of electron emitter, the present state of the art and its effect on device performance, and the possible developments in the near future are discussed.