Metal-interface amplifiers of the kind described by S pratt, S chwarz and K ane have been fabricated with thin Al and Al 2O 3 films and a thick Au emitter. Grounded-base and grounded-emitter collector characteristics are similar to those of an n-p-n junction transistor. Power gain of the order of 100 has been obtained from these structures at room temperature and at a frequency of 1 kc/s. Experiments involving variations in Al film thickness, and variations in oxide thickness have been performed. Several anomalous features of the device, such as the decrease in tunneling resistance with increasing Al film thickness, and the decrease of α and β under d.c. conditions, are reported. Measurements of two- and three-terminal transfer characteristics have been examined in order to elicit information concerning the mechanism responsible for power gain. The measurements and experiments have been evaluated with a view to establishing whether a hot-electron or depletion-layer transistor model is effective. In order to accommodate the depletion-layer transistor model of H all, pinholes ranging from a few hundred angstroms to several microns have been incorporated into the structure. The measurements and experiments reported here suggest that there is merit in H all's proposed model. While a major portion of the current transferred from Au to Ge probably takes place by means of injection into the space-charge region of the Ge, none of the measurements reported rules out the possibility that a small portion of the current may be carried by hot electrons.