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The basic idea in these devices is to achieve a good high-frequency response by using processes which are localized in the high-field region of reverse-biased p-n junctions. Structures have been proposed previously which use one injecting contact in the space-charge region. These devices have disadvantages which may be overcome by going to devices with two contacts in the space-charge region. These latter devices have been successfully built and tested. Electrons are injected into the space-charge region of a reverse-biased junction. The injection current is modulated by another nearby contact. This modulating contact is a small alloyed p-type region which is biased in "reverse" and consequently draws negligible current. Its function, in a sense, is analogous to the grid in a vacuum tube. In such a device, the second contact has the additional function of shielding the injecting contact from voltage changes across the main p-n junction. The high degree of shielding already achieved in the first few experimental units reflects itself in the fact that the output impedance is of the order of 30 megohms. The measured input impedance is, at present, also about 30 megohms. The input impedance can probably be made even higher. The first experimental devices show a low-frequency power gain at least as high as that obtained with our present-day transistors. The input and output circuits are coupled only through a low interelectrode capacity.