Practical RF Handbook, 4th Edition

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Active components for RF uses

The simplest semiconductor active device for RF applications is the diode, which like its thermionic forebear conducts current in one direction only. Arguably, semiconductor diodes are not active devices, simply non-linear passive ones, but their mode of operation is so closely linked with that of the transistor that they are usually considered together. The earliest semiconductor diode was of the point contact variety - the user-adjusted crystal and cat’s whisker used in the early days of wireless. Later, new techniques and materials were developed, enabling robust pre-adjusted point contact diodes useful at radar frequencies to be produced. Germanium point contact diodes are still produced and are useful where ...