Microwave
Solid state microwave devices

The development of semiconductor electronics in the 1950s led to the first solid state microwave devices which worked by a new principle; negative resistance (some of the prewar microwave tubes had also used negative resistance). The feedback oscillator and two-port amplifiers which were used at lower frequencies became unstable at microwave frequencies, and negative resistance oscillators and amplifiers based on one-port devices like diodes worked better.

The tunnel diode invented in 1957 by Japanese physicist Leo Esaki could produce a few milliwatts of microwave power. Its invention set off a search for better negative resistance semiconductor devices for use as microwave oscillators, resulting in the invention of the IMPATT diode in 1956 by W.T. Read and Ralph L. Johnston and the Gunn diode in 1962 by J. B. Gunn. Diodes are the most widely used microwave sources today.

Two low-noise solid state negative resistance microwave amplifiers were developed; the ruby maser invented in 1953 by Charles H. Townes, James P. Gordon, and H. J. Zeiger, and the varactor parametric amplifier developed in 1956 by Marion Hines. These were used for low noise microwave receivers in radio telescopes and satellite ground stations. The maser led to the development of atomic clocks, which keep time using a precise microwave frequency emitted by atoms undergoing an electron transition between two energy levels. Negative resistance amplifier circuits required the invention of new nonreciprocal waveguide components, such as circulators, isolators, and directional couplers. In 1969 Kaneyuki Kurokawa derived mathematical conditions for stability in negative resistance circuits which formed the basis of microwave oscillator design.
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