Growing Helical Density Waves in Semiconductor Plasmas

An experimental and theoretical investigation is made of growing screw-shaped plasma density waves in a semiconductor bar subjected to parallel electric and magnetic fields. A particularly simple mode is used, requiring only a thermal-equilibrium electron-hole plasma, low-recombination surfaces, and moderate fields. It is shown that in extrinsic material the growth is spatial, corresponding to stable traveling-wave amplification, while for nearly equal densities of positive and negative carriers the wave is absolutely unstable and corresponds to the oscillistor phenomenon. Experimental observations of the waves were made in germanium at and above room temperature for frequencies from 20-400 kc and with electric and magnetic fields from 25-60 V/cm and 0-11 kG, respectively. The growth rates and phase characteristics were found to be in excellent agreement with theory and gain in excess of 35 dB/cm was obtained. At higher temperatures, corresponding to nearly intrinsic material, evidence of instability was found in accordance with the theoretical prediction.

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