Ultrasonic Amplification in Extrinsic Semiconductors

When a semiconductor, with only one type of carrier, is placed in crossed electric and magnetic fields, an amplification of sound waves similar to that discussed by Dumke and Haering for semimetals can take place. The sound wave is amplified instead of absorbed, when the drift velocity in the crossed fields, $\frac{\mathrm{cE}}{H}$, is in the direction of propagation of the wave and exceeds the wave velocity $V_s$. When the carrier concentrations and the sound wave frequency are such that the screening of the ionic currents by the electronic currents breaks down, the amplification factor can become very large.