Effect of Trapping of Free Carriers in CdS Ultrasonic Amplifier

Abstract

Bunched electron waves in CdS ultrasonic amplifier are formed by two kinds of space charge waves, that is, mobile electron waves due to free electrons and bound electron waves at trapping centers. When the product of the relaxation time of electron trapping and the angular frequency of sound is an order of unity, the phase difference between the mobile and the trapped waves becomes appreciable, and the fraction, f, which gives the ratio between the mobile and total space charges, takes a complex form. Under this condition the dependence of amplification factor, α′, deduced by use of the complex f, on the applied dc voltages becomes considerably unsymmetrical about a certain value of the dc voltage at which α′=0. Futher, the frequency characteristics of α′_max should deviate from the theory derived by Hutson and White. The experimental results agree: quite well with these theoretical predictions.