The influence of boundary conditions and the bias on amplification from negative differential mobility elements

We demonstrate from a study of the small signal impedance properties of supercritical negative differential mobility (NDM) element that the various modes of device amplification are determined primarily by cathode conditions and the bias. These conclusions are drawn from 1) a detailed analytical investigation where cathode conditions are represented by prespecified values of electric field, and 2) by synthesizing previously published studies. Briefly, it is shown that multiple (bias induced stable-unstable-stable) amplifying states will arise from NDM elements with cathode fields Ecwithin the NDM region. (Here, the different states occur when the device dc current density is, respectively, less than, approximately equal to, or greater than a critical value of current Jc. Jcis determined by Ec.) Single amplifying states will arise from elements with values of Ecthat are either less than the NDM threshold field value, or within the saturated drift velocity region. (In the latter case, time-dependent values for Ecare required.) The study proposes that general features of the space-charge profiles and the cathode conditions may be ascertained from measurements of 1) the small signal device impedance as a function of bias level, and 2) the dc current versus voltage relation.