Dependence of current switching dynamics on contact conductivity in semiconductor superlattices

Numerical simulation results are presented for a discrete drift-diffusion electronic transport model appropriate to weakly-coupled semiconductor superlattices [1]. Sequential resonant tunneling between adjacent quantum wells is the primary conduction mechanism for this model which also incorporates an effective contact conductivity $\sigma _{c}$. We study the dependence on $\sigma _{c}$ of time-averaged current-voltage characteristics and transient current response to abrupt steps in applied voltage. For intermediate values of $\sigma _{c}$, three qualitatively distinct transient responses -- each associated with a different mechanism for the \textit{relocation} of a static charge accumulation layer [1] - are observed for different values of voltage step $V_{step}$; these involve, respectively, 1) the motion of a single charge accumulation layer, 2) the simultaneous motion of one depletion and two accumulation layers [2], and 3) the simultaneous motion of two accumulation layers. The range of $V_{step}$ values for each mechanism and the relocation times associated with each are studied as a function of $\sigma _{c}$; a critical value of $\sigma _{c}$ is identified above which the second relocation mechanism is not observed for any value of $V_{step}$. Relocation times are found to depend sensitively on specific values of $\sigma _{c}$ and $V_{step}$. [1] L. L. Bonilla and H. T. Grahn, Rep. Prog. Phys. \textbf{68}, pp. 577-683 (2005), and refs. therein. [2] A. Amann, A. Wacker, L. L. Bonilla, and E. Schoell, Phys. Rev. E \textbf{63}, 066207 (2001).