Multistability and state-switching in series-coupled resonant tunneling diodes

Resonant tunneling diodes (RTDs) embedded in an electrical circuit are known for their neuron-like response characteristics, which makes them promising candidates for neuromorphic applications. This paper investigates the dynamical response of series-coupled RTDs and systematically analyzes the impact of coupling and inhomogeneities on the solution structure. We further propose a scheme for controlled switching between coexisting stable states which allows to realize tunable memory elements in these circuits. The coupled RTD system exhibits a rich bifurcation structure, showing different degrees of multistability between symmetric and antisymmetric solutions. Limit-cycle branches and their dependence on the system parameters are analyzed using numerical continuation methods. A central focus is placed on the role of symmetry. For two identical RTDs, the system possesses a $\mathbb{Z}_2$ exchange symmetry, which governs the emergence of symmetry-breaking bifurcations and multistable states. The analysis is further generalized to $N$ coupled RTDs, revealing the underlying $S_N$ symmetry structure and its influence on the organization of equilibrium branches, paving the way for neuromorphic network operation.