Unified admittance model for resonant tunneling diodes: Incorporating the space-charge dynamics of the quantum well and collector

Abstract

In this paper, we resolve a long-standing controversy: how and to what extent electron transport in the collector of a double-barrier resonant tunneling diode (RTD) affects its dynamics, particularly the rate of charge-relaxation processes and the RTD admittance. Here, we present a general analytical small-signal RTD admittance model that consistently incorporates space-charge effects in both the quantum well (QW) and the collector depletion region (spacer). If electron transport through the collector is sufficiently fast—this condition is met in contemporary subterahertz and terahertz RTDs under typical operating conditions—the general model reduces to a simple four-element 𝑅⁢𝐿⁢𝑅⁢𝐶 equivalent circuit with a certain charge-relaxation time constant (𝜏), which, in general, largely deviates from the tunnel electron lifetime in the RTD QW. Collector space-charge effects influence both dynamic and static RTD characteristics. Specifically, in the negative differential conductance (NDC) region, RTDs exhibit increased 𝜏 and capacitance, resulting in slower operation. Additionally, the RTD 𝐼-𝑉 curve tilts to the right and the NDC increases. However, collector space-charge effects are typically negligible when electrons travel ballistically through the collector—this outcome of our analysis is important for the practical modeling and design of RTDs. Nevertheless, these effects become significant in RTDs with long collector spacers, high current density, and electron transport at saturation velocity, though such RTDs are unusual at subterahertz and terahertz frequencies.