High-frequency conductivity-based terahertz gain models in quantum semiconductor superlattices: A comparative study

The development of high-power, stable and portable terahertz (THz) sources that can operate at room temperature remains one of the biggest challenges in THz and solid-state physics. Despite modern semiconductor devices such as resonant tunnelling diodes and quantum cascade lasers demonstrating a significant progress, they still face several limitations related to a low power output, temperature sensitivity and the lack of frequency tunability. In this respect, semiconductor superlattices operating in the miniband transport regime continue to represent promising quantum materials for the realization of the desirable THz gain. In this study, we briefly overview basic semiclassical models describing the high-frequency conductivity of superlattices. We cover the popular model of Ktitorov et al. and the lesser-known and more advanced model of Ignatov and Shashkin, and also make their comparative analysis with reference to the classical quasistatic model of gain in devices with the negative differential conductivity. This work aims to offer a simple introduction to these models and their practical relevance to THz device design and development.