Suppressed absolute negative conductance and generation of high-frequency radiation in semiconductor superlattices

– We show that space-charge instabilities (electric field domains) in semiconductor superlattices are the attribute of absolute negative conductance induced by small constant and large alternating electric fields. We propose the efficient method for suppression of this destructive phenomenon in order to obtain a generation at microwave and THz frequencies in devices operating at room temperature. We theoretically proved that an unbiased superlattice with a moderate doping subjected to a microwave pump field provides a strong gain at third, seventh, etc. harmonics of the pump frequency in the conditions of suppressed domains. Introduction. – There exists a strong demand for miniature, solid-state, room temperature operating sources and detectors of THz radiation (0.3-10 THz). The need is caused by a rapid progress of THz sciences and technologies ranging from the astronomy to the biosecurity [1]. Semiconductor superlattices (SLs) [2], operating in the miniband transport regime, are interesting electronic devices demonstrating properties of both nonlinear and active media. Nonlinearity of voltage-current (UI) characteristic of SL gives rise to a generation of harmonics of microwave and THz radiation [3, 4]. On the other hand, Bloch oscillations of electrons within a miniband of SL cause an appearance of negative differential conductance (NDC) for dc fields (voltages) larger than the critical Esaki-Tsu Ec [2]. Remarkably, the static NDC should be accompanied by a small-signal gain for ac fields of very broad frequency range from zero up to several THz [5]. The feasible device, employing such active media properties of dc-biased SL at room temperature, is known as THz Bloch oscillator. However, an existence of static NDC causes space-charge instabilities and a formation of high-field domains inside SL [6]. These domains are believed to be destructive for the Bloch gain. Recent top class experiment demonstrates only decrease in THz absorption, but still not gain, in an array of short dc-biased SLs [7]. Therefore, it is important to consider modifications of this canonic scheme of generation in order to suppress electric domains but preserve the high-frequency