# Characteristics of Resonant Tunneling in Nanostructures with Spacer Layers
## 1 Introduction  
The creation of compact, coherent radiation sources in the terahertz frequency range (0.1-10 THz) operating at room temperature and also having high sensitivity and speed is of great practical interest [1; 2]. This interest is caused by the possibilities of their wide application in various fields such as medicine, meteorology, security systems, wireless high-speed communications, spectroscopy, etc. [2; 3]. The terahertz frequency range lies between long-wave infrared radiation and ultrahigh-frequency millimeter waves. Therefore, scientific research in the fields of electronics and photonics is carried out to create devices operating in this frequency range. Among electronic devices for generating terahertz radiation, Gunn diodes, high electron mobility transistors (HEMs), impact ionization avalanche transit-time diodes (IMPATTs), and tunnel injection transit-time negative resistance diodes (TUNNETTs) are usually considered; quantum cascade lasers (QCLs) are popular among optical devices [3]. One of the possible candidates as a source of terahertz radiation is a resonant tunneling diode (RTD) related to solid-state, compact electronic devices operating at roomtemperature. For the first time, the generation of electromagnetic radiation on an RTD was obtained at a temperature of about 200 K in 1984 [4], while the frequency and output power were small and amounted to 18 GHz and several uW, respectively. Subsequently, by reducing the thickness of the quantum well and barriers [5] and optimizing the thickness of the collector spacer [6] and the size of the slot antenna [7], it was possible to achieve a significant increase in the oscillation frequency. At the moment, the oscillation frequency of RTDs of the order of 2 THz has been experimentally obtained [8; 9; 10], which is a record for electronic devices operating at room temperature. Despite the recent obvious progress in the frequency characteristics of the resonant tunnel diode, the generation power in the terahertz frequency range remains small (about 1 uW). In this regard, theoretical studies aimed at a deeper physical understanding of various features of resonant tunneling processes in nanostructures acquire an important role. Detailed theoretical studies of RTDs, carried out earlier [11; 12; 13; 14; 15; 16; 17], made it possible to make significant progress in understanding the resonant tunneling processes in a constant and alternating high-frequency electric field. However, these studies did not take into account the important features inherent in experimental structures, such as the charge accumulation effects, the presence of spacer layers, and the highly doped contacts in the structure, so they can only serve as a reference point for further research. The parameters of the active region (quantum well and barriers) represent the main importance in controlling the characteristics of resonant tunneling diodes since they determine the width and energy of the resonant levels. Spacer layers, which are located between the active region and the doped contact areas, also have a great influence on the processes of resonant tunneling (see Figure 1). Initially, spacer layers were used to reduce the diffusion of donor impurities into the undoped active region. However, it turned out that the role of spacer layers is not limited to this application, and they may have a significant impact on both the static and dynamic characteristics of the resonant tunneling diodes. Experimental studies of the effect of spacer layers on the static characteristics of RTDs were carried out in [18; 19; 20; 21; 22]. The attention was mainly paid to the influence of the thickness of the spacer layers on the peak and valley currents. In [18], an InGaAs-/InAs-based structure demonstrated that the peak and valley currents decrease with the increasing thickness of the spacer layers (the thickness of the emitter and collector spacers together varies within 1 and 5 nm).