## 1 Introduction  
Metal-oxide-semiconductor field-effect transistors (MOSFETs) have been scaled down over the last half-century to achieve high density, high performance, and high cost-effectiveness [1; 2; 3]. In spite of the advantages of scaling, some issues have arisen as the device sizes continue to shrink. For instance, the power consumption and operating temperature of the transistor have increased significantly [2; 3]. In addition, the leakage current has increased to the extent that it may surpass the dynamic power consumption [2; 3]. Various techniques have been introduced to reduce both the leakage current and power consumption, but those efforts have thus far been restricted by the theoretical limit of subthreshold swing (SS) (i.e., SS \(\sim\)60 mV/decade at 300 K) of MOSFET [2]. In order to improve the subthreshold swing of the transistor, various novel steep-switching devices have been explored. The major novel devices are classified as follows: negative capacitance FET (NCFET) that exhibits steep slope by using the negative capacitance effect of ferroelectric layer [4; 5; 6; 7; 8], phase FET, which consolidates additional components to use the unique properties of phase transition material like abrupt and reversible resistivity switching with a conventional thermionic emission process through low-drive voltage [9; 10], Nano-Electro-Mechanical (NEM) relay using the mechanical operation for causing connection and disconnection of the channel [11], impact ionization MOS (I-MOS) device controlled by impact ionization which is generated by a high electric field [12; 13; 14], and tunnel FET (TFET), utilizing the band-to-band tunneling (BTBT) operational mechanism [15; 16; 17]. Of these, field-effect transistors (FETs) using a positive feedback mechanism have attracted particular attention. Feedback FETs (FBFETs) show excellent subthreshold swing (\(\sim\)0 mV/decade at 300 K) and high on-/off current ratios (\(\sim\)10\({}^{10}\)) [18; 19]. For those reasons, many researchers have expressed interest in FBFETs and proposed various types of FBFETs, such as positive feedback devices (PF devices) [20; 21; 22], zero-impact ionization, zero subthreshold swing FETs (Z\({}^{2}\)-FETs) [23; 24; 25; 26; 27; 28; 29], and zero subthreshold swing, zero-impact ionization, and zero-gate FETs (\(Z^{3}\)-FETs) [30; 31; 32]. In this study, we will summarize the characteristics of FBFETs (i.e., operating mechanism, structure modeling, and transfer characteristics). Compared to conventional FETs, the superior properties of FBFETs can be utilized in a circuit composed of inverters or especially for neuromorphic devices. One of these properties involves a saturation region that is clearly distinguished according to gate voltage (V\({}_{\text{g}}\)). Especially at threshold voltage (V\({}_{\text{th}}\)), the power supplied to a device in both the on-state and off-state can be very low, and the power consumption is low both during operation and in the off-state until operation. FBFETs have demonstrated their potential as memory devices. A representative example is the capacitor-less dynamic random-access memory (DRAM). The 1T-DRAM was first proposed two decades ago [33], however, it still remains in the academic research stage. These devices exhibit fatal drawbacks such as low reliability, high power consumption, or lack of compatibility with standard process technologies such as meta-stable DRAM (MSDRAM) [34; 35; 36] and A2 RAM [37; 38]. Heretofore, the conventional 1-transistor 1-capacitor DRAM (1T-1C DRAM) succeeded in overcoming difficulties in the development process without serious problems. The 1T-DRAM has not yet demonstrated the ability to become commercially available to replace the DRAM [37; 38; 39; 40; 41; 42]. However, the DRAM is facing huge challenges that are proving hard to overcome. The primary issue is that scaling the capacitor in the bit-cell of the DRAM can no longer keep up with the access transistor in the bit-cell [41; 42].