Total Gain Recovery in Floating Gate Thin-Film Transistors for Neuromorphic and Edge Computing
Authors: Bestelink Eva , de Sagazan Olivier , Sporea Radu Alexandru 
Published: December 2020

We propose a floating-gate (FG) thin-film transistor architecture which alleviates a significant limitation present since the inception of FG field-effect transistors, namely the loss of gain due to parasitic capacitive coupling on the FG [1].

The interest in large area electronics has grown beyond traditional applications, such as display and sensing arrays, with recent trends including neuromorphic and edge computing. However, the challenges of fabricating robust thin film transistor (TFT) circuits have remained despite the many significant achievements in material systems and process development. In order to realise the full benefit of low-cost, high-throughput manufacturing methods, device shortcomings need to be addressed [2]. In FG devices, specifically, one major limitation is the degradation of gain compared to conventional devices, as a result of capacitive coupling of the channel and drain potentials to the floating gate. The problem is far more severe in contact-controlled TFTs, such as the staggered-electrode, high-gain source-gated transistor (SGT) [3, 4], and crippling in conventional TFTs manufactured in materials such as InGaZnO [5].

SGT operation differs from conventional field-effect TFTs, whereby the semiconductor is fully depleted at the source edge by an energy barrier (e.g. Schottky), which is reverse biased by the application of drain voltage. Charge injection is modulated via the gate/source overlap, producing very low saturation voltage and ultra-low output conductance gd
[3]. This latter feature yields the distinctive and extremely high intrinsic gain observed in such devices. A large voltage drop occurs across the source depletion region, producing the notable early saturation and, consequently, the channel potential is maintained at a value close to the drain potential for operating conditions in which a conventional TFT would still be working in the linear region. As such, when a floating gate is used in a SGT, the FG couples strongly to both the drain and the channel. FG potential varies considerably with drain voltage, thus increasing charge injection along the source, ultimately raising gd
. While gd
is still orders of magnitude lower than that of a TFT or FG TFT (see Figure 1), many analog applications would benefit from a solution to this problem [1].

Here, we present a staggered-electrode contact-controlled device, the multimodal transistor (MMT), which shares SGT charge injection principles, however the switching mechanism of its channel is controlled by a separate gate [6]. In a FG MMT, the source gate responsible for charge injection and channel switching gate are designed as FGs (FG1 and FG2, respectively) and a main control gate (CG) may overlap both FGs. FG MMTs have been fabricated in low T°C technology [6] with Ni source and drain contacts. ICP-CVD (Inductively Coupled Plasma Chemical Vapour Deposition) via the Corial 210D reactor was used to deposit µ-Si as active layer, as well as the insulators of the gate stack (Figure 1a). Temperature never exceeded 250°C in any of the processing steps.

The MMT exhibits the same high-gain properties of the SGT and preserves it even in FG configuration. With the channel and drain potentials coupling to FG2, its potential is raised significantly higher than FG1 (see Figures 1b and 1c for TCAD simulation with Silvaco Atlas), which is effectively shielded. As charge injection is exclusively modulated by FG1, the low saturation and extremely low gd
are maintained, in contrast to the FG SGT (see Figure 1c). Thus, extremely high gain can be obtained from the FG MMT structure, with a wide range of applicability to traditional analog applications such as displays and sensors, as well as emerging neuromorphic circuits.

Figure. 1 a) Photomicrograph of a µ-Si floating gate MMT (FG MMT); b) FG2 potential is significantly raised, while FG1 is shielded from parasitic capacitive coupling from the channel and drain potential, keeping charge injection at the source constant with drain voltage. c) Output conductance gd
comparison confirms FG MMT gd
is significantly lower than that of the equivalent FG SGT.

References: [1] E. Rodriguez–Villegas, Low Power and Low Voltage Circuit Design with the FGMOS transistor. London: The IET, 2006. [2] Y. Kuo, Electrochem. Soc. Interface, vol. 22, no. 1, pp. 55–61, 2013. [3] R. A. Sporea et al., Sci. Rep., vol. 4, pp. 1–7, 2014. [4] J. Zhang et al., Proc. Natl. Acad. Sci. U. S. A., vol. 116, no. 11, pp. 4843–4848, 2019. [5] T. Qin et al., Acta Phys. Sin., vol. 67, no. 047302, pp. 1–7, 2018. [6] E. Bestelink et al., SID, vol. 31.1, 2020.

Figure 1