Compact modeling of Optically-Gated Carbon NanoTube Field Effect Transistor

Background Carbon Nanotube Field Effect Transistors (CNTFETs) have high charge sensitivity at room temperature [1]. By using this sensitivity, some nonvolatile memory devices have been demonstrated with charge trapping in SiO 2 gate insulator [2, 3]. Besides, a new design of synapse-like circuit requires a multi-level nonvolatile memory [4]. For this application, and according to its high charge sensitivity, Optically-Gated Carbon Nanotube Field Effect Transistor (OG-CNTFET) appears as a good candidate thanks to optical writing and electrical erasing abilities both with single and multiple drain current levels [5, 6]. By coating a thin layer of photosensitive polymer such as poly3-octylthiophene-2,5-diyl (P3OT) over the nanotube, a CNTFET becomes an OG-CNTFET [5], as presented in Fig. 1a. Compared to the conventional CNTFET, the OG-CNTFET reveals a significant increase of the drain current below the threshold gate bias voltage. If this device is under significant powerful illumination, the gate bias will no longer control the conductivity of the CNT channel, and the optical gate will dominate the functionality. This property of variable conductance is of particular interest for neural network designs to define a third logic level. In this work, we present a compact model for OG-CNTFET. Indeed, compact modeling (i.e. SPICE-like) is a key issue for predicting the ultimate performances of these novel nano-devices in a circuit environment using standard simulation tools. Electron trapping/releasing mechanism When a thin film of P3OT coats on a CNTFET, this polymer acts as a p-type doping under no illumination condition [5]. We suppose that the CNT channel is electrostatically doped due to negative charges trapped at the polymer/SiO 2 interface in the nanotube vicinity [1]. When an OG-CNTFET is under illumination with a wavelength that can be absorbed by the polymer, electron-hole pairs are generated in the P3OT layer, and a minor part of them can be separated [7]. This polymer is known for trapping only electrons [8]. If the gate is biased positively, these trapped electrons can be homogenously attracted by the electrostatic field to the P3OT/SiO 2 interface. The resulting charge density contributes to modulate the channel conductivity by screening the back gate bias. Fig. 1b describes this so-called optical gating effect. When the light is turned off, the trapped electrons are not released immediately; they are hold for about ten hours [5, 7]. The optical gating effect can be removed more rapidly by applying a negative gate voltage or a positive drain-source one …