Multi-scale analysis of radio-frequency performance of 2D-material based field-effect transistors

Abstract

Two-dimensional materials (2DMs) are a promising alternative to complement and upgrade high-frequency electronics. However, in order to boost their adoption, the availability of numerical tools and physically-based models able to support the experimental activities and to provide them with useful guidelines becomes essential. In this context, we propose a theoretical approach that combines numerical simulations and small-signal modeling to analyze 2DM-based FETs for radio-frequency applications. This multi-scale scheme takes into account non-idealities, such as interface traps, carrier velocity saturation, or short channel effects, by means of self-consistent physics-based numerical calculations that later feed the circuit level via a small-signal model based on the dynamic intrinsic capacitances of the device. At the circuit stage, the possibilities range from the evaluation of the performance of a single device to the design of complex circuits combining multiple transistors. In this work, we validate our scheme against experimental results and exemplify its use and capability assessing the impact of the channel scaling on the performance of MoS₂-based FETs targeting RF applications.

Fig. 1. The multi-scale approach consists on combining a small-signal equivalent circuit and a numerical simulator to describe the behavior of a 2DM-based FETs. The small-signal elements are extracted from the numerical solver and fed into the small-signal equivalent circuit. Importantly, the metal-2DM contact resistances as well as the gate resistance are also included.

Fig. 2. Small-signal equivalent circuit suited to 2DM-based FETs. The equivalent circuit of the intrinsic device is framed in blue. The small-signal elements are: g_m = ∂I_DS/∂V_gs transconductance; g_DS = ∂I_DS/∂V_DS output conductance; and C_gs, C_gd, C_sd, and C_dg intrinsic capacitances. R_g is the gate resistance and R_d and R_s account for the contact resistances of the drain and source, respectively. They connect the intrinsic (noted G, D and S) and extrinsic (G,e, D,e and S,e) gate, drain and source terminals.

Fig. 3. Experimental data of a MoS₂ FET reported in (ref. 7) (symbols) and results from the numerical solver (line). The operating point for RF experimental measurements is indicated by a square.

Fig. 4. (a) Current gain h₂₁ and (b) Mason's unilateral gain U calculated using the multi-scale (MS) approach (blue lines) and compared against the experimental values extracted from (ref. 7) (red lines with symbols). The arrows indicate the values of f_T and f_max.

Fig. 5. Cut-off frequency f_T as a function of the gate length for different drain biases (solid lines with markers). The 1/L_g² trend and the physical limit (that also serves as a guideline for the 1/L_g scaling) are also depicted (dashed lines). Solid and hollow markers correspond to experimental works.

Fig. 6. Maximum oscillation frequency f_max as a function of the gate length for different drain biases (solid lines with markers). The 1/L_g and trends are indicated by dashed lines. Solid and hollow markers correspond to experimental works.