Physics-Based Analytical Channel Charge Model of In_xGa_1-xAs/In_0.52Al_0.48As Quantum-Well Field-Effect Transistors From Subthreshold to Strong Inversion Regimes

This paper presents a physics-based analytical channel charge model for indium-rich In _x Ga _1-x As/In _0.52 Al _0.48 As quantum-well (QW) field-effect transistors (FETs) that is applicable from the subthreshold to strong inversion regimes. The model requires only seven physical/geometrical parameters, along with three transition coefficients. In the subthreshold regime, the conduction bands ( $E_{C}$ ) of all regions are flat with finite and symmetrical QW configurations. Since the Fermi–level ( $E_{F}$ ) is located far below $E_{C}$ , the two-dimensional electron-gas density ( $n_{2{-}DEG}$ ) should be minimal and can thus be approximated from Maxwell–Boltzmann statistics. In contrast, the applied gate bias lowers the $E_{C}$ of all structures in the inversion regime, yielding band-bending of an In _0.52 Al _0.48 As insulator and In _x Ga _1-x As QW channel. The dependency of the energy separation between $E_{F}$ and $E_{C}$ on the surface of the In _x Ga _1-x As QW channel upon $V_{GS}$ enables construction of the charge–voltage behaviors of In _x Ga _1-x As/In _0.52 Al _0.48 As QW FETs. To develop a unified, continuous and differentiable areal channel charge density ( $Q_{ch}$ ) model that is valid from the subthreshold to strong inversion regimes, the previously proposed inversion-layer transition function is further revised with three transition coefficients of $\eta $ , $\alpha $ and $\beta $ in this work. To verify the proposed approach, the results of the proposed model are compared with those of not only the numerically calculated Qch from a one-dimensional (1D) Poisson–Schrödinger solver, but also the measured gate capacitance of a fabricated In _0.7 Ga _0.3 As QW metal-insulator-semiconductor FET with large gate length, yielding excellent agreement between the simulated and measured results.