Quantum capacitance of vertical tunnel field-effect transistors: A first-principles study

The vertical two-dimensional (2D) van der Waals (vdW) heterostructure has been intensively studied for the application of tunnel field-effect transistor (TFET) devices. Despite the similarities between TFET and capacitor architectures, the correlations between quantum capacitance and quantum transport characteristics have been rarely discussed. Carrying out first-principles finite-bias non-equilibrium TFET simulations within the multi-space constrained-search density functional theory (MS-DFT) formalism we have recently developed [1], we elucidate the quantum transport and quantum capacitance properties of the graphene-based TFET in atomistic details. We show that the total capacitance of graphene-based TFET significantly deviates from the classical geometric capacitance due to the low quantum capacitance of graphene electrodes. Under applying the gate-bias, we extract electrode-specific quantum capacitances and find that electrodes exhibit negative quantum capacitances raising the total capacitance at the resonant-tunneling regime. Finally, we extend the study for the defective channel case and study how a point defect introduced within the inner channel region affects the capacitance and transport properties. Our findings provide fundamental insight into the non-equilbrium device characteristics of low-dimensional quantum devices and point towards a future direction for the design of 2D vdW heterojunction devices.