First integration of MOSFET band-to-band-tunneling current in BSIM4

Static leakage currents represent a major issue in nano-scale CMOS. In digital VLSI circuits, the most relevant contributions to the overall leakage current are: sub-threshold conduction, gate current and band-to-band-tunneling (BTBT) current, which flows from drain (source) to the substrate through the reverse biased diffusion junctions [1]. While the last one has been recognized as an important effect in digital nano-CMOS, yet no compact model of it has ever been included in the industry-standard device model BSIM4 [2]. In this work, we show that the lack of a BTBT current model leads to discrepancies between SPICE and device-level simulations and, that adding a BTBT current source into BSIM4 DC model can correct this. The new current source follows a widely accepted physical model of the BTBT phenomenon [3] with a rectangular junction approximation [1]. Test case results show a good agreement between the new circuit-level simulations and the device-level extracted currents. The Band-To-Band-Tunneling phenomenon Fig. 1: Electron tunneling In a reverse biased p–n junction, a high electric field causes a large current to flow through the junction. As shown in fig. 1, this phenomenon is due to the tunneling of electrons from the valence band of the p-region to the conduction band of the n-region (causing the generation of holes in the pregion) [3] [5] [6]. For BTBT to happen, the necessary condition is that the total voltage drop across the junction (applied voltage plus built-in voltage) is higher than silicon’s band-gap [6], as seen in fig. 1. 2D doping profiles for channel, source and drain regions can be approximated as bi-dimensional gaussian functions [1]: