Band-to-Band Tunneling (BTBT) is the quantum mechanical process where electrons tunnel directly from the valence band of one semiconductor region to the conduction band of an adjacent region — it is a major source of reverse-junction leakage at high doping levels and the switching mechanism in tunnel FETs designed for ultra-low power logic.

What Is Band-to-Band Tunneling?

• Definition: A two-band tunneling process where an electron in the filled valence band tunnels across the forbidden bandgap to an empty conduction band state when the two bands are brought into alignment by a strong electric field.
• Field Requirement: BTBT requires a very high electric field (typically above 10^6 V/cm in silicon) to bend the bands so that the valence band maximum on one side aligns with the conduction band minimum on the other side within a short tunneling distance.
• GIDL Mechanism: Gate-Induced Drain Leakage occurs when high drain voltage combined with a below-threshold gate voltage creates a strong lateral field in the gate-drain overlap region, triggering BTBT that generates electron-hole pairs contributing to off-state leakage.
• Exponential Field Dependence: BTBT current depends exponentially on the electric field, making it highly sensitive to junction abruptness, doping concentration, and applied voltage.

Why Band-to-Band Tunneling Matters

• OFF-State Leakage: BTBT at the drain junction is a significant component of transistor off-state current in advanced nodes, contributing to static power consumption and limiting achievable V_DD reduction.
• SRAM Retention: GIDL-induced leakage raises the minimum supply voltage below which SRAM cells cannot retain data, setting a lower bound on SRAM V_DD in near-threshold computing.
• Tunnel FET Operation: Tunnel FETs exploit BTBT as their switching mechanism — source-channel band alignment is controlled by the gate voltage, turning BTBT on and off. This enables sub-60mV/decade subthreshold swing theoretically, promising lower power operation.
• Scaling Challenge: As junctions become more abrupt and doped more heavily at advanced nodes, electric fields at the drain junction increase, worsening BTBT leakage and making voltage scaling more difficult.
• Power Device Implications: In high-voltage power devices, BTBT contributes to avalanche pre-breakdown leakage and sets constraints on maximum allowed field in the drift region.

How Band-to-Band Tunneling Is Modeled and Managed

• Non-Local BTBT Models: Accurate BTBT simulation requires non-local models that track the tunneling path between starting and ending k-states across the band gap, as implemented in Synopsys Sentaurus and Silvaco Atlas.
• Junction Engineering: Lower peak electric fields through graded junction profiles and halo optimization can reduce BTBT leakage without sacrificing short-channel electrostatic control.
• Tunnel FET Design: Optimal tunnel FET design uses low-bandgap source materials (SiGe, Ge, InGaAs) with high-k gate dielectrics to increase BTBT probability in the ON state while maintaining OFF-state control.

Band-to-Band Tunneling is both a leakage problem and a switching opportunity in advanced devices — managing it requires careful junction design in conventional MOSFETs while harnessing it as the core switching mechanism in tunnel FETs for ultra-low power circuit applications.