Quantum Tunneling is the quantum mechanical phenomenon where electrons pass through a potential barrier despite lacking sufficient classical energy — a critical leakage mechanism in nanoscale transistors and the operating principle of tunnel FETs and flash memory.

Types of Tunneling in Semiconductors

• Direct Tunneling: Electron tunnels directly through a thin barrier (< 3-4 nm gate oxide). Exponentially dependent on barrier thickness.
• Fowler-Nordheim (FN) Tunneling: Electron tunnels through triangular barrier under high electric field. Mechanism for flash memory erase/program.
• Band-to-Band Tunneling (BTBT): Electron tunnels from valence band to conduction band across reverse-biased junction. Key leakage in scaled MOSFETs.

Gate Oxide Tunneling (Direct)

• For SiO2: Significant tunneling starts below 3 nm (1999, ITRS).
• At 1.2 nm SiO2: Gate leakage ~10 A/cm² — unacceptable for standby power.
• Solution: High-k dielectrics (HfO2, k=22) — physically thicker but equivalent capacitance, lower tunneling.
• High-k allows 2–3nm equivalent oxide thickness (EOT) with 4–5nm physical thickness.

BTBT Leakage in Scaled MOSFETs

• Short channels create high electric fields at drain-body junction.
• BTBT generates electron-hole pairs → subthreshold leakage.
• Major contributor to off-state current (Ioff) in sub-20nm nodes.
• Mitigated by: lightly doped drain (LDD), graded junctions, higher bandgap materials.

Tunnel FET (TFET)

• Exploits controlled BTBT for switching — steep subthreshold slope < 60 mV/dec.
• Theoretical advantage: Ultra-low power switching.
• Challenge: Low on-current — not yet competitive with MOSFET at high speeds.

Quantum tunneling is both a fundamental challenge and an engineering tool in advanced semiconductors — managing it defines gate dielectric selection, and harnessing it enables next-generation steep-slope devices.