quantum tunneling transistor

**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.