leakage current

**Leakage Current** — unwanted current that flows through transistors even when they are "off," consuming static power and creating a fundamental scaling challenge. **Types of Leakage** - **Subthreshold Leakage**: Current through the channel when $V_{gs} < V_{th}$. Exponentially depends on $V_{th}$: 10x increase for every ~100mV decrease in $V_{th}$ - **Gate Leakage**: Quantum tunneling through the thin gate oxide. Solved by high-k dielectrics (hafnium oxide replaced SiO2) - **Junction Leakage**: Reverse-bias current through source/drain-to-body junctions - **GIDL (Gate-Induced Drain Leakage)**: Band-to-band tunneling at drain-gate overlap **Impact at Advanced Nodes** - At 7nm and below, leakage power can be 30–50% of total chip power - A modern 5nm chip with billions of transistors: Leakage alone can be 10–50W - This is why power gating (shutting off unused blocks) is essential **Mitigation** - Multi-$V_{th}$ libraries: Use HVT cells on non-critical paths - Power gating: Cut VDD to idle blocks - Body biasing: Raise $V_{th}$ dynamically when performance isn't needed - FinFET/GAA: Better gate control reduces subthreshold leakage - High-k gate dielectric: Eliminated gate leakage as a concern **Leakage current** is the primary reason chip power hasn't scaled linearly with Moore's Law — managing it is a central challenge of modern semiconductor design.