Threshold Voltage

Threshold Voltage ($V_{th}$) — the minimum gate voltage required to create a conducting channel between source and drain in a MOSFET, the most fundamental transistor parameter.

Definition

• Below $V_{th}$: Transistor is "off" (only leakage current flows)
• Above $V_{th}$: Transistor is "on" (strong inversion — current flows freely)
• Typical values: 0.2–0.5V for modern process nodes

What Determines $V_{th}$

• Gate oxide thickness (thinner → lower $V_{th}$)
• Channel doping concentration (higher doping → higher $V_{th}$)
• Gate material work function (metal gate engineering)
• Body bias / back-gate voltage

Multi-$V_{th}$ Design

• HVT (High $V_{th}$): Slow switching but very low leakage. Used for non-critical paths
• SVT (Standard $V_{th}$): Balanced performance and leakage
• LVT (Low $V_{th}$): Fast switching but high leakage. Used for critical timing paths
• ULVT (Ultra-Low): Fastest, highest leakage. Sparingly used

Scaling Challenge

• $V_{th}$ must decrease as supply voltage decreases (maintain ON/OFF ratio)
• But lower $V_{th}$ → exponentially higher leakage current
• This is the fundamental power-performance tradeoff at every node

Threshold voltage engineering is at the heart of every process technology — it determines both the speed and power of every transistor on the chip.