What is threshold voltage in MOSFETs?

Threshold voltage (Vth) is the minimum gate-to-source voltage required to create a conducting channel between source and drain. Below Vth, the transistor is in subthreshold region with exponentially decreasing current. Above Vth, strong inversion occurs and transistor conducts normally. Vth depends on oxide thickness, doping levels, body bias, and temperature. Multi-threshold CMOS (MTCMOS) uses high-Vth transistors for low leakage in non-critical paths and low-Vth for high speed in critical paths.