Table of Contents
What Is Threshold Voltage?
The threshold voltage (Vth) of a MOSFET is the minimum gate-to-source voltage required to create a conducting channel between the source and drain terminals. Below this voltage, the transistor is in cutoff and essentially no current flows. The threshold voltage is one of the most critical parameters in MOSFET design and determines switching speed, power consumption, and noise margins in digital circuits.
The threshold voltage depends on several physical factors: the flat-band voltage (determined by the work function difference between the gate material and substrate), the surface potential needed to invert the channel, the depletion charge in the substrate, and the oxide capacitance. Temperature and body bias also affect Vth.
Threshold Voltage Formula
Factors Affecting Vth
| Factor | Effect on Vth |
|---|---|
| Higher substrate doping NA | Increases Vth |
| Thicker oxide tox | Increases Vth |
| Higher temperature | Decreases Vth |
| Reverse body bias VSB | Increases Vth (body effect) |
| High-k dielectric | Allows lower Vth with same leakage |
Typical Values
- Standard CMOS (180nm): Vth ~ 0.4 to 0.6V
- Low-power CMOS: Vth ~ 0.5 to 0.7V (high Vth to reduce leakage)
- High-performance CMOS: Vth ~ 0.2 to 0.3V (low Vth for speed)
- Power MOSFETs: Vth ~ 1.0 to 4.0V
Frequently Asked Questions
What is the body effect?
The body effect is the change in threshold voltage caused by a voltage difference between the source and body (substrate) terminals. When VSB > 0, the depletion region widens, increasing Vth. This is characterized by the body effect coefficient gamma.
How does scaling affect threshold voltage?
As MOSFETs are scaled down, Vth must also decrease to maintain performance at lower supply voltages. However, lower Vth increases subthreshold leakage current exponentially, creating a tradeoff between performance and power consumption that is a central challenge in modern CMOS technology.