Mosfet Operation: Vgs, Vds, and Vth Explained

[Genji Shimada]

Let's say I have a Mosfet with Vth=4V. According to the circumstances for it to be in linear mode Vds<Vgs-Vth, if I apply Vgs of 10volts, my Vds must be lower than 6V in order for the transistor to operate just under saturation. Is that right? And if I want to use it as a switch, I would apply Vds>6V to saturate it, right?

 

[CraigHB]

MOSFETs are most typically used in power switching applications. It's possible to operate them in the linear region and they are sometimes, especially when built on a chip that performs a particular power management function. In that case transistor design is somewhat different from a discrete MOSFET used in power switching.

Vgs(th) provides a rough indicator of voltage required to saturate the device. Vgs(th) occurs at the top of the linear region, but the value provides an indication for tun-on voltage. Logic level MOSFETs are typically fully on at 4.5 Volts Vgs with a Vgs(th) around 1.5V. Some can be found with threshold voltages around a half Volt that may be fully on as low as 2.5 Volts Vgs. It's pretty much independent of Vds. Drain-source current is primarily a function of Rds and voltage across the drain-source terminals.

If you look at a power MOSFET data sheet you'll sometimes see a curve for Rds versus Vgs. Not all power MOSFET data sheets show that graph but they always at least list a Rds for a particular Vgs such as 4.5V or 10V. The data sheet implies the turn-on voltage when specifying Rds(on). If you see a Rds(on) spec for 4.5V that would be the minimal Vgs you want to use in the application. Of course you can go higher as long as voltage is within tolerance of the part. You can also go a little lower. If you look at a Rds versus Vgs curve you'll see it's hyperbolic with a knee at some voltage below the Rds(on) spec. As long as Vgs is past that knee the part will work well as a switch. For example there's a knee in the curve around 3V for a part with a 4.5V Rds(on) spec and a knee around 1.5V for a part with a 2.5V Rds(on) spec.