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Nonlinear Devices - the MOSFET

The last true device to talk about is the field effect transistor (FET), specifically the metal-oxide-semiconductor type (MOSFET). In the previous post, we finished on the idea of a BJT as a current controlled current source. Like BJTs, FETs also come in two types, nMOS and pMOS, which just have opposite behaviors. I’ll be focusing on nMOS in this post. A FET is very similar to a BJT, except we can think of it as a voltage controlled current source.

Again, the simplest way to think of FETs (or any transistor!) is to imagine them as electronic switch. We have three terminals: gate (perpendicular in the schematic), source (with the arrow), and drain (the other one). By changing the voltage at the gate, we change the current that is allowed to flow through the drain-source connection. For an nMOS, increasing the gate voltage increases the drain current. Let’s view it in a schematic form.

fet-example

Just to repeat myself, we have three modes to choose from, all of which are dependent on where we put the voltages Vd, Vg. If they’re both very small, the transistor is just off, and doesn’t do anything. If we raise both Vg and Vd, but keeping Vg larger, we move into linear mode. Here the output current is dependent on both the input voltage and output voltage. In the final state, we raise them both but keep Vd greater than Vg, we go into saturation mode, where the output is only a function of the input voltage. In text form, it’s a bit dense, so let’s view it in a tabular way (where Vd, Vg, Vs is the voltage at the drain, gate, source, respectively).

Node Voltages Mode Name Function
Vs > Vg > Vd (Cut) Off Not much! Transistor just sits there doing nothing, conducting no current
Vs < Vg > Vd Linear Here, the output current has a linear dependency on the output voltage, as well as the input voltage
Vs < Vg < Vd Saturation The output current now is only dependent on the input voltage to the gate

mosfet-iv Image courtesy Wikpedia.

The axes here are Vds - the voltage across the drain-source junction, and Id, the current through the drain-source. Each increasing line shows an increase in the gate-source voltage Vgs (it’s actually offset by a voltage Vth, but not important for now). With very low gate voltage, we’re in the cut off mode, and the device doesn’t do much of anything. With a very low voltage across the junction, we have a linear curve. We can increase the output current by either changing the voltage across that C-E terminal or the voltage at the gate. After a certain point, the line becomes horizontal, and we only have the gate current to change our output. This is the saturated mode! The red line shows the curve where we transition from linear to saturation.

In active mode, the device is called a “voltage controlled current source”, which makes sense, since the current output is just a function of our voltage input!