MOSFETS

Before we proceed any further, we should briefly explain what a Transistor is and what a semiconductor is.

Semiconductor : A substance that has the electrical conductivity(how well electrons flow) between a conductor(i.e. Copper) and an insulator(i.e. Fiberglass)

Transistor : A semiconductor device that can be used to amplify or switch electrical signals. They can be split into Bipolar Transistors(BJT) and Field Effect Transistors(FET). For the purposes of this document we’re discussing FETs

Think of Transistors like a switch that isn’t manually controlled by a human, instead we can electrically control them. BJTs are often regarded as “current controlled switches” and FETs as “voltage controlled switches”. By adding some current and voltage to the control pin(Gate in a FET, Base in BJT), it allows current to flow, this is part of how a Transistor acts as an amplifier.

As we continue on in this document, we will only be discussing MOSFETs

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As you can see from the diagrams provided, there are 4 parts to a MOSFET(the Bulk is usually referred to as the “Body)

N and P channel MOSFETs differ from their construction, as you can see from the diagram above(Left is N-CH and Right is P-CH). n-type and p-type silicon refer to how the silicon is doped(adding different substances with silicon to alter its charge characteristics).

MOSFET Operation(Assuming a N-CH Enhancement Mode FET, will explain what this means later)

Let’s define some terminology that you often see in MOSFET datasheets:

  • Vgs - Voltage between the Gate and Source terminal of the MOSFET

  • Vds - Voltage between the Drain and Source terminal of the MOSFET

  • Vth - Threshold voltage a MOSFET(compared to Vgs normally, will be explained

  • Rds(on) - Resistance between the drain and source terminals when the MOSFET is fully on.(usually in the magnitude of milliOhms) for NMOS, PMOS is much higher because it’s hole carrier based)

Cutoff Region

If Vgs ≤ Vth , the MOSFET acts as an “Off” Switch(no current flow, open circuit)

Think of it like a water pipe with a control knob that controls the flow of water. If the knob is closed(or below a certain amount), water doesn’t flow because there’s not enough pressure.


Linear Region

If Vgs ≥ Vth and Vds ≤ Vgs - Vth the MOSFET acts as an variable resistor, “partially” on.

Think of it like a water pipe with water flowing, but how far you turn the knob controls the rate that water is flowing through because of the pressure the water pump provides.

 

Saturation Region

If Vgs ≥ Vth and Vds ≥ Vgs - Vth the MOSFET acts as “fully” on.

Think of it like a water pipe with water flowing, and you’ve turned the knob all the way, so moving the knob more doesn’t change the water flow rate anymore.

 

Enhancement vs Depletion MOSFET

  • Enhancement-mode MOSFETS are commonly used as switching elements in most integrated circuits and are off at zero gate-source voltage

    • NMOS can be turned on by pulling the gate voltage higher than the source voltage, and PMOS can be turned on by pulling the gate voltage lower than the source voltage

  • Depletion-mode MOSFETS are less common than enhancement-mode MOSFETS and are normally on at zero gate-source voltage

    • These can be used as load “resistors” in logic circuits

  • The mode of a MOSFET can be determined by the sign of the threshold voltage on the datasheet. For an N-type FET, enhancement-mode devices have positive thresholds whereas depletion-mode devices have negative thresholds; the reverse is true for P-type FETs

    • It could also just say what it is on the datasheet! (If it is depletion-mode, it will be explicitly stated somewhere. If it just doesn’t say, it's in enhancement-mode)

 

 

If this is too much words for you, use this table(On Switch = switch closed, Off Switch = switch open)

If you want to see more details on the applications of MOSFETs(H-Bridges, Gate Drivers, Power MOSFETS), I invite you to take a look at the Electrical Knowledge database from our friends at WARG. They have a great knowledge database available
FETs