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Chapter 18. MOSFETs

Introduction

As said in the previous lesson, the acronym FET stands for Field Effect Transistor; MOS stands for Metal Oxide Silicon.

There are enhancement and depletion MOSFETs. Each type is available in an N-channel and P-channel flavour. This means that there are four types of MOSFETs available:

JFETs and MOSFETs have many properties in common:

  • Both have a very high input resistance.

  • Both have three terminals: Drain, Gate and Source. Some MOSFETs have an extra Bulk terminal.

  • Both can look like a voltage-controlled current source; the current source between the Drain and the Source is controlled by the voltage across the Gate and the Source. The ratio dID/dVGS is called the forward transfer admittance, symbol yfs.

  • Both can also look like a voltage-controlled resistor; VGD must be between 0V and the pinch-off voltage.

JFETs and depletion MOSFETs have even more in common:

  • At VGS=0V, the D-S channel is conductive. (Enhancement MOSFETs need a certain G-S voltage before a Drain current can flow.)

  • To close an N-channel, VGS must be negative; increasing VGS will increase ID.

Of course, there are also differences between JFETs and MOSFETs. MOSFETs make perfect switches. The channel resistance in 'on mode' is very low, generally less than 10 ohms. To switch an N-channel enhancement MOSFET on, simply apply a high enough voltage across the Gate and Source. You cannot do that with a JFET, because a JFET's VGS must be negative (N-channel); when VGS is positive, the G-S diode will be forward-biased, dramatically decreasing the input resistance!

And although MOSFETs make perfect switches, they can be used in amplifiers as well.

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