TF EIM Chapt6

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Field Effect Transistors (FET, JFET, MOSFET)

Properties

FETs differ from the bipolar transistors in the las chapter in that the current from a FET is only due to the majority charge carriers in the semiconductor while bi-polar transistors current is produced from both carrier types; electron and hole.

  • higher input impedance than bi-polar
  • less gain than bi-polar


JFET

JFET [math]\equiv[/math] Junction Field Effect Transistor


In a bi-polar transistor you have a depletion region with mixed charge carriers

TF EIM BipolarJunction.png TF EIM BipolarJunctionDiodeRep.png TF EIM BipolarJunctionCircuit.png
pnp bi-polar transistor Equivalence circuit Circuit diagram


In the Junction Field Effect Transistor you have a single charge carrier with the minority charge carriers forming a choke point for the majority carrier current flow. It is similar to "pinching" a garden hose when water is flowing through it.


TF EIM JFETnchan.gif TF EIM nchanDiodeRep.jpg TF EIM JFETnchanCircuit.jpeg
JFET Equivalence circuit Circuit diagram

The semiconductor material of the gate is the opposite of the channel. Here the n-p (or p-n) junction is between the gate and the channel.

The JFET operates by reverse biasing the gate-channel junction (diode) so the gate current doesn't flow in the direction indicated by the circuit diagram symbol. This means that the current through the gate is small (nAmps). As a result the input impedance looking into the gate is high (M[math]\Omega[/math]) for the equivalent circuit.

The current junction rule is

[math]I_D = I_S + I_G \approx I_S[/math]

for the Bi-Polar transistor

[math]I_C=I_E +I_B \approx I_E[/math]

FET resistor

The FET acts like a resistor.

Consider the following circuit

Let

[math]V_{DD} =[/math] the drain driving voltage

[math]R_D[/math] = resistor between the drain and V_{DD}

if [math]\rho[/math] = resistivity of the n-type semiconductor

then

r = [math]\rho \frac{\ell}{A}[/math] = resistance of the JFET
[math]I_D = \frac{V_{DD}}{r+R_D}[/math]

If you reverse bias the gate then the depletion region at the p-n junction expands into the n-type material thereby reducing the cross-sectional area (A) of the channel.

MOSFET

MOSFET[math] \equiv[/math] Metal-Oxide-Semiconductor Field Effect Transistor

Forest_Electronic_Instrumentation_and_Measurement