The Operation of a JFET and FET

Engineering
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Discuss the operation of a JFET, including the pinch off and gate-source cutoff voltage. Discuss how you can tell whether FET is operating.

May 19th, 2014

 The Junction Field Effect Transistor (JUGFET or JFET) has no PN-junctions but instead has a narrow piece of high resistivity semiconductor material forming a “Channel” of either N-type or P-type silicon for the majority carriers to flow through with two ohmic electrical connections at either end commonly called the Drain and the Source respectively.

There are two basic configurations of junction field effect transistor, the N-channel JFET and the P-channel JFET. The N-channel JFET’s channel is doped with donor impurities meaning that the flow of current through the channel is negative (hence the term N-channel) in the form of electrons.

JFET Channel Pinched-off

jfet channel pinched off

In this pinch-off region the Gate voltage, VGS controls the channel current and VDS has little or no effect.

jfet model

JFET Model

The result is that the FET acts more like a voltage controlled resistor which has zero resistance when VGS = 0 and maximum “ON” resistance ( RDS ) when the Gate voltage is very negative. Under normal operating conditions, the JFET gate is always negatively biased relative to the source.

It is essential that the Gate voltage is never positive since if it is all the channel current will flow to the Gate and not to the Source, the result is damage to the JFET. Then to close the channel:

  • No Gate voltage ( VGS ) and VDS is increased from zero.
  • No VDS and Gate control is decreased negatively from zero.
  • VDS and VGS varying.

The P-channel Junction Field Effect Transistor operates the same as the N-channel above, with the following exceptions: 1). Channel current is positive due to holes, 2). The polarity of the biasing voltage needs to be reversed.

JFET operation is like that of a garden hose. The flow of water through a hose can be controlled by squeezing it to reduce the cross section; the flow of electric chargethrough a JFET is controlled by constricting the current-carrying channel. The current also depends on the electric field between source and drain (analogous to the difference in pressure on either end of the hose). Construction of the conducting channel is accomplished using the field effect: a voltage between the gate and source is applied to reverse bias the gate-source pn-junction, thereby widening the depletion layer of this junction (see top figure), encroaching upon the conducting channel and restricting its cross-sectional area. The depletion layer is so-called because it is depleted of mobile carriers and so is electrically non-conducting for practical purposes.

May 20th, 2014

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