Mar 152019

FET characteristics

The N-channel JFET consists of a silicon bar of N-type semiconductor with two P type regions on both sides. JFET is a tri-terminal device whose terminals are called drain, source and gate.

The Outline of an n-channel JFET is described on picture (a),  n-channel JFET symbol is shown on picture (b), channel JFET symbol is shown on picture (c).

JFET biasing requires that the pn junctions are reverse-biased. In order to have a current flowing through the channel, on a n-channel JFET or NJFET.

The Drain voltage must be greater than the Source voltage. In addition, to have the p-n junction reversed biased, the gate must have a more negative voltage than the source. Both cases are shown in the picture.

FET symbols - Fet characteristics

Main difference between JFET and BJT

Note: BJT = Bipolar Junction Transistor (NPN or PNP transistor)

The JFET electric characteristics curves are similar to the bipolar transistor curves. However, the JFET devices are controlled by a voltage, and bipolar transistors are controlled by current.

NJFET_characteristics - Fet characteristics

JFET has 3 main parameters:

  • ID (Current flowing from drain to source),
  • VGS (Gate to Source voltage) and
  • VDS (Drain to Source voltage).

There are four basic operation regions: cut off, triode or Ohmic and active or saturation. We will make a brief description of each of these regions for an NJFET. (See the image above)

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