53-jfet-transistor

JFET Transistor is electronic component with field effect. It has three terminals: source, drain and gate.

Main current in JFET transistor flows from drain to source. Space between source and drain is called channel. Depending on semiconductor type, channel can be of either P or N type. In JFET transistor, small arrow symbolize gate, and it is pointing from P to N side of the PN junction. In figure bellow, on the left hand side there is symbol of N channel JFET, and on the right hand side there is symbol of P channel JFET.

jfet-symbols

Amplification is achieved with electric field effect. PN junction between gate and the channel is reverse biased. Space charge region is spreading into less doped part of the semiconductor, i.e into the channel. By changing voltage level between gate and source, space charge region vary and effective width of the channel vary. On this way, resistance of the channel is changing and this is how gate to source voltage controls drain to source current. If gate-to-source voltage is such that PN junction is reverse biased but channel is not pinched through out all it’s length, JFET is conducting current. It can conduct in:

  • Ohmic mode
  • Saturation mode

Ohmic mode is when drain to source voltage is much lower than gate to source voltage, when pinch is not formed into channel yet. This mode corresponds to “ON” state of the electric switch.

Saturation mode is when drain to source voltage can’t be neglected, pinch is formed, but it is not covering all channel. In that mode JFET operates similar to bipolar transistor in active mode (current amplification is highest).

If voltage gate to source is higher in amplitude than pinch voltage, pinch covering whole channel, and JFET is not conducting. This mode corresponds to “OFF” state of the electric switch.

Transfer function of N channel and P channel JFET is given respectively in Figure bellow.

transfer-jfet

Maximal current through JFET is when VGS=0. This current is called drain-to-source saturation current (IDSS). In normal operation VGS<0 for N channel and VGS>0 for P channel JFET. If, for example polarity of VGS was direct, whole current would flow from drain to gate, instead from drain to source and JFET wouldn’t operate as amplifying component.

JFET transistors are widely used in many schemes, such as:

 

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